Consumption of underutilised grain legumes and the prevention of type II diabetes and cardiometabolic diseases: evidence from field investigation and physicochemical analyses

doi:10.21203/rs.3.rs-1963563/v1

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Consumption of underutilised grain legumes and the prevention of type II diabetes and cardiometabolic diseases: evidence from field investigation and physicochemical analyses

https://doi.org/10.21203/rs.3.rs-1963563/v1

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Background

Grain legumes (GLs) are used in food as well as in traditional medicine due to their therapeutic properties. This study was carried out to correlate grain legume consumption to the prevention of type II diabetes and some cardiometabolic diseases (high blood pressure, obesity, and stroke).

Materials and methods

An exploratory survey was carried out among the population (n = 468) of Diamaré’s division (Maroua, Cameroon). A correlation study was carried out to demonstrate the link between the consumption of certain GLs and the prevention of metabolic diseases. A compositional analysis of selected GLs was performed according to the referenced methods.

Results

Significant correlations were observed between the consumption of soybean, Bambara beans, and cowpea and the prevention of certain metabolic diseases. Remarkable protein content was noted in soybean’s whole flour (WF) (38.133 ± 0.366 g/100g DM), followed by the underutilised cultivar BVB of Bambara bean (26.434 ± 0.366 g/100g DM). The cowpea seed has the highest total carbohydrate and vitamin C content of 59.294 ± 0.129 g/100g DM and 327.715 ± 0.336 µg EAA/100g DM, respectively, followed by the underutilised cultivar BVB with values of 57.696 ± 0.097 g/100g DM and 261.951 ± 0.376 µg EAA/100g DM. The three underutilised Bambara bean cultivars’ WF have the most remarkable dietary fibre content (18–28%). Soybean’s WF and those of the three underutilised Bambara bean cultivars contained the highest levels of total phenolic compounds and flavonoids, i.e., 206.089 ± 0.087 mg GAE/100 DM, 143.868 ± 0.299 mg QE/100g DM for soybean, respectively. The highest free radical scavenging activity (DPPH) was noted with the polyphenolic compounds’ extracts from soybean’s WF, i.e., 21.193 ± 0.049 µg TE/100g DM, followed by those from the underutilised BVB cultivar with a value of 16.594 ± 0.160 µg TE/100g DM. The most notable ferric reducing antioxidant power (FRAP) was obtained with soybean (34.481 ± 0.029 µg TE/100g DM), followed by the cultivar GHB (32.036 ± 0.076 µg TE/100g DM).

Conclusion

The significant levels of protein, phenolic compounds, and dietary fibre in these seeds confirmed these correlations and their potential therapeutic effects and nutritional properties.

underutilised grain legumes

survey

bioactive compounds

preventive nutrition

Noncommunicable diseases

Noncommunicable diseases (NCDs) are the leading cause of morbidity and mortality worldwide, killing more people each year than all other causes combined. The available data show that almost 80% of deaths from NCDs occur in developing countries [1, 2]. The prevalence of these NCDs in the world is a real public health problem. They are mainly linked to smoking, alcohol, a lack of physical exercise, and a poor diet. The poor diet is correlated to the proliferation of diabetes, high blood pressure, cancer, and obesity. These metabolic diseases are the cause of the thousands of deaths recorded worldwide and particularly in low-income countries [1, 2]. According to the Food and Agriculture Organisation of the United Nations [2], more than 820 million people suffer from malnutrition worldwide. Globally, an estimated 422 million adults were living with diabetes in 2014, up from 108 million in 1980. The global prevalence of diabetes has almost doubled since 1980, from 4.7 to 8.5% of the adult population.

In Cameroon, NCDs are the cause of 239,000 cases of death estimated in 2012 by the World Health Organisation. These NCDs include diabetes (2 to 5% or 2,340 people aged 30 to 69), and cardiovascular diseases (11% of the population) [3]. In addition to these diseases, Cameroon also faces the problem of severe malnutrition, particularly in the Adamawa, East, North, and Far-North regions [4].

According to the diagnostic report of the territory carried out by the Ministry of the Economy, Planning and Territorial Development-Cameroon [5], the Far-North region is one of the largest pulse production areas in Cameroon. In this region, grain legumes (GLs) are affordable. However, having been consumed for several millennia, these pulses have gradually been forgotten and are used mainly in the manufacture of food for livestock. Despite being high in nutrients, easily accessible, and beneficial to health, legumes promote agricultural sustainability and aid in the mitigation of climate change [2, 6].

Data presented in WHO reports [1, 3] show that NCDs affect all countries, with low- and middle-income countries bearing the heaviest burden of mortality and disease. The cost of combating NCDs at the individual and national levels, as well as the loss of productivity due to premature death, are major impediments to poverty reduction and sustainable development. This work is aligned with the goals of fighting poverty, eliminating hunger, ensuring food security, improving nutrition, and promoting sustainable agriculture in order to meet the first three United Nations Sustainable Development Goals (SDGs). Similarly, by promoting economically accessible GLs (some of which are underutilised) with bioactive potential, this work aligns with the third SDG of empowering people to live healthy lives and promote well-being at all ages. As well by encouraging the consumption of GLs in order to prevent and/or treat NCDs. It is with this in mind that the present study will first involve carrying out field investigations in order to identify the GLs varieties (produced and consumed) with technological, nutritional, and therapeutic properties. Secondly, to establish correlations between primary field data and statistical data of certain metabolic disease frequencies (obtained from hospitals), this is in order to establish correlations between GLs consumption and the prevention of metabolic diseases. And finally, to analyse the physicochemical characteristics of some selected GLs based on the survey’s results, in order to correlate the probable therapeutic properties of these GLs with the bioactive molecules that they contain.

Survey

Study area

Primary data was collected through surveys carried out in the Far North Region (Cameroon), more precisely in the nine (9) districts of the Diamaré’s division. This region is one of the ten regions of Cameroon and the most populous, located in the north of the country and bordering Chad and Nigeria. Its capital is Maroua [5].

Survey sheet and procurement method

According to Combessie [7], the survey sheet is made up of open and closed questions that are asked to the interviewee and the answers to which are noted or ticked off on the sheet by the interviewer. The preferred method of handover during our field investigation is face-to-face.

The survey was conducted ethically and with the consent of the participants. The ethics committee of the Faculty of Science of the University of Maroua (0020/004/UMA/FS/D/VD-RC) and the Governor of the Far North Region (Cameroon) (0000437/AR/K/SG/DAESC) approved the conduct of the field survey. The statistical data of patients in the hospitals was obtained following the approval of the Regional Delegate of the Ministry of Public Health of the Far North region (Cameroon) (00692/L/19/MINSANTE/SG/DRSPEN/MRA).

Sampling

The sample was calculated by following Eq. 1 [8]:

The Diamaré division’s population is estimated at 4 186 844, according to the MINEPAT report [5]. The application of Yaman's formula [8] estimates the sample to be 399,9618 (approximately 400) individuals. According to the inclusion criterion, only people aged 15 or more are interviewed. In addition, in the MINEPAT report [5], individuals aged 15 and over constitute 50% of the population of the Diamaré’s division. Thus, the minimum sample size is 199,9809 (approximately 200) individuals.

Data analysis

Statistical processing of the data collected as well as the entry and production of raw tables and figures were carried out with the software Statistical Package for the Social Sciences for Windows (SPSS 22.0, IBM SPSS Inc.), Sphinx Plus² v5 and STATISTICA 10.0 (Stat Soft, TIBCO Software Inc.). Quantitative variables were expressed as numbers and percentages (frequencies). The significance of the statistical classics was expressed as a function of the Pearson and R² Correlation.

Physicochemical properties

Reagents and Chemicals

Standards (98% >) such as Gallic Acid, Quercetin, Catechin, Trolox, Orcinol, as well as Vanillin and Trolox were purchased through Sigma-Aldrich Chemical Company (Mumbai, India). Sodium carbonate, methanol, ethanol, and other solvents were obtained from the commercial source Fisher (New Jersey, USA).

Seed samples

At the end of the field investigations and the analysis of the primary data, a few cultivars with bioactive potential were subjected to physicochemical analyses. The samples consist mainly of seeds of Bambara bean (coded BVB, BU, and GHB), soybean (coded BS), and cowpea (coded RC). These samples were collected in the Diamaré’s division.

Whole flour production

Seeds were cleaned and then weighed using an electronic balance with a sensitivity of 0.0001g (Entris Sartorius). They were ground using a grinder (brand) and sieved using a 200µm mesh according to the protocol described by Mune and Sogi [9] with slight modifications.

Proximate composition

Moisture, fat, ash, protein, and total dietary fibre in the samples were determined using AOAC standard analytical methods [10]. Moisture content was determined by drying the GL in an oven at 105°C until a constant weight was obtained in accordance with AOAC 925.40 procedures [10]. The ash content of the seeds was determined by incinerating them in a muffle furnace at 550 ° C in accordance with AOAC 942.05 procedures [10]. The nitrogen (N) content was determined using the Kjeldahl method, according to AOAC 984.13 procedures. The protein content was calculated as N x 6.25. The lipid content was determined according to AOAC 963.15 methodology. The total carbohydrate content was determined according to the phenol-sulfuric method described by DuBois et al. [11] with some modifications.

Phenolic compounds extraction

Polyphenols were extracted as described by Laya & Koubala [12] with slight modifications. 200 mg of GL flour was mixed with 10 ml of 80% methanol and stirred for 24 hours at room temperature (25 ± 4°C). The mixture was filtered through Whatman 4 filter paper, and the filtrate was collected. This filtrate was then diluted to a tenth (1/10) and stored at 4°C until use.

Total polyphenolic content

The polyphenol content of the methanolic extract was determined according to the Folin-Ciocalteu colorimetric method described by Singleton et al. [13]. The absorption of phenolic compounds was measured at 745 nm using a UV-VIS spectrophotometer (PRIM Light & Advanced, Germany). Gallic acid (0-250 µg/mL) was used to plot the standards curve. The results were expressed in milligrams of gallic acid equivalent per 100 grams of dry matter (mg GAE/100 g DM).

Flavonoid content

The flavonoid content of the methanolic extract was determined using the vanillin-HCl reagent as described by Brainbridge et al. [14]. Absorbance was measured at 430 nm. A standard curve was plotted with quercetin (0-100 µg/mL). The results were expressed in milligrams of quercetin equivalent per 100 grams of dry matter (mg QE/100 g DM).

Tannin content

The tannin contents of the methanolic extract were determined using aluminium chloride as described by Gaytan-Martínez et al. [15]. The tannin content was measured at 500 nm. The standards curve was plotted with catechin (0–100 µg/mL), and the results were expressed as milligrams of catechin equivalent per 100 grams of dry matter (mg EC/100 g DM).

Vitamin C content

Based on the reduction of potassium permanganate's absorbance at 525 nm, the vitamin C content was determined using the techniques outlined by Barros et al. [16] and Barrett and Lloyd [17]. 0.2g of the sample was mixed with 5 mL of 0.5% oxalic acid, stirred in the dark for 24 hours, and filtered through Whatman No. 4 filter paper. Following that filtration, 1.5 mL of KMnO₄ (0.1%) had been dissolved in H₂SO₄, mixed, and homogenised with an aliquot of 0.5 mL of the sample. A UV-VIS spectrophotometer (PRIM Light, Germany) was used to measure the mixture's absorbance at 525 nm. The vitamin C quantification was calculated from the curve plotted using the L-ascorbic acid standard (0–80 µg/mL). The results were expressed in milligrams of L-ascorbic acid equivalent per 100 grammes of edible portion (µg AAE/100 g DM).

Antioxidant properties

DPPH (2,2-Diphényl-2-picrylhydrazyl)

DPPH was carried out as described by Sun et al. [18] with slight modifications. 500 µL of methanolic sample (0.1 g/mL) or standard was mixed with 1500 µL of 1 mM DPPH, and the mixture was stirred for 10 min before incubating for 40 min at room temperature (25 ± 4°C) in the dark. The experiments were done in triplicate. Absorbance of the mixture was measured at 517 nm. The results were expressed in microgram Trolox equivalent per 100 grams of dry matter (µg TE/100 g).

FRAP (Ferric Reducing Antioxidant Power)

FRAP was determined as described by Benzie and Strain [19]. Briefly, 500 µL of methanolic sample (0.1 g/mL) or standard was mixed with 1500 µL of FRAP reagent (250 µL of acetate buffer 0.3 M, pH 3.6, 225 µL of 2,4,6-Tris(2-pyridyl)-s-triazine 0.01 M in 40 mM HCl, and 225 µL of FeCl₃ 140 mM). The experiments were done in four repetitions. Then, the mixture was shaken and incubated for 40 min at room temperature (25 ± 4°C). The absorbance of the mixture was measured at 593 nm. The results were expressed in milligram Trolox equivalent per 100 grams of dry matter (µg TE/100 g DM).

Data analysis

All experiments were carried out at least three times. The obtained results were expressed as the mean ± standard deviation. Means were compared using one-way ANOVA and the Duncan, Tukey, and Student-Newman-Keuls (S-N-K) tests in post hoc. At a 0.05 threshold, the means were classified into homogeneous subclasses. For this, the Statistical Package for the Social Sciences (SPSS 22.0 for Windows) was used.

Survey

Study area and profile of the sample

Primary data was collected through the survey carried out in the Far-North Region (Cameroon), precisely in the nine districts of the division of Diamaré (Figure S1, supplementary material). This data was collected from households, hospitals, and markets. The sample is composed of 468 respondents randomly selected through survey cards. The characteristics of the respondents are presented in Table 1.

Table 1

Sample characteristics
Characteristics	Number	% Frequency
Gender
Male	246	52,56%
Female	222	47,44%
Age (in years)
15–24	140	29,91%
25–34	174	37,18%
35–49	100	21,37%
50–64	50	10,68%
65+	4	0,85%
Socio-economic profession
Farmer	67	14,32%
Trader	104	22,22%
Inactive	34	7,26%
Households	112	23,93%
Student	144	30,77%
Teacher	5	1,07%
Doctor	4	0,85%
Education
College graduate	158	33,76%
Less than high school	129	27,56%
Higher school graduate	106	22,65%
Some college	75	16,03%

GLs varieties: production and consumption

Soybean [Glycine max (L.) Merr.]

Soybeans, Glycine max (L.) Merr., are a legume rich in protein and other nutrients beneficial to health. Soybeans are the fourth most consumed GL in the Diamaré’s division as shown by the results of the survey (in Figure S2, supplementary material) according to the different survey sites (9 districts of the Diamaré’s division). This GL is widely consumed in the Maroua I, II, and III districts at an average frequency of 3 to 6 times (day) per week. Soybean is consumed in the form of soy flour (soy milk) mainly. However, in the Ndoukoula, Dargala, and Pette Districts, soybeans are consumed once to twice a month and/or less (rarely and/or occasionally).

Bambara bean [Vigna subterranea (L.) Verdc.]

Bambara bean (BB) is an herbaceous plant in the legume family of Fabaceae. BB [Vigna subterranea (L.) Verdc.] is the third legume, the most important in production and consumption terms after groundnuts and cowpeas [20]. The plant is cultivated for its seeds rich in proteins, carbohydrates, and trace elements [6, 21, 22].

According to the survey results (Fig. 1) and the observations of the Food and Agriculture Organisation of the United Nations [20], BBs are the third most consumed legume in Diamaré. Field investigations revealed several varieties (cultivars) of BB depending on seed coat colour. The field investigation’s results show the different most consumed BB cultivars according to the survey area. These most consumed cultivars are constituted by the beige-coloured BB cultivar in the localities of Maroua (I, II, III), Bogo, Pette, and Gazawa Districts, with a frequency of 2 to 6 times a week (very often), according to field investigations (Figure S3A, supplementary material). However, in the localities of Méri and Dargala, the red BB variety is the most consumed at a frequency of 2 to 6 times (day) per week. The least consumed are the red (uniform, variegated black), brown, and black BB cultivars. These underutilised GLs are consumed at a frequency of 2 to 6 times per month (Figure S3, Figure S4, supplementary material).

Bean [Phaseolus vulgaris (L.)]

Beans are one of the most consumed GLs in the Diamaré’s division after BB and soybeans. Three cultivars were encountered during the field investigations, in particular the red, black, and green cultivars.

The field investigation’s results show that the red bean is widely consumed in Maroua (I, II, and III) at 5 to 6 times (day) per week (Figure S5, supplementary material). This is justified by the accessibility and availability of this GL. These results are in agreement with the statistical observations of the Food and Agriculture Organisation of the Nations [20] and the MINEPAT diagnostic report [5].

Groundnut [Arachis hypogea (L.)]

According to the survey results, this GL is highly consumed on a regular basis (6/7 days) (Figure S6, supplementary material). Groundnuts are one of the most widely produced GL in the world. It is, therefore, the most consumed GLs [20]. This GL is consumed in the form of seeds, oil, and other forms (caramels). These results would be justified by the fact that this legume is the most produced in terms of quantity. The results agree with the observations of FAO [20] and MINEPAT Cameroon [5].

Cowpea [Vigna unguiculata (Linn.) Walp.]

Cowpea is a species of plant of the Fabaceae family and the Vigna genus native to tropical Africa, several subspecies of which are cultivated as food plants for their seeds, close to beans, or for their pods. It is the main food legume in tropical Africa, providing an economical source of high-quality protein [9, 23]. These investigations show that white cowpea is the variety most consumed in the nine localities (Figure S7, supplementary material). The least consumed varieties of cowpeas in these different localities are red and black cowpeas.

GLs varieties: nutritional and therapeutic uses

Pulses in diet

GLs such as groundnuts, cowpeas, Bambara bean, and beans have pride of place in food and cooking in the Far North region. The GLs consumption’s frequencies in the Diamaré’s division are shown in Fig. 1.

Groundnuts are the most consumed GL in the Diamaré’s division with a frequency of 23%, according to the results presented in Fig. 1. Followed by cowpea and BBs with respective frequencies of 22% and 20%. Soybeans, beans, and sesame follow successively behind Bambara groundnut. These results are in agreement with the observations of the Food and Agriculture Organisation of the United Nations [20] as well as the diagnostic report of the territory carried out by the Ministry of Economy, Planning and Land Planning – Cameroon [5]. These results rank Bambara groundnut in third place, after groundnuts and cowpeas. These observations could be explained by the fact that these legumes are the most produced [20].

Pulses for therapeutic use

Beyond the strict coverage of nutritional needs, GL has undeniable advantages for health. They are used due to their therapeutic properties. During the field investigations, the local populations provided information concerning the potential therapeutic effects of some GL, summarised in Table 2.

Table 2

Therapeutic uses frequencies of some grain legumes
Varieties	Uses	Survey areas									Totals
Varieties	Uses	Maroua 1	Meri	Maroua 2	Maroua 3	Bogo	Gazawa	Pette	Ndoukoula	Daragala	Totals
Soybean	Against gastrointestinal acidity	0,452ⁱ²	0,000^f3	3,620^d1	0,000^d3	0,000ⁱ³	0,452^f2	0,000^e	0,000^f3	0,452ⁱ²	4,977
	Child nutrition	5,882^c6	1,357^d7	14,027^a2	1,357^b7	19,457^a1	7,692^a5	13,575^a3	14,027^a2	10,860^b4	88,235
	Weight gain	1,357^e2	0,000^f4	0,905^f3	0,000^d4	0,905^g3	1,810^e1	0,000^e4	0,905^d3	0,905^g3	6,787
Bambara bean	Helps the brain to memorise	0,958^g3	0,000^f6	0,639^g4	0,000^d6	0,000ⁱ⁶	0,319^g5	0,958^d3	1,278^c2	4,473^c1	8,626
	Antidiabetic	0,958^g5	0,958^e5	4,153^c2	0,000^d6	2,236^d4	6,390^b1	0,958^d5	0,000^f6	2,556^e3	18,211
	Against skin allergy	0,319^j1	0,000^f2	0,000^h2	0,000^d2	0,000ⁱ²	0,000^h2	0,000^e2	0,000^f2	0,000^k2	0,319
	Intelligence	1,278^f6	5,751^b2	6,070^b1	0,000^d8	0,958^f7	1,917^d5	3,834^b3	1,278^c6	2,236^f4	23,323
	Against gastrointestinal acidity	0,319^j2	0,000^f3	2,556^e1	0,000^d3	0,000ⁱ³	0,000^h3	0,000^e3	0,000^f3	0,000^k3	2,875
	Against back pain	0,319^j2	0,958^e1	0,000^h3	0,000^d3	0,000ⁱ³	0,000^h3	0,000^e3	0,000^f3	0,000^k3	1,278
	Improves eyesight	0,639^h4	0,000^f5	0,000^h5	0,000^d5	5,112^b1	0,000^h5	3,195^c2	0,000^f5	2,875^d3	11,821
	Against cataracts	0,000^k2	0,000^f2	0,000^h2	0,000^d2	0,319^h1	0,000^h2	0,000^e2	0,000^f2	0,000^k2	0,319
	Used in mysticism	7,987^a1	1,597^c3	4,153^c2	1,597^a3	0,000ⁱ⁵	0,000^h5	0,000^e5	0,639^e4	0,000^k5	15,974
	Hypotensive	0,639^h4	0,000^f6	0,000^h6	0,958^c3	2,875^c2	0,000^h6	0,000^e6	3,834^b1	0,319^j5	8,626
	Used in case of digestive disorder	0,000^k2	0,000^f2	0,000^h2	0,000^d2	1,597^e1	0,000^h2	0,000^e2	0,000^f2	0,000^k2	1,597
	Child nutrition	0,319^j1	0,000^f2	0,000^h2	0,000^d2	0,000ⁱ²	0,000^h2	0,000^e2	0,000^f2	0,000^k2	0,319
	Against dizziness	0,319^j1	0,000^f2	0,000^h2	0,000^d2	0,000ⁱ²	0,000^h2	0,000^e2	0,000^f2	0,000^k2	0,319
	Consolidation of the brain after a shock	0,319^j4	0,000^f5	0,000^h5	0,000^d5	1,597^e2	3,195^c1	0,000^e5	0,639^e3	0,639^h3	6,390
Cowpea	Food	3,846^d1	0,000^f2	0,000^h2	0,000^d2	0,000ⁱ²	0,000^h2	0,000^e2	0,000^f2	0,000^k2	3,846
	Against allergy	0,000^k2	0,000^f2	0,000^h2	0,000^d2	0,000ⁱ²	0,000^h2	0,000^e2	0,000^f2	46,154^a1	46,154
	Antidiabetic	3,846^d1	0,000^f2	0,000^h2	0,000^d2	0,000ⁱ²	0,000^h2	0,000^e2	0,000^f2	0,000^k2	3,846
	Anti-venomous	0,000^k2	38,462^a1	0,000^h2	0,000^d2	0,000ⁱ²	0,000^h2	0,000^e2	0,000^f2	0,000^k2	38,462
	Accelerates wound healing	7,692^b1	0,000^f2	0,000^h2	0,000^d2	0,000ⁱ²	0,000^h2	0,000^e2	0,000^f2	0,000^k2	7,692
Results with the same numbers (1, 2, 3) on the same row are not significantly different (P < 0.01)
Results with the same letters (a, b, c) on the same column are not significantly different (P < 0.01)

Therapeutic uses of soybean

According to the field surveys, people use soybeans not only to cover their nutritional needs but also for therapeutic purposes (Figure S10, supplementary material). It emerges that Diamaré's population uses soybeans to fight against undernutrition children's problems with an 88.24% frequency. Soymilk is also used by the population for gastrointestinal acidity (stomach ailments). These results are in accordance with FAO’s observations [24]. Studies carried out by FAO in 2004 have confirmed the current and likely future importance of soybeans for human nutrition globally. These analyses show the contribution of soybeans to the fight against hunger in food insecure countries. Soybeans are composed of approximately 40% protein and 20% oil. This legume's high energy content makes it a highly energetic food. Soybeans can play an important role in alleviating protein-calorie malnutrition [23, 24]. In addition, it is easy to grow, relatively drought-tolerant, and households can store it easily [23, 24]. The soybean benefits are not taken lightly in a country where more than half of children under five in rural areas are malnourished or stunted.

Therapeutic uses of Bambara beans

Beyond the coverage of energy and nutritional needs, BBs constitute a nutraceutical for Diamaré's populations. BB is used in traditional medicine to treat various ailments such as diabetes, high blood pressure, and dementia. In addition, it is also used to improve eyesight and enhance memory (intelligence).

The primary data processed on the BBs’ therapeutic properties are presented in Fig. 2. It emerges in general terms and in all the districts that Bambara groundnut is used to improve and/or enhance memory (24.265%), to help diabetics and hypertensives control their blood sugar (18.015%) and blood pressure (9.559%), respectively. This survey highlights the therapeutic potentials of Bambara groundnut seeds in the prevention of neurodegenerative diseases, diabetes, cardiometabolic diseases, and many other ailments. The primary collected data are in agreement with the in vivo work carried out on the study of the neuroprotective effects of BB’ extracts [25]. According to Abaissou et al. [25], aqueous extracts have a favourable effect on intelligence and memorisation. Likewise, the potential antidiabetic and antihypertensive properties have been confirmed by Mune et al. [26]; Arise and Malomo [27]. Angiotensin Converting Enzyme and Dipeptidyl Peptidase IV inhibitory effects of BB protein hydrolysates were investigated in this study [26]. In addition to protein and protein hydrolysates properties, dietary fibre may explain BBs' anti-diabetic and antihypertensive properties. A diet high in fibre or whole grains has been shown in several large cohort studies to reduce the risk of coronary heart disease [28, 29]. Furthermore, according to WHO [28], dietary fibre consumption will help with glycaemic index and obesity management.

Other studies also look at the therapeutic properties of BBs. This GL is used as a treatment for diarrhoea and to relieve nausea associated with pregnancy [30]. These observations agree with the present results (used in the context of digestive disorders at a frequency of 1.838%). Other prophylactic and therapeutic uses of BBs include use against kwashiorkor protein deficiency and treatment of cataracts [30]. These results agree with the obtained results from the survey (Table 2).

Therapeutic uses of Cowpea

In addition to its nutritional use, cowpea is also used by the Diamaré’s population for therapeutic purposes. Cowpea seeds are mainly used to treat cases of allergies or bites from poisonous reptiles. The seed is crushed and placed on the injury to act as a sponge. A small portion of the population pointed to the use of cowpeas in the prevention of type II diabetes (Figure S12, supplementary material). The obtained results are in conformity with Berhaut and Masunov's [31] and Michels et al.'s [32] works. Their studies made it possible to identify the therapeutic virtues of cowpea. Cowpea powder applied to a wound has a sanitising effect.

Other therapeutic properties mentioned by the local population include the use of cowpeas for wound healing. Locally, cowpea seeds are used not only in cases of venomous reptile bites, but also to accelerate wound healing. This can be mainly justified by the richness of this seed in vitamin C (Table 3). A multitude of studies have pointed to a close relationship between vitamin C and the wound healing process. Indeed, Mohammed et al. [33] established that ascorbic acid supplementation induces the expression of self-renewal genes and promotes fibroblast proliferation. Vitamin C has a favourable impact on the spatio-temporal expression of transcripts associated with early resolution of inflammation and tissue remodelling. In addition, Barrios-Garay et al. [34] have also shown from in vivo animal studies that vitamin C appears to accelerate bone formation due to increased osteoblastic proliferation and differentiation and its antioxidant function when pro-oxidant substances are added. These studies corroborate with the results obtained from the survey on the therapeutic uses of cowpea seeds.

Table 3

Proximate composition, vitamin C content, polyphenol compounds content, and antioxidant properties of GLs’ whole flours
Varieties	BVB	BU	GHB	BS	RC
Dry matter	91.088 ± 0.082^c1	93.921 ± 0.723^a1	92.195 ± 0.070^b1	92.902 ± 0.088^{a. b1}	89.409 ± 0.479^d1
Moisture	8.912 ± 0.082^b2	6.079 ± 0.723 ^d2	7.805 ± 0.070^{b c2}	7.098 ± 0.088^c2	10.591 ± 0.479^a2
Ash	4.258 ± 0.354^a3	4.134 ± 0.273^a3	3.457 ± 0.012^b3	3.649 ± 0.332^b3	3.415 ± 0.047^b3
Fat content (%)	7.935 ± 0.047^c4	7.552 ± 0.137^d5	8.710 ± 0.087^b5	9.594 ± 0.054^a4	7.570 ± 0.076^d5
Protein content (%)	26.434 ± 0.366^b3	19.034 ± 0.366^d4	18.743 ± 0.457^d4	38.133 ± 0.366^a3	22.880 ± 0.366^c3
Carbohydrate (%)	57,696 ± 0,097^b2	53,711 ± 0,000^d2	55,959 ± 0,064^c2	49,931 ± 0,029^e2	59,294 ± 0,129^a2
Vitamin C (µg AAE/100g)	261.951 ± 0.376^b1	161,538 ± 0.171^d1	207,207 ± 0.138^c1	148,492 ± 0.073^e1	327,715 ± 0.336^a1
Soluble fibre (%)	1.240 ± 0.086^a5	1.057 ± 0.081^b6	0.694 ± 0.127^c6	0.387 ± 0.046^d6	0.539 ± 0.058^d6
Insoluble fibre (%)	24.933 ± 0.349^b3	26.510 ± 0.047^a3	17.544 ± 0.068^c3	8.419 ± 0.050^e5	13.488 ± 0.464^d4
Total dietary fibre (%)	26.173 ± 0.436^b3	27.567 ± 0.128^a3	18.238 ± 0.195^c3	8.806 ± 0.096^e5	14.027 ± 0.522^d4
Total polyphenol content ⁱ	179.893 ± 0.080^c1	178.094 ± 0.075^d1	191.163 ± 0.015^b1	206.089 ± 0.087^a1	158.292 ± 0.027^e1
Flavonoid content ⁱⁱ	117.446 ± 0.075^c2	112.431 ± 0.299^d2	119.467 ± 0.150^b2	143.868 ± 0.299^a2	61.757 ± 0.075^e2
Tannin content ⁱⁱⁱ	31.842 ± 0.075^b3	32.042 ± 0.025^b3	31.019 ± 0.100^c3	33.139 ± 0.025^a3	28.823 ± 0.250^d3
DPPH*	16,594 ± 0,160^b2	15,383 ± 0,076^c2	15,612 ± 0,104^c2	21,193 ± 0,049^a2	9,954 ± 0,076^d2
FRAP*	31,581 ± 0,104^c1	30,027 ± 0,050^d1	32,036 ± 0,076^b1	34,481 ± 0,029^a1	26,456 ± 0,029^e1
BVB: beige Bambara bean variegated with black - BU: Black Bambara bean - GHB: beige Bambara bean grey hilum - BS: beige soybean - RC: red cowpea
ⁱmg Gallic Acid Equivalent /100gDM
ⁱⁱmg Quercetin Equivalent /100gDM
ⁱⁱⁱmg Catechin Equivalent /100gDM
*µg Trolox Equivalent/100 DM
µg AAE/100g: µg ascorbic acid equivalent per 100 of dry matter
Results with the same letters (a, b, c) on the same row are not significantly different (P < 0.05).
Results with the same numbers (1, 2, 3) on the same column are not significantly different (P < 0.05).

Both young pods and young leaves, when crushed or peeled, are effective against oedema, whitlow, and itching. Berhaut [31] shows that cowpeas can be used to solve memory problems. Studies have linked regular consumption of this GL to several benefits, such as better blood sugar control, a decreased risk of cardiovascular disease, and a decreased risk of colorectal cancer [32].

Correlations between field survey data and hospital data

The survey conducted to identify GLs with therapeutic properties allowed data to be collected correlating the beneficial effects of GL consumption with the prevention of certain metabolic diseases. Data obtained from the population on GLs consumption (frequencies) has been correlated with statistical data (from hospitals) on the prevalence of metabolic diseases (type II diabetes, high blood pressure, obesity, and stroke).

GLs consumption and the prevention of type II diabetes

The correlations between some GLs' consumption frequencies (for the varieties most consumed by the population) and the type II diabetes frequencies are presented in the graphs grouped together in Fig. 3 (A, B, and C). The soybean consumption could prevent the onset of type II diabetes as correlation results presented in Fig. 3A. This is justified by the strong link between soybean consumption and type II diabetes prevalence, with a correlation coefficient of R² = 0.8799.

The BBs (beige and red) consumption, widely consumed (indifferently from one locality to another) in Diamaré, could also help to optimise blood sugar control, according to the correlation results. Strong correlations were obtained (R² = 0.9804 and 0.9239, respectively, for beige and red cultivars) between BB consumption and the prevention of type II diabetes (Fig. 3B and C).

The correlations between some GLs' consumption frequencies (for the varieties least consumed by the population) and the type II diabetes prevalence are presented in Fig. 4. It emerges from this analysis that black BB consumption is strongly correlated with the prevention of type II diabetes (R² = 0.9992) (Fig. 4A). Regarding the underutilised cowpea, in particular the red cultivar, its consumption will have a positive effect on the prevention of type II diabetes with a correlation coefficient of R² = 0.9467 (Fig. 4B).

GLs consumption and the prevention of high blood pressure

The results of the correlation analysis evaluating the link between the GL consumption and the prevention of high blood pressure are grouped in Figs. 5 and 6, respectively for the most and least consumed varieties.

Soybean and beige BB are among the most consumed GLs in Diamaré. Their consumption was strongly correlated with the prevention of hypertension with R² correlation coefficients of 0.9234 and 0.9651, respectively for soybean and beige BB (Fig. 5A, B).

The relationships between red cowpea consumption and the prevention of high blood pressure are presented in Fig. 6. This consumption is greatly linked to the high blood pressure control with a correlation coefficient R² = 0.9632.

GLs consumption and prevention of stroke-obesity

Notable coefficients were obtained by analysing the link between GLs consumption and the prevention of stroke and obesity. The correlation study shows that there is a strong connection between soybean consumption and the prevention of obesity and stroke, with an R² coefficient of 0.9656 (Fig. 7A). The R² of the red BB cultivar is 0.8063 (Fig. 7B).

An underutilised cultivar of cowpea (red) has been correlated with the prevalence of obesity and stroke. It emerges from this analysis that red cowpea consumption could prevent obesity and strokes with a correlation coefficient of R² = 0.9135 (Fig. 8). Studies have been carried out in this direction, thus confirming the results obtained in this present work [35, 36].

Physicochemical characterisation

GLs cultivars

The exploratory survey carried out in the Diamaré’s division of allow to choose the seeds to be analysed. Choice based on availability, consumption frequency, and potential bioactive properties. Thus, five (5) GLs were retained namely soybean, BBs (three cultivars), and red cowpea.

GLs flours production

To obtain the flour, the five (5) GLs varieties were separated, cleaned, and crushed. According to the dry weight of the varieties BVB, BU, GHB, BS, and RC, the yields of flour production from the chosen seeds are, in order, 91.25%, 94.63%, 92.31%, 94.03%, and 94.51%. The material losses experienced throughout the grinding and sieving procedures would explain for the variations in yields that were observed.

Proximate composition

Dry matter (DM), moisture and ash content

The values of the DM, moisture, and ash content of whole flours (WFs) are shown in Table 3. DM contents vary significantly (P < 0.05) between seeds and even within a variety (in the case of BB). The WFs’ DM content values vary between 89.409 ± 0.479% (RC) and 93.921 ± 0.723% (BU). The DM contents of the BB cultivars are more or less high than those of Adeleke et al. [37] on three BB cultivars.

The WFs' moisture content change significantly (P < 0.05) between the samples (Table 3). Following the BB cultivars, the RC variety (10.591 ± 0.479), the BVB (8.912 ± 0.082), and the GHB (7.805 ± 0.070) varieties have the highest moisture content values. The latter (7.098 ± 0.088) does not significantly differ from the BS variety. The BS variety’s moisture content is higher than that obtained by Liu et al. [38]. The BU’s BB cultivar has the lowest moisture content (6.079 ± 0.723%). The flour is more stable and has a longer shelf life as a result of its comparatively low moisture content. Oyeyinka et al. [39] reported comparable outcomes using whole flour from two BB cultivars (red and beige).

The WF's ash content ranges from 3.42% (RC) to 4.26% (BVB). These results (Table 3) show values that are quite higher than those of Oyeyinka et al. [39]. These values are significantly higher than the BB cultivars from South Africa [40]. The BB, soybean, and cowpea varieties employed in this study could potentially be used as a dietary source of minerals due to their high ash content.

Protein and fat contents

The total protein and total lipid values for the five GL varieties are presented in Tables 4. Significant differences at P < 0.05 are observed between these varieties.

The total protein contents vary considerably (P < 0.05) according to the different GL varieties (Table 3). Of all the studied GLs, the BS variety’s WF has the high protein content at 38.133 ± 0.366 g/100g DM. The BVB cultivar has the high total protein content of the three BB’s cultivars, at 26.434 ± 0.366 g/100g DM. Compared to the cultivars BU and GHB, the variety NR's wholemeal has a much greater protein content (22.880 ± 0.366 g/100g DM).

The total protein content obtained with the BVB cultivar’s WF are far superior to the results obtained by Mune et al. [21], Adeleke et al. [37] and Saho [41]. Studies conducted [37] on a commercial BB variety (18.50 g/100g DM) and two BB cultivars grown in Nigeria (20.14 and 20.73 g/100g DM). The protein contents of the BU and GHB (Bambara bean) cultivars are in agreement with the FAO table [23] and similar to the results obtained by Mune et al. [21] and Adeleke et al. [37]. The BVB cultivar’s WF has a higher protein content. The RC variety’s protein content are on average similar to those obtained by Soaud et al. [42] on 14 cowpea cultivars but much higher than those obtained by Saho [41]. The protein content of BS is higher than the provided value by FAO [23] in the food composition table (32.00 g/100g DM) and that obtained by Gao et al. [43] (35.00 g/100g DM).

The genetic variety of the examined species, the type of crop, the post-harvest treatments, the environmental circumstances (climates and type of soils), and the type of technical treatment used can all explain the observed protein content variances in the various GLs.

The WFs' fat content ranges from 9.594 g/100g DM to 7.552 g/100g DM. For BS, GHB, BVB, RC, and BU, the resulting fat contents are 9.594 ± 0.054, 8.710 ± 0.087, 7.935 ± 0.047, 7.570 ± 0.076, and 7.552 ± 0.137 g/100g DM, respectively. Lipid content is highest in soybean flour. The results for the median total lipid content are for the cultivars GHB and BVB. The total lipid levels of the BU and RC cultivars did not differ significantly (P ≤ 0.05).

In comparison to commercial varieties examined by Adeleke et al. [37], Mune et al. [21], and data from the FAO Food Composition Table, the obtained values for the three cultivars of BBs are higher [23]. Oyeyinka et al. [39] obtain yielded comparable results on two cultivars of Zimbabwean BBs, beige (7.09%) and red (7.58%),.

The BS variety has a fat content of approximately 9.594 ± 0.054 g / 100g DM. It is less than the FAO Food Composition Table value [23]. Taghdir et al. [44] obtained much lower values than the obtained with BS. The lipid contents of the RC variety (7.570 ± 0.076 g / 100g DM) are significantly higher than those obtained by Antova et al. [45] on some Bulgarian cowpea varieties or by Bai et al. [46] on 24 Chinese cowpea cultivars. Saho [41] obtained slightly higher fat content results than the studied cowpea on a white variety of cowpea grown in Mokolo (Far North, Cameroon).

Based on this comparison, it is clear that pedoclimatic and environmental factors have a significant influence on the chemical composition of GLs. Furthermore, genetic factors can be cited as parameters of compound variation from seed to seed.

Carbohydrate content

The total carbohydrate content of the different GLs' WFs ranges between 49.931 ± 0.029 and 59.294 ± 0.129 g/100g DM. The RC variety's WF has the highest carbohydrate value (59.294 ± 0.129 g/100g DM), while the BS variety has the lowest (49.931 ± 0.029 g/100g DM).

The carbohydrate contents of the three BB cultivars' WFs ranges from 53.711 ± 0.000 (BU) to 57.696 ± 0.097 (BVB) g/100g DM. The carbohydrate content of the studied BBs is higher than those obtained by Adebowale et al. [47] on two BB cultivars. They are, however, comparable to Adeleke et al. [37] and similar with FAO food composition table [23].

The total carbohydrate contents of the RC (59.294 ± 0.129 g/100g DM) and BS (49.931 ± 0.029 g/100g DM) varieties are slightly similar to the FAO food composition table reference values [23]. The total carbohydrate content of the NR variety is 59.430 g/100g DM, which is similar to the value obtained by Mune et al. [21]. The SB variety's total carbohydrate content is slightly higher than that obtained by Bainy et al. [48] on 12 soybean varieties. The differences in total carbohydrate content observed between the different GLs could be attributed to genetic predisposition, the type of technological treatment used, and the location of cultivation.

Thus, the high carbohydrate content of these GL varieties reflects their nutritional value. Carbohydrates are required for bodies to function properly. They are primarily a large, rapidly usable energy source that is required for cell function [49].

Soluble (SF), insoluble (IF) and total dietary fibre (TDF)

The soluble and insoluble fibre contents of the different flours fluctuate considerably at P ≤ 0.05 (Table 3). The WFs’ SF content of the studied GLs varies between 0.387 ± 0.046 and 1.240 ± 0.086 g/100g DM. The BB cultivar BVB has the highest soluble fibre content. The values obtained with the varieties BS (soybean, 0.387 ± 0.046%) and RC (cowpea, 0.539 ± 0.058%) are not significantly different. IF content of the GLs’ WFs ranges significantly from 8.419 ± 0.050 to 26.510 ± 0.047 g/100g DM, at P ≤ 0.05. The BU’s BB cultivar has the highest insoluble fibre content, followed by the BVB cultivar (24.933 ± 0.349 g/100g DM). The fibre contents (soluble and insoluble) obtained with the BB cultivars are higher than those of Diedericks and Jideani [50] on a BB cultivar. Those obtained with the soybean variety (BS) are similar to the results obtained by Coletti et al. [51] on some soybean cultivars. Bai et al. [46] reported similar values to those obtained with the cowpea variety (RC).

The WFs’ dietary fibre content of the five GLs varies significantly (P ≤ 0.05) between 9.270 ± 0.005 (BS) and 29.018 ± 0.047 (BU) g/100g DM (Table 3). Of all the studied varieties, the three BB cultivars’ WF have the highest TDF. The black BB (BU)’s flour has the highest TDF content. The BVB and GHB cultivars’ flours come second and third, respectively, in terms of TDF content (27.550% and 19.198%). The RC variety’s WF has a TDF content of about 15%. The lowest dietary fibre content was obtained with soybean flour (BS, 9.270 ± 0.005 g/100g DM).

The TDF obtained with the BU and BVB’s BB cultivars are significantly higher than those obtained by Diedericks and Jideani [50] on a BB variety. The value obtained with the black cultivar (BU) is also higher than the FAO food composition table [23]. While the BVB cultivar’s TDF content is in line with the [23] food composition table data.

The TDF content obtained with soybean flour is significantly higher than that obtained by Armstrong et al. [52]. However, they are slightly lower than the results obtained by Ferreira et al. [53] on 80 Brazilian soybean varieties.

The result obtained with cowpea (RC) is higher than the FAO [23] food composition table value (12.6%). However, the results obtained by Bai et al. [46] on the 24 cowpea varieties are much higher than the ones obtained in this study.

From this comparison, the differences observed may be associated with the geographical area of cultivation and production of these GLs. They can also be justified by the genetic nature of the legume cultivar, as in the case of BBs. The variation in TDF content of the studied GLs could also be explained also by the climatic conditions and soil characteristics. Thus, the high TDF content of some GLs could be exploited as a source of dietary supplements.

Vitamin C (Vit-C)

The vitamin C content of the different studied GLs is presented in Table 3. The WFs’ vit-C content vary significantly between 148.492 ± 0.073 and 327.715 ± 0.336 µg AAE/100g DM. The BS variety’s WF has the lowest vit- C content, while the RC variety has the highest. The three BB cultivars contain 261.951 ± 0.376, 207.207 ± 0.138 and 161.538 ± 0.171 µg AAE/100g DM, respectively for BVB, GHB and BU. The vit-C content obtained with the BB cultivars is similar to that of the three BB cultivars studied by Adeleke et al. [37]. The values obtained with the cowpea variety (RC) are higher than those of the cowpea varieties studied by Bai et al. [46] but similar to some Indian cowpea cultivars studied by Devi et al. [54]. The results obtained with the soybean variety (BS) are in line with those obtained with six yellow soybean cultivars (India) by Kumar et al. [55]. The observed differences between the obtained values could be explained by the analytical methods used. In addition, these differences could also be explained by the influence of geographical, pedoclimatic, and genetic factors.

Phytochemical compounds

Total polyphenolic content (TPC)

The total polyphenolic compound, flavonoid, and tannin contents of the five GLs varieties’ WFs are presented in Table 3. Significantly (P < 0.05) differences were observed between these values.

The TPC of the five GLs varieties varies significantly (P < 0.05) between 158.292 ± 0.027 and 206.089 ± 0.087 mg GAE/100g DM. The beige soybean variety (BS) has the highest TPC (i.e., 206.089 ± 0.087 mg GAE/100g DM at P 0.05), while the red cowpea variety (RC) has the lowest. Indeed, soybean contains anthocyanins, flavonols, flavones, isoflavones, and chalcones, as well as their acetic, p-hydroxybenzoic, caffeic, coumaric, and other derivatives [56]. Researchers have focused on isoflavones because they are the most abundant.

The three BB cultivars have significantly different values at P < 0.05, 178.094 ± 0.075, 179.893 ± 0.080, and 191.163 ± 0.015 mg GAE/100g DM, respectively for BU, BVB, and GHB. The studied varieties demonstrate GLs' ability to accumulate phenolic compounds. These results outperform those obtained by Avanza et al. [57] with some cowpea cultivars. They are, however, partially similar to the results of Oyeyinka et al. [39], Sombié et al. [58], and Yadav et al. [56] obtained with some cowpea and BB varieties.

The observed variation in polyphenol content between GLs, according to Zhao et al. [59] and Moreira-Arajo et al. [60], may be due to genetic and pedoclimatic factors. This difference may also correlate with morphological variation between cultivars and with the extraction method used.

Flavonoid content

The flavonoid contents of the studied GLs differ considerably (P < 0.05) between 61.757 ± 0.075 and 143.868 ± 0.299 mg QE / 100g DM (Table 3). The BS variety has the highest flavonoid content, with 143.868 ± 0.299 mg QE / 100g DM. The median flavonoid content of WFs differ significantly between the three BB cultivars (GHB > BVB > BU at P < 0.05). The RC variety has the lowest flavonoid content. These results are similar to those of Sombié et al. [58] and far superior to those of Yadav et al. [56]. These observed differences could be explained by seed genetic factors, environmental, climatic, agrologic, and morphological factors [42].

Tannin content

The tannin content of studied GL varieties is presented in Table 3. For WFs, these values differ significantly at P < 0.05. The maximum and minimum tannin contents are 33.139 ± 0.025 mg CE/100g DM and 28.823 ± 0.250 mg CE/100g DM, respectively. The tannin content of the BS variety is particularly high. The tannin content of the three BB cultivars is higher than the obtained values with cowpea variety. These results are significantly higher than those reported by Oyeyinka et al. [39]. These differences could be explained by the fact that tannins are polyphenols associated with seed coat colour [61]. Because they were grown in the same location under the same environmental conditions, the difference between local races of BBs could be due to genetic factors.

Antioxidant properties

The antiradical activity (DPPH) of the polyphenolic compounds extract fluctuates significantly (P < 0.05) between the studied GLs. The free radical scavenging activity of the WFs varies between 21.193 ± 0.049 and 9.954 ± 0.076 µg Trolox Equivalent/100g DM. The highest activity is observed with the wholemeal of the BS variety (i.e., 21.193 ± 0.049 µg TE /100g DM), followed by the three BB cultivars GHB, BVB and BU (16.594 ± 0.160, 15.612 ± 0.104 and 15.383 ± 0.076 µg TE /100g DM, respectively). The RC variety shows the lowest anti-free radical activity of 9.954 ± 0.076 µg TE /100g DM. The results obtained with the BS variety are largely superior to those obtained by Villalobos et al. [62]. Lower results were obtained by Mune et al. [26] on red BB cultivar.

The ferric reducing antioxidant power (FRAP) of polyphenolic compounds extracted from GLs’ WF varied significantly (P < 0.05) between 26.456 ± 0.029 (RC) and 34.481 ± 0.029 (BS) µg TE/100g DM. The three BB cultivars have median values of iron reducing antioxidant activity of 30.027 ± 0.050 (BU), 31.581 ± 0.104 (BVB), and 32.036 ± 0.076 (GHB) µg TE/100g DM. The strong ferric reducing antioxidant power observed with soybean’s WF can be explained by the high presence of the different polyphenolic compounds previously measured (total polyphenols, flavonoids and tannins), but mainly isoflavones.

Consumption of GLs and the prevention of type II diabetes

Descriptive statistics tools and chemical composition were used to study the strength of the link between seed consumption and the prevention of type II diabetes (figure 9), high blood pressure, obesity and stroke (figure 10). Strong and negative correlations were obtained with the most consumed GLs varieties. While strong and positive correlations were found with the least consumed GLs varieties. In vivo, in vitro, and in silico studies have confirmed the correlations between the GLs’ consumption and the prevention of the aforementioned diseases. Correlations regarding antidiabetic properties agree with the works of Flight & Clinton [63], Remond & Warland [64], and Mune et al. [26]. Works which highlight the positive impacts of GLs’ consumption (soybeans, cowpeas and Bambara groundnut) in the prevention of type II diabetes. Indeed, GLs administered (during a meta-analysis) alone or as part of low glycaemic index or high fibre diets improve the main markers of glycaemic control, namely blood levels of glycated hemoglobin or HbA1c or fructosamine, and fasting blood sugar [63,64]. These studies have shown that consuming GLs can optimise medium-to long-term glycaemic control by enhancing the effects of insulin.

In addition, the significant protein content in these various GLs widely justifies their nutritional and therapeutic values. Indeed, GLs are known for their high protein content, due to these proteins fulfil various biological functions. Certain peptides mainly have a metabolic role (enzymes, oxygen transport, etc.), or a defensive role (enzyme inhibitors, lectins, entomo-toxic, antifungal, antimicrobial proteins). Numerous studies on peptides derived from the hydrolysis of dietary proteins have revealed their importance in the regulation of type II diabetes [26,65,66]. Other studies have also shown that a diet rich in plant foods, including GLs, reduces the risk of developing type 2 diabetes, improves blood sugar levels, and controls blood lipid constants [67]. These studies confirm the antidiabetic effects of the studied GLs. These effects can also be justified by the presence in soybean, Bambara groundnut, or cowpea of bioactive proteins, peptides, or amino acids endowed with an inhibitory property of Dipeptidyl peptidase-IV [26].

Diabetes is associated with hyperglycaemia and hyperlipidaemia-induced oxidative stress. Oxidative stress is caused by an imbalance in the production and detoxification of reactive oxygen species (ROS) in biological systems [68]. Many diabetes-related complications are caused by oxidative stress, which activates major signalling pathways and transcriptional factors (e.g., activating protein (AP)-1 and Kelch-like ECH-associating protein 1-NFE2-related factor 2 (Keap1-Nrf2)) [68–70]. As a result, there is a compelling case for using therapeutic antioxidants to treat and prevent diabetic complications. In view of their biological potential, polyphenolic compounds, particularly flavonoids, could play a fundamental role in antioxidant mechanisms. In addition, chronic nonspecific inflammation, including inflammatory cytokine expression primarily from adipose tissue, occurs during the development of type 2 diabetes. As a result, the reported anti-inflammatory properties of anthocyanins may be critical for the treatment of this disease [68–70]. These compounds may thus contribute considerably to combating oxidative stress, thus improving the control of diabetes.

Furthermore, dietary fibre regulates blood sugar, lowering hyperglycaemia and hyperinsulinemia, which is especially important in diabetic and pre-diabetic patients [29,71]. Short-chain fatty acids (SCFAs), also known as volatile fatty acids, are produced when fermentable fibres are fermented. SCFAs play a role in a variety of health-promoting physiological processes, including blood sugar stabilisation via pancreatic insulin release and hepatic control of glycogen breakdown. These volatile fatty acids also increase the expression of glucose transporter genes in the intestinal mucosa, which controls glucose absorption [28,29,71,72].

These correlations provide the arguments leading to the valorisation of GLs in general and of those underutilised in particular in the formulation of functional foods with the above-mentioned bioactive properties.

Consumption of GLs and the prevention of high blood pressure

The above-mentioned chemical compounds and correlations regarding GL consumption and the blood pressure control are in agreement with the works of He et al. [73], Koné et al. [30], Arise et al. [74], and Mune et al. [26]. In the studies by Arise et al.; and Mune et al. [26,74], Bambara groundnut protein hydrolysates were obtained using different proteases (subtilisin, trypsin, and pepsin) and their peptide fractions (molecular weight: 10, 5, 3, and 1 kDa) showed antihypertensive and antioxidant properties. The results showed the hydrolysates exhibited strong inhibitory activities against Angiotensin-Converting Enzyme (ACE), anti-free radical activity with DPPH, and the metal chelating power. In addition, the work of He et al. [73] showed that soy protein supplementation resulted in a reduction in systolic and diastolic blood pressure. Antihypertensive effects in humans have not been proven for most of the peptides obtained by processing food proteins. In recent years, it has been reported that the biologically active peptides from foods might have beneficial effects on human health, especially as immunomodulating, antihypertensive, and osteoprotective substances [26,75].

The probable antihypertensive properties of these GLs would also be justified by the involvement of protein hydrolysates (peptides), endowed with bioactive properties, in the regulatory mechanisms of the renin-angiotensin system. This is in addition to the inhibition of ACE through the regulation of vascular functions (vasodilation and vasoconstriction) [27,76]. In addition, the antioxidant potential of protein hydrolysates would explain the antihypertensive properties of studied GLs. Indeed, the relationship between antioxidant and antihypertensive activity is that antioxidant deficiency leads to increased blood pressure (hypertension). Thus, one of the main adverse effects of angiotensin II is the increase in the formation of reactive oxygen species [77]. In addition, the oxidation of lipids, specifically low-density lipoproteins (LDL), contributes to the development of arteriosclerosis. In many cases, this pathology accompanies and promotes the state of hypertension [77].

In addition, legumes are rich in potassium, magnesium and other minerals that have a positive impact on the maintenance and regulation of blood pressure. Indeed, epidemiological, observational, and controlled clinical trials (in the USA) show that increased sodium intake is associated with higher blood pressure [64,78]. Reducing sodium intake in hypertensive patients, particularly in salt-sensitive patients, significantly lowers blood pressure by 4-6/2 to 3 mm Hg [64,78]. Similarly, a significant reduction in blood pressure with increased dietary K+ intake. High dietary calcium levels are not only associated with lower blood pressure, but also with a reduced risk of developing hypertension [64,78].

The antihypertensive potential of GLs would also be justified by their polyphenol compounds. Numerous studies, including the present one, have shown that polyphenolic compounds, mainly flavonoids, are endowed with free radical scavenging properties (DPPH) and a strong ferric reducing antioxidant power (FRAP). These antioxidant properties have an important place in the mechanisms of blood pressure regulation [77].

Similarly, the potential antihypertensive properties of these GLs are also explained by the presence of dietary fibre in remarkable quantities. Indeed, the consumption of dietary fibre contributes to reducing the risk of systolic and diastolic hypertension [79]. Short-chain fatty acids (SCFAs), produced during fibre fermentation, are involved in many physiological processes that promote good blood pressure management. These fatty acids decrease hepatic cholesterol synthesis and reduce blood levels of LDL-cholesterol and triglycerides responsible for atherosclerosis [29,71].

All of these studies, including the present work, suggest that an increased intake of soybean, Bambara groundnut, and cowpea protein (and their hydrolysates) may play an important role in the prevention and/or treatment of type II diabetes and high blood pressure (figure 10).

Consumption of GLs and the prevention of obesity and stroke

Strong correlations were obtained between the GLs’ consumption and the prevention of obesity and stroke. Numerous studies suggest that soy protein and its isoflavones may play a beneficial role in weight management. Several nutritional intervention studies in animals and humans indicate that consumption of soybean and BB protein reduces body weight and fat mass in addition to lowering plasma cholesterol and triglycerides [35,36]. Several potential mechanisms by which proteins from these legumes can reduce body fat and blood lipids are discussed and include a wide range of biochemical and molecular activities that favourably affect fatty acid metabolism and cholesterol homeostasis (figure 10) [35,36]. Protein hydrolysates derived from GLs could be used to support their potential therapeutic properties in the prevention of cardiometabolic diseases. In some in vitro and in vivo studies, these enzymatic hydrolysates demonstrated hypocholesterolaemia and hypolipidemic activity related to LDL-cholesterol control. Indeed, it has been proposed that soy proteins can reduce obesity by modulating the expression of SREBPs (sterol regulatory element binding proteins), a family of transcription factors that regulates many genes involved in fatty acid and cholesterol synthesis [36,80].

Lipids play an important role in the proper functioning of the body (storage, structural and functional properties). Muench et al. [81] suggest that GLs may be beneficial for cardiovascular health and general health due to their high content of polyunsaturated fatty acids and low content of saturated fatty acids [21]. Both n−6 and n−3 polyunsaturated fatty acids have been associated with lower cardiovascular risk. Within the n−6 series, linoleic acid seems to decrease cardiovascular risk. Long-chain fatty acids (eicosapentaenoic and docosahexaenoic acids) in the n3 series are associated with lower risk, particularly for fatal coronary outcomes, whereas the role of linolenic acid is less clear [82].

Phenolic compounds constitute a class of organic molecules that arouse a great deal of interest due to their beneficial effects on health (antimicrobial properties, antioxidant capacities) [58,69,83]. Indeed, their role as a natural antioxidant is arousing growing interest in the prevention and treatment of cancerous, inflammatory, cardiovascular, and neurodegenerative diseases [58,69]. The presence of these polyphenolic compounds explains the use in traditional medicine of GLs as a cure for the prevention of certain metabolic and neurodegenerative diseases (Bambara groundnuts, particularly as shown in table 2). Recent in vitro studies have also shown that polyphenolic compounds, in particular flavonoids, have anti-amyloid and antidiabetic properties.

The GLs' anti-obesity properties could be explained by the antioxidant potential of proteins, protein hydrolysates, and/or polyphenolic compounds. Obesity is defined by an abdominal visceral accumulation of white adipose tissue, which causes chronic low-grade inflammation and oxidative stress [84]. Obesity and its associated pathologies are thought to be linked by oxidative stress. Oxidative stress has been shown to stimulate preadipocyte proliferation and differentiation, as well as the growth of mature adipocytes [84,85].

The abundant presence of dietary fibre, particularly in the BB cultivars, may justify the use of this GLs in traditional medicine. Dietary fibre, indeed, lowers total cholesterol levels of low-density lipoprotein (LDL), which may lower the risk of cardiovascular disease. The unique physical and chemical properties of dietary fibre that provide early signals of satiety as well as an enhanced and prolonged feeling of fullness [29,71] explain its role in the regulation of energy consumption and the development of obesity.

Field investigations carried out through the survey of consumers and producers of GLs were a decision-making tool. They made it possible to identify the GLs' consumption frequencies according to the survey areas. The field investigations also facilitated the circumscription and identification of GLs with technological, nutritional, and therapeutic interest. Correlations establish the strong link between the consumption of certain GLs varieties (underutilised or not) and the prevention of metabolic diseases, including type II diabetes, high blood pressure, obesity and stroke. This study revealed that the consumption of soybean, Bambara groundnut, and cowpeas is of significant health benefit in terms of the prevention of metabolic diseases and, furthermore, the prevention of neurodegenerative diseases due to their therapeutic uses and chemical composition. These GLs, easily and economically accessible, could be extremely helpful in controlling diabetes, obesity, and cardiovascular disease. In view of the richness of the studied GLs in proteins and the potential bioactivities of these proteins, raw materials and/or protein hydrolysates (peptides) are promising materials of particular interest to the pharmaceutical industries in the fight and/or prevention of metabolic diseases (type II diabetes, high blood pressure, obesity, stroke), food insecurity, and the prevention and/or reversion of neurovegetative diseases. And finally, in the formulation of a new functional ingredient.

BB	Bambara bean
BS	Beige Soybean
BU	Uniform black Bambara bean
BVB	Beige Bambara bean variegated with black
CE	Catechin Equivalent
DM	Dry matter
GAE	Gallic Acid Equivalent
GHB	Beige Bambara bean grey hilum
GL	Grain legume
NCD	Noncommunicable disease
QE	Quercetin Equivalent
RC	Red cowpea
TDF	Total dietary fibre
Vit-C	Vitamin C
WF	Whole flour

Acknowledgments

The Applied Biochemistry and Biological Chemistry Lab of the University of Maroua is duly acknowledged for providing the facilities for carrying out the present research work.

Fundings

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Consent for publication

All participants gave their consent before participating in the survey. In addition, they also consented to the publication of the data collected.

Ethics approval and consent to participate

Competing interests

The authors have no conflict of interest to declare.

Availability of data and materials

All data are available.

Authors' contributions

MH and MMMA designed the experiments, MH, FOV, and BJP performed the analysis, MH wrote the manuscript and performed the data analysis, MMMA, MSR, MZH, and BB made the proofreading, all the author reviewed the main manuscript.

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Consumption of underutilised grain legumes and the prevention of type II diabetes and cardiometabolic diseases: evidence from field investigation and physicochemical analyses

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Abstract

Background

Materials and methods

Results

Conclusion

Figures

Introduction

Materials And Methods

Survey

Study area

Survey sheet and procurement method

Sampling

Data analysis

Physicochemical properties

Reagents and Chemicals

Seed samples

Whole flour production

Proximate composition

Phenolic compounds extraction

Total polyphenolic content

Flavonoid content

Tannin content

Vitamin C content

Antioxidant properties

DPPH (2,2-Diphényl-2-picrylhydrazyl)

FRAP (Ferric Reducing Antioxidant Power)

Data analysis

Results

Survey

Correlations between field survey data and hospital data

Physicochemical characterisation

GLs cultivars

GLs flours production

Proximate composition

Phytochemical compounds

Total polyphenolic content (TPC)

Flavonoid content

Tannin content

Antioxidant properties

Discussion

Conclusion

Abbreviations

Declarations

References

Additional Declarations

Supplementary Files

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