In the developing world, vitamin A is usually consumed in the form of β-carotene, which is present in orange and dark green leafy vegetables such as sweet potatoes, squash, orange fruits, and plant leaves. Availability of such foods is often seasonal and their consumption is sporadic in food-insecure environments. The primary sources of zinc in the diets of the poor are whole-grain cereals and legumes. These foods have a high concentration of phytates, which form insoluble complexes with zinc and, consequently, reduce its bioavailability [3]. As a result, typical diets often do not provide adequate zinc. Iron is primarily obtained from legumes in the developing world, but the quantity and bioavailability of iron in these sources is often insufficient to prevent iron deficiency [4].

Cassava is the staple food of 250 million of the poorest Africans [5]. Cassava has a relatively high tolerance to drought. The cyanide-rich peel of the cassava tuber protects it from boring pests. Once cassava matures as a crop, it remains viable and edible in the ground for up to 3 years. These characteristics make cassava a preferred crop to afford food security to vulnerable subsistence farming communities in Africa [6, 7]. While cassava is an excellent energy source, it contains <1 μg/100 g β-carotene, ≈3 μg/100 g zinc, and ≈4 μg/100 g iron [8].

Kenyans derived 59% of their energy from cassava, while 15% of dietary energy of Nigerian children was derived from cassava (Table 2).

Micronutrient intakes were inadequate in a large fraction of the children (Table 3), and 89% of Kenyan children and 31% of Nigerian children obtained >25% of their dietary energy from cassava. The dietary diversity score was 4.0±1.4 for Nigerian children and 4.5±1.3 for Kenyan children (Table 3).

54 Kenyan children were surveyed twice. On the first survey day vitamin A, zinc and iron intakes were 420±550 μg/d, 3.4±4.7 mg/d and 6.1±2.5 mg/d, respectively (mean ± SD), and on the second survey day vitamin A, zinc and iron intakes were 450±420 μg/d, 3.1±1.3 mg/d and 6.6±2.8 mg/d. The intraindividual SDs, calculated from the second measurement, for vitamin A, zinc and iron were 310 μg/d, 1.4 mg/d and 2.0 mg/d, respectively.

The fraction of dietary energy provided by cassava was negatively correlated with vitamin A intake (r=−0.15, P<0.0001), with zinc intake (r=−0.11, P<0.0001) and with iron intake (r=−0.36, P<0.0001).

Considering both populations together, 33% of children had an adequate intake of vitamin A, 45% had an adequate intake of iron, and 37% had an adequate intake of zinc. If the micronutrient content of cassava was to be increased 20-fold for β-carotene, and 4 -fold for iron and zinc, the fraction of this population that would have an adequate vitamin A intake would be 61%, for iron it would be 86%, and for zinc it would be 67%.

Results of the cost analysis in Kenya indicate that adequate dietary β-carotene can be provided for $0.22/d, adequate dietary iron for $0.25/d, adequate dietary zinc for $0.74/d, and adequate amounts of vitamin A, zinc and iron for $0.74/d. For a Kenyan child consuming cassava as a staple, if the β-carotene content is increased 20-fold, zinc content increased 4-fold and iron content 4-fold, the cost of providing all of these micronutrients and adequate daily macronutrients falls to $0.16/d.

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 [12, 13]. A series of consecutive recalls is more accurate than a single one for determination of an individual’s dietary intake. Repeating the recall in 54 children showed that the populations estimates of micronutrient intake were likely to be accurate within 10%, since mean intake values varied by 10% or less. It is established, however, that when considering a population, a single dietary recall provides an accurate estimate of the nutrient intake and nutrient deficits of the population as a whole [13, 21]. The dietary recall method used in this study alerted participants of the survey date in advance. This has been shown to improve the accuracy of the recall, but it also offers participants the opportunity to change their dietary intake in anticipation of the survey. This has been noted to a source of bias in developed world dietary surveys, but is thought to be less of a problem in populations that consume a monotonous diet in the developing world. The study population was predominantly Africans, who, either, grow their own crops, or obtain their food from extended family members who grow crops. Thus, our findings should not be extended to populations with more dietary diversity, such as wage earners or urban dwellers who primarily purchase food. 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.

Since vitamin A supplements are routinely given to children in the developing world [22], vitamin A intake may be low while vitamin A status may not be. Among the individuals surveyed in Kenya, 33% reported receiving vitamin A supplements in the 6 months prior to the survey. National coverage figures for Nigeria indicate that 55% of children receive annual vitamin A supplements [23]. However, our survey was designed to identify children with inadequate vitamin A intake, not poor vitamin A status. It is acknowledged that food is the preferred vehicle to receive micronutrients, and these surveys do accurately report what micronutrients were received from food sources [24].

The diets in both Kenya and Nigeria lacked diversity when assessed by a standardized scoring system (Table 3). While many of the cereals and tubers consumed by these populations had a low content of vitamin A, zinc and iron, cassava provided the smallest quantities of these micronutrients when expressed per unit energy (Table 2). Nigerian children received more iron and zinc in their diets, probably because they consumed a more diverse group of cereals, most of which contained more zinc and iron than cassava. Cassava has the reputation of being a staple food of individuals living with food insecurity. In this study, cassava consumption correlated with inadequate micronutrient intake, this is certainly a consequence of its low micronutrient content, but also may be determined by factors such as socio-economic status, which were not included in the regression modeling.

In Kenya, the dietary cost analyses indicated a 4.6-fold increase in spending (relative to the cost of simply meeting macronutrient requirements) is needed to provide adequate amounts of zinc, iron and vitamin A. In Nigeria, only a 1.2-fold increase in spending (relative to the cost of simply meeting macronutrient requirements) is needed to provide adequate amounts of zinc, iron and vitamin A. Due to the high cost of these micronutrient-rich foods, it may well be more difficult for rural Kenyans to choose to increase the micronutrient content of their diet than Nigerians.

The proportions of children with inadequate micronutrient intakes were quite high in both populations surveyed. Inadequate dietary intake of a micronutrient does not correspond to clinical nutrient deficiency, although it does correlate with this health parameter. Assessment of clinical nutrient deficiency includes the biochemical measurement of a biological sample from the child. For zinc, iron and vitamin A, methods of biochemical assessment have limitations that prevent their widespread use [25–27]. Unfortunately, published rates of nutrient deficiency are also very high in Nigerian and Kenyan children. In a sample of 555 rural Kenyan children, 52% of children were iron deficient, 66% were zinc deficient, and 89% were vitamin A deficient [28]. In Nigeria, one survey found that 37% of pre-school aged children were iron deficient [29], and 26–75% of pre-school aged children were vitamin A deficient [30, 31].

Supplementation/fortification research to reduce micronutrient deficiency has been very successful. A meta-analyses of eight studies of vitamin A supplementation found that supplementation resulted in a 23% reduction in child mortality [32]. Zinc supplementation reduced the incidence and duration of diarrhea by 15–24%, as well as reducing the rate of lower respiratory infection and death [33]. The use of iron fortified maize flour resulted in an 89% reduction in iron-deficiency anemia in children [34].

Translating the research about supplementation into effective public health programs has been challenging. The most successful program is UNICEF’s biannual administration of vitamin A capsules [22]. This program has a 68% global coverage and saves hundreds of thousands of lives annually. However, supplementation has the lowest coverage among rural communities in Africa, where children are most likely to die as a result of vitamin A deficiency, and requires billions of dollars to sustain. There have not been widespread public health programs that have increased zinc and iron intake.

Given that public health micronutrient supplementation programs are expensive and leave gaps in the coverage of vulnerable populations, alternatives which incorporate more micronutrients into staple crops are being explored. Staple crops have been bred to increase essential mineral and β-carotene content [34]. However, the degree of nutrient enhancement that can be achieved by breeding is typically less than 2-fold for minerals and 10-fold for carotenoids. Yellow cassava roots, for example, have a 10-fold increase in ß-carotene, but no varieties of cassava have been identified with substantial zinc or iron content.