This study found that cassava intake is inversely correlated with protein intake and diet quality in Nigeria and Kenya. Those individuals who consume cassava as a staple do not compensate for the very low P:E of cassava by including sufficient amounts of protein-rich foods such as legumes and fish in the diet.
A limitation of this study is the uncertainty associated with a single 24 hour recall to determine dietary intake, as dietary intake is known to vary substantially day-to-day [17, 18]. A series of consecutive recalls is more accurate than a single one for determination of an individual's dietary intake. The value of our single dietary recall was assessed by repeating the recall in 54 children, and a 7% variation in protein intake was found for this group of children. However, it is established that when considering a population, a single dietary recall provides accurate estimate of the nutrient intake and deficits of the population as a whole [18, 26]. Another limitation is that our study population was predominantly of rural Africans, who, for the most part, were subsistence farmers, thus our findings should not be extended to laborers who primarily purchase food, nor to urban dwellers. Thirdly, this assessment was restricted to 2-5 year old children and therefore it might not be applicable to other age groups who obtain more of their food prepared outside the home. The analyses exploring the relationship between stunting and protein intake is limited in that this study only provides information at a single point in time, stunting is a dynamic process that certainly is influenced not only by diet, but the frequency of acute infections.
The typical protein content of cassava is about 1.3 g/100 g. A serving of cassava that constitutes 25% of an average energy intake of a 2-5 year old provides only 1.1 g protein, about 8% of the estimated protein requirement. The Protein Digestibility Corrected Amino Acid Score (PDCAAS) for cassava is 57, one of the lowest on any staple foods, with lysine and leucine being the limiting amino acids [24, 27]. These facts suggest that it is difficult for children to get adequate dietary protein from cassava, and children that do receive a significant fraction of their dietary protein from cassava are at risk for inadequate essential amino acid intake.
The protein intake of the Nigerian population studied was greater than that of the Kenyan population. This was achieved by incorporating more low cost cereals in the diets, such as maize, rice and sorghum, rather than foods nutritionists traditionally identify as good sources of dietary protein, such as fish or legumes.
Dietary diversity has been shown to be linked to household food security, child growth, and diet quality. Diets were found to be less diverse as reliance on cassava as a staple food increased. This association identifies cassava consumers as a vulnerable population that may benefit from interventions to improve nutrition.
Our finding that dietary protein intake is a predictor of height is consistent with previous results in other settings. In China, height correlated with dietary protein intake, when controlled for dietary energy intake, socio-economic status and income . Another study compared adult heights by country with dietary energy and protein intake, income, ethnic group and socio-economic status, and only dietary protein intake was associated with height . A study from Peru found that animal source dietary protein was strongly associated with height and weight, but not with dietary energy . Studies from Bangladesh and the Philippines show that dietary protein intake, but not dietary energy intake, is associated with child growth [29, 30]. Certainly these associations between growth and protein intake do not prove a causal relationship, and these previous studies did not account for the micronutrient content of the diets as confounding factors. But the cumulative evidence from observational human studies and animal intervention studies strongly supports the notion that if the P:E of the diet falls below a certain level during the time when statural growth occurs, the result is stunting. The WHO estimates of adequate protein intake and energy requirements suggest that a P:E < 5% in humans is detrimental. While it is rare to see such low P:Es, our data indicate that such diets are not uncommon among populations that consume cassava as a staple food.
Interventions aimed at increasing dietary protein intake to improve growth and reduce morbidity in children include nutritional education and counseling and supplemental feeding programs. Typically studies have found very modest correlations between nutrition knowledge and dietary behavior [31–33]. A study in infants analyzed the effect of maternal nutritional counseling over 12 months and found no increase in weight or height gain and no decrease in morbidity . Supplemental feeding programs usually involve blended flours, which are combinations of cereals, legumes, and other foods to provide high quality protein derived from plants, given over a period of several weeks to a vulnerable population . Although these programs are extensive, expensive, and have been used over many decades, there is no evidence that supplemental feeding reduces or reverses stunting . Given the meager effectiveness of education and supplemental feeding in the past, it seems unlikely that they would be helpful to very poor populations that consume cassava as a staple food in the future.
The protein content of existing cassava cultivars varies between 1-4% , with a typical value of 1%. Hence breeding of the most protein-rich cassavas with African germplasm for consumption as a staple will not result in cassava hybrids that will ameliorate dietary protein deficiency. A novel research initiative, BioCassava Plus, aims to achieve increased protein content through genetic engineering of local cassava varieties of cassava consuming populations. If the P:E of cassava were increased 4-8 fold using genetic enhancement of cassava, the fraction of the population with inadequate protein intake might be reduced to half of that found in this study.