Search results. Three hundred and sixty-six unique titles and abstracts were identified through the literature search. Of those, 204 studies clearly failed to meet inclusion criteria and 162 full-text articles were assessed. Sixty-two studies were deemed eligible and included in the final analyses (Figure 1).
Results overview. The majority of investigations (70%) were conducted in a single country (Nigeria). Each study was placed into one of three main categories according to the primary nature of the investigation: (a) descriptive study of anthropometric characteristics (31 articles; 50%); (b) descriptive study of macro- or micronutrient levels (22 articles; 35%); and (c) interventional studies (9 articles; 15%). The studies are summarized in Tables 1-3 and described in greater detail below.
Descriptive studies of anthropometric characteristics. Half of all studies identified focused on anthropometric characteristics. The studies typically assessed height, weight, and body mass index (BMI). Other measurements included head circumference, arm span, and various body composition parameters.
The majority (23/31; 74%) of studies were conducted in Nigeria, including fourteen investigations from the major metropolitan area of Lagos. Other studies involved populations in Democratic Republic of Congo (DRC),[22,23] Ghana, Tanzania, Egypt, and Algeria. In addition, a multi-country study involved patients in Cameroon, Ivory Coast, Gabon, Mali, and Senegal. The majority of reports focused on children and adolescents; only one study exclusively involved adults. Most studies evaluated approximately 50-200 patients and a similar number of age- and gender-matched healthy controls; two large studies enrolled over 1,000 SCD patients each.[25,28] Several studies used WHO growth standards for comparison rather than a non-sickle cell disease control group. Male and female subjects were generally equally represented in the study populations.
The two largest studies found significant growth defects compared to healthy controls. A multi-country study of more than 3,500 SCD patients (aged 10-24 years) with nearly 1,000 controls in Cameroon, Ivory Coast, Gabon, Mali, and Senegal was designed to evaluate determinants of vascular complications. Anthropometric analyses showed that SCD patients were significantly shorter and had lower BMI than controls; weight was not reported. A caveat of the study was that the control group was significantly older (median age 24 vs 16 for patients) and more likely to be female (60% vs 54% for patients) than the SCD group. Another large study followed a cohort of approximately 1,000 SCD patients aged 6 months to 48 years over five years and found SCD to be significantly associated with stunting, underweight, and wasting, with the most pronounced effects associated with adolescent age and male gender.  Adult men were seven times more likely than adult women to be underweight and were significantly more likely to be stunted and wasted. Females demonstrated improved catch-up growth compared with males following growth deficits that were identified during adolescence.
The studies involving smaller sample sizes showed greater variation in the results, but some trends emerged. Several reports recapitulated the finding that males were more likely to show growth defects than females.[30–33] Multiple studies also noted that growth deficits became more pronounced with age. For example, a study that involved young patients aged 6-35 months showed no association with wasting, stunting, or underweight status. A study of 233 children aged 2-17 years with SCD in Lagos, Nigeria found that the factor most significantly associated with both wasting and stunting was older age. Additional studies similarly detected one or more growth deficits in adolescents but not in younger children.[35–40] In several Nigerian studies, SCD patients were found to be underweight or to have low BMI, but showed no difference in height compared to controls [33,38,41–44]; while other Nigerian studies showed differences in both weight and height.[29,31,32,43,45,46] These variable findings may have resulted from the fact that the studies were not powered to detect significant differences in height specifically. Reports from DRC, Egypt, and Ghana found that children with SCD had a higher prevalence of stunting compared to controls, but did not always show differences in wasting or BMI.[23,24,26] SCD was also associated with delayed puberty.[26,47] Three studies from Nigeria reported the presence of overweight and obesity among patients with SCD although in lower proportions (an average of less than 3% of the sample population).[30,42,48]
Descriptive studies of macro- or micronutrient levels. The second most common group of studies identified involved assessment of biomarkers from serum samples for macronutrients or micronutrients. Most studies were conducted in Nigeria (17/22, 77%); other studies took place in DRC, Tanzania, Egypt, Kenya, and Malawi. Nutritional parameters measured included proteins/amino acids, fatty acids, vitamins, and minerals. The majority of studies included less than 100 patients. Both children and adults were studied, with male and female subjects generally equally represented.
Serum protein levels were investigated in one small study (13 children with SCD and 17 healthy controls) in Nigeria in which no significant differences were reported in the concentrations of total protein or albumin between SCD patients and controls. However, serum prealbumin levels were significantly lower for the population of patients with SCD, which was hypothesized to result from poor nutrition or existing disease-related inflammation. The serum concentrations of all amino acids except alanine, glutamic acid, and proline were significantly reduced in SCD patients. A small study involving 11 participants in Tanzania measured the nutrition status of SCD patients who died compared with those who were alive at the end of the study period. Those who suffered mortality had a significantly lower BMI, plasma taurine levels and arginine bioavailability before succumbing.
Proportions of fatty acid and the state of metabolism were evaluated in four related studies of young SCD patients in Nigeria. [31,45,53,54] These reports found perturbed pathways of fatty acid elongation and desaturation in children with SCD. Specifically, arachidonic acid, eicosapentanoic acid (EPA), and decosahexanoic acid (DHA) were significantly reduced, whereas saturated (palmitic acid) and monounsaturated (oleic acid) were significantly elevated in patients compared to controls. Another study in a population of 26 SCD patients aged 11-43 in Enugu, Nigeria, recapitulated the finding that EPA and DHA fatty acids are reduced in SCD patients. The authors of these studies hypothesized that reduced polyunsaturated fatty acids in the phospholipids of the cell membrane of SCD patients could lead to their being more rigid, thereby contributing to disease symptoms. A study of 30 children with SCD in Egypt found that patients also had significantly lower cholesterol, triglycerides, and LDL (but not HDL) in blood plasma compared to healthy controls.
Acknowledging that interpretation of plasma concentrations of vitamins and minerals can be problematic in patients with ongoing inflammation, analysis of serum vitamin levels in SCD patients generally indicated lower concentrations of vitamin A,[55,56] vitamin C, and vitamin E,[50,51,55,56]. One study of 14 SCD patients in Kenya found no association of HbSS genotype and low concentrations vitamin A. Three reports of a related study population in Ilese, Nigeria, investigated vitamin D status of young SCD patients.[57–59] When compared to healthy controls, mean 25-hydroxyvitamin D levels were significantly lower in SCD patients and suboptimal vitamin D levels were seen in greater than 10% of patients. However, no SCD patients with severe vitamin D deficiency (defined as <20 ng/ml) were observed. A limitation of the latter two studies was the lack of a healthy comparator group: each used vitamin D deficiency cut-off values for a healthy population in other published studies as reference.
Selected minerals were evaluated in eight small studies of SCD patients and compared to healthy controls. Serum iron concentration was reduced in patients compared to controls in all studies that evaluated it.[60–62] Serum or plasma zinc was also generally reduced in SCD patients,[49,61–64] although zinc was elevated in one population of 59 Nigerian adult SCD patients. Measures of other minerals showed mixed results. Magnesium levels were either reduced,[62,63] elevated or unchanged[60,62,65] in SCD patients compared to healthy controls. Similarly, copper was reduced, elevated,  or unchanged[60,62] in SCD patients. Other minerals measured in only a few studies included manganese, chromium, selenium, potassium, rubidium, cadmium, and calcium.
Interventional studies. A very small number of clinical studies involving nutritional interventions in SCD patients in African countries were identified. There were two randomized, placebo-controlled trials.[66,67] The first was a study of 128 SCD patients aged 2-24 years in Sudan investigating the impact of one year of treatment with oral omega-3 capsules containing EPA and DHA fatty acids (using age- and weight-dependent dosing) compared to placebo. The hypothesis was that omega-3 fatty acids could reduce red blood cell aggregation, adherence, and inflammation that occur during sickle cell disease-mediated vaso-occlusive crises. The treatment group had significantly fewer clinical vaso-occlusive events, as well as reduced rates of severe anemia and need for blood transfusions. The study was not powered to detect changes in other outcomes such as stroke, sequestration crisis, or vascular necrosis. A second study of 125 SCD patients in Nigeria (involving children aged 1 year and above) tested the effect of lime juice on SCD parameters. All patients in the treatment group (n=65) as well as controls (n=60) were given folic acid, vitamin B complex, and proguanil, with the treatment group also given twice-daily oral lime juice with weight-based dosing ranging from 5-15 mL. Each child was assessed monthly for 6 months. The group receiving lime juice was reported to have significantly fewer pain episodes, febrile illnesses, and hospital admission rates. There was no change in transfusion rate, organomegaly, or jaundice. The positive effect was postulated to result from vitamin C, amino acids (in particular, phenylalanine), and flavonoids contained in the juice, but no direct evidence for this was provided.
A third trial utilized a double-blind, random order design. Ready-to-use-supplementary food was studied in 119 children with SCD in Tanzania. Two different formulations of the supplements were compared: a commercially available (Nutriset, France) RUSF fortified with vitamins and minerals according to recommended daily allowances and an “enhanced” version of the same RUSF that was additionally fortified with arginine and citrulline. Arginine is the substrate for endothelial nitric oxide synthase, a natural vasodilator, and has been implicated in pathophysiology of SCD complications. In the cross-over study design, children received each treatment for 4 months, with 4-month washout periods following the intervention. Ready-to-use-supplementary food led to small weight gains, an increased arginine bioavailability ratio, and improved measures of endothelial function compared to baseline; addition of arginine and citrulline to the supplement did not provide additional benefits.
Other interventional studies investigated the use of vitamin D supplementation and administration of ready-to-use-supplementary food (RUSF). A small treatment arm was nested in a Nigerian study comparing blood levels of vitamin D and pro-inflammatory cytokines. The hypothesis was that low vitamin D levels might lead to a pro-inflammatory environment that exacerbates SCD symptoms. Twelve children with SCD who were determined to have low vitamin D levels were given 3 months of oral vitamin D supplementation (2000 U). At the end of treatment, mean serum 25-hydroxyvitamin D levels were significantly increased compared to baseline, levels of several proinflammatory cytokines were significantly decreased, and the levels of anti-inflammatory cytokine IL-11 were significantly increased.
Several longitudinal studies evaluated multi-faceted interventions that included nutritional supplementation but did not test the effects of nutrition directly. A large study in Nigeria enrolled more than one thousand patients (6 months-62 years of age) over a period of 8 years and involved multiple interventions including health education, nutritional supplementation (folic acid, vitamins B and C), vaccines, malarial prophylaxis, and improved access to analgesics, antibiotics, and antimalarials. Over the study period, there was a marked decline in mortality, hospitalization rates, and blood transfusions. A longitudinal study in Benin enrolled 236 children (aged 8 months-12 years) with SCD. Severe SCD-related adverse outcomes, such as vaso-occlusive crises, hospitalization, and transfusions during the study period of up to 6.5 years were compared to parents’ reports of outcomes prior to the study period. The interventions included health and nutrition education, folic acid supplementation, vaccines, malarial prophylaxis, and pneumococcal prophylaxis with oral penicillin. Disease severity appeared to be lower during the study period than prior to the intervention, although the study was limited since the period before the intervention had not been observed by the investigators. Another study in Benin evaluated prenatal care for 108 pregnant women with HbSS and HbSC genotypes; the intervention included the provision of education about SCD and nutrition, malaria prevention, early detection of bacterial infections, and restricted use of blood transfusion. The nutritional education was described by the authors as “drawing attention to affordable local products that have a high nutritional value.” The maternal mortality rate decreased (compared to a small historical data set) to a rate comparable with the overall maternal mortality rate for the maternity unit where the study was conducted (1.8% for SCD mothers vs 1.2% overall). However, the fetal loss rate of 11.9% for SCD mothers remained higher than the average for the hospital maternity unit (6.3%). For all of these studies, the marginal impact of the nutritional interventions was unfortunately not possible to determine.