Model development
Following the review of Waijers et al. (25)., the following steps were considered in the development of the nutriRECIPE-Index: definition of the aim of the index, choice of component type (e.g., nutrient, nutrient ratios or food groups), selection of the components (in this case various nutrients), target sizes and scaling, weighting, practical test and adjustment, and validation.
The aim of the nutriRECIPE Index is the assessment (and thus the possible optimisation) of individual recipes for meals, considering 19 macro- and micronutrients and five BPC groups with high public health relevance. Table 1 provides an overview of the nutrients and BPC groups included. Moreover, the nutriRECIPE-Index distinguishes between moderation and adequacy components as proposed by Thiele et al.(34) because a one-sided consideration of an advantageous or disadvantageous effect is less promising(35). While every nutrient can have beneficial or adverse effects, the nutriRECIPE-Index includes components with a minimum and maximum target value, considering current recommendations and nutrition patterns. The reference values for macro- and micronutrients of the German Nutrition Society (DGE), the Austrian Nutrition Society (ÖGE) and the Swiss Nutrition Society (SGE)(36, 37) served as references. The reference value for protein was modified in line with more recent studies that found more beneficial effects when protein is ingested in amounts of 1.2 g per kg body weight per day(38, 39, 40, 41). The German Nutritional Society (DGE) and the World Health Organisation (WHO) have specified the minimum recommended protein intake as 0.8 g/kg body weight (BW)(36, 42), but the suggested optimal protein intake of 1.2 g/kg BW is used as the basis for calculating the nutriRECIPE-Index. It is also important to note that lunch is often the most protein-rich meal of the day. The maximum sugar in a meal followed the recommendation of the WHO(43), which corresponds with the recommendations of the consensus paper of the DGE on sugar consumption(44).
Table 1
Nutrients integrated into the nutriRECIPE-Index, their target values and weighting factors. Requirement values for macronutrients and micronutrients according to recommendations of DGE and WHO, Values for Bioactive Plant Compounds are calculated based on DGE-meal plans for one week, which incorporate the “Five-A-Day” recommendation for fruits and vegetables
|
Nutrient
|
Daily
recommendation
|
Unit
|
Average intake/day
|
Weighting factor
|
Favourable nutrients
|
|
|
|
|
|
Carbohydrates
|
Fibre
|
30
|
g
|
24.00
|
1.25
|
Proteins
|
Protein
|
84
|
g
|
78.75
|
1.07
|
Fats
|
Mono- and poly-unsaturated fatty acids
|
20
|
% of energy
|
20.00
|
1.00
|
Vitamins
|
Vitamin D
|
20
|
µg
|
3.35
|
3.00
|
|
Vitamin E
|
14
|
mg
|
14.70
|
0.95
|
|
Thiamine
|
1,2
|
mg
|
1.55
|
0.77
|
|
Riboflavin
|
1,4
|
mg
|
1.95
|
0.72
|
|
Vitamin B6
|
1,5
|
mg
|
2.30
|
0.65
|
|
Folic acid
|
300
|
µg
|
314.00
|
0.96
|
|
Vitamin B12
|
3
|
µg
|
5.40
|
0.56
|
|
Vitamin C
|
110
|
mg
|
152.00
|
0.72
|
Minerals
|
Calcium
|
1000
|
mg
|
1581.00
|
0.63
|
|
Magnesium
|
350
|
mg
|
412.50
|
0.85
|
|
Iron
|
15
|
mg
|
13.70
|
1.09
|
|
Iodine
|
200
|
µg
|
102.50
|
1.95
|
|
Zinc
|
10
|
mg
|
10.90
|
0.92
|
Unfavourable nutrients
|
|
|
|
|
|
Carbohydrates
|
Added sugars
|
50
|
g
|
92.00
|
1.84
|
Fatty acids
|
Saturated fatty acids
|
10
|
% of energy
|
10.00
|
1.00
|
Minerals
|
Salt (sodium)
|
6
|
g
|
7.44
|
1.24
|
Bioactive Plant Compounds
|
|
|
|
|
|
|
Carotenoids
|
20
|
mg
|
5–6
|
1.00
|
|
Glucosinolates
|
40
|
mg
|
< 50
|
1.00
|
|
Phenolic Acids
|
200
|
mg
|
200–300
|
1.00
|
|
Polyphenols
|
300
|
mg
|
100–150
|
1.00
|
|
Phytosterols
|
450
|
mg
|
170–440
|
1.00
|
While other indices (e.g., the Healthy Eating Index or the Menu Sustainability Index) usually assume linear correlations between the level of nutrient implementation and the health effect, the nutriRECIPE-Index is based on the concept of diminishing marginal utility and the logarithmic relationship of Bernoulli(45). Moreover, the degree of fulfilment is not observed in absolute terms but is related to the energy content of the menu, so that the nutrient density is evaluated as a result. Figure S1 in the supplementary material shows how the results differ when analysing the amount of nutrients in a recipe considering its energy content, in comparison with just assuming that a menu should include a third of the amount of nutrients in a daily recommendation.
For calculating the degrees of fulfilment, the following formulas are used:
For favourable nutrients: f(x) = ln(x) + 1
For unfavourable nutrients: f(x) = -ln(x).
If the density of a favourable nutrient is high enough that the menu includes the daily requirement of that nutrient, the function gets the value "1". If the maximum recommended daily intake of an unfavourable nutrient is not exceeded, the function gets the value "0". However, if the density for a specific nutrient is tripled, so that one-third of the energy is already sufficient to cover the daily requirement, the logarithmic function returns the value "2.1", which is also the unweighted maximum value for favourable nutrients (upper cut-off). On the other hand, exceeding the recommended daily amounts of unfavourable nutrients leads to malus points, whereby the unweighted minimum value is "-2.1" (lower cut-off).
According to Waijers et al. (25), de facto non-weighting the various compounds of a score is also a weighting, namely, where all components are weighted equally. To avoid this bias, the nutriRECIPE-Index applies a moderate weighting taking into account the degree of supply within the target group (in this publication, the population of Germany) based on officially documented intake data from the last nutrition survey in Germany(46) and based on the method of ecological scarcity. (47). The more inadequate the supply in the overall population with regard to the nutrient is, the lower the weighting of the nutrient and the higher the impact in the assessment. On the other hand, if the supply in the general population is in accordance with the corresponding recommendation, the weighting factor is 1. At the same time, the supply level of the nutrient supply can change the number of bonus points by multiplying the unweighted maximum value of "2.1" or minimum value of "-2.1" by the corresponding weighting factor for the particular nutrient. Table 1 gives an overview of the weighting factors used.
The formula of nutriRECIPE scores for nutrients with beneficial or adverse effects was as follows:
Nmenu – nutrient content in a menu
Nrec– recommended nutrient intake per day
Erec – recommended energy intake per day
EMenu – energy content of one menu
Nact – average nutrient intake per day
The sum of the single nutrient scores results in the nutriRECIPE-Index. The higher the index value, the more nutrients are contained in the dish in a balanced ratio. Given the weightings, a total score of 100% is possible, even if not all respective dietary target values are met. With bonus points, it is also possible for a menu (e.g., if the nutrient reference values are exceeded) to have a nutriRECIPE score higher than 100%.
a)
Considering bioactive plant compounds:
A particular focus during the index development was the inclusion of BPC. As no food-specific reference databases exist, a separate database has been created containing information on the occurrence and content of BPC in fruits, vegetables, nuts and oil seeds(49). The eBASIS database(50) served as a basis. Data for phytosterols in bread and cereals were added from Normen et al. (51). Retention factors of BPC for different cooking methods were taken from the review of Palermo et al.(52) and for phytosterols from Thanh et al.(53). The nutriRECIPE score ultimately includes aggregated sum values for the five main classes of BPC: carotenoids, phenolic acids, polyphenols, glucosinolates, and phytosterols.
b)
Recipe origin and validation
Using the nutriRECIPE Index, a complete 6-week menu cycle of a university canteen (Chemnitz in Saxony, Germany) was evaluated, which allows adjustments to the model to review its usability. This menu included standard as well as MensaVital recipes. MensaVital® is a trademark used for certain dishes in German student canteens that claim to be physiologically balanced(48). Validation was carried out by comparing the results of the analysis of the recipes using two established and validated tools for the assessment of nutritionally balanced meals in canteens, the Healthy Meals Index(18) and the Nutri-Score(19).
For effective data acquisition and processing, the recipes were recorded in a self-generated MS Access® database. The data of the German Nutrition Database Federal Food Code (BLS) Version 3.02 and the data of the BPC database were stored and linked via the BLS code number.