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Micronutrients are essential vitamins and minerals which are unable to be synthesised by the body and must be obtained from dietary sources. Approximately two billion people, across countries of all levels of development, consume diets with insufficient quantities of micronutrients (Bhutta & Salam, 2012; Péter et al., 2014). Deficiencies during childhood contribute to impairments of growth, immune function and physical development and are a major factor in nutrition-related deaths globally (Viteri & Gonzalez, 2002). While external clinical manifestations such as goitre and blindness may prompt recognition of a public health problem, subclinical deficiencies affect much larger segments of a population, often where energy requirements are met and people are not considered to be “hungry” in a classical sense (Kennedy, Nantel, & Shetty, 2003). Deficiencies of iodine, iron, vitamin A and zinc are considered the most prevalent and important of the numerous micronutrient deficiencies of public health significance (Tulchinsky, 2010). This chapter focuses on two of these, iron and vitamin A, due to the particular value of foods of animal origin in meeting human physiological requirements for each, and the availability of information on the prevalence of deficiencies through national surveys.

Vitamin A is a group of fat-soluble retinoid compounds needed for normal vision, growth and development, immune function, epithelial cell integrity and reproduction. There is strong evidence that improved vitamin A status is associated with large reductions in all-cause mortality, morbidity and vision problems in children under five years of age (Imdad et al., 2011;

Mayo-Wilson, Imdad, Herzer, Yakoob, & Bhutta, 2011). Reasons for deficiency in young children have been suggested to include: (a) low levels of vitamin A in breast milk due to maternal

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deficiency, (b) early introduction of complementary foods low in vitamin A, (c) less bioavailable forms of vitamin A in diets associated with poverty, and (d) anorexia, malabsorption and increased catabolism of vitamin A resulting from a high burden of disease (Miller et al., 2002).

Supplementation is recommended in all countries where the under-five mortality rate exceeds 70 deaths per 1000 live births, an internationally-accepted proxy for a high risk of vitamin A deficiency in children (UNICEF, 2007). It has been cautioned that widespread supplementation programs may impede efforts towards more sustainable food-based approaches and fortification programs (Darnton-Hill, 1999; Darnton-Hill, Neufeld, Vossenaar, Odendarp, &

Martinez, 2017; Latham, 2010).

Dietary sources of vitamin A include pre-formed retinyl esters in ASF (including liver, milk and eggs) and pro-vitamin A precursors, such as β-carotene, in plant-source foods (including dark green leafy vegetables and deep yellow fruits and vegetables) (Ross, 2010). Absorption of pre-formed vitamin A is by far more efficient than that of carotenes (Allen & Haskell, 2002). A revision of former guidelines has seen a dramatic drop in the previously-estimated bioconversion ratio of 6 µg β-carotene to 1 µg retinol (FAO & WHO, 1988), to 26:1 for dark-green leafy vegetables and 12:1 for fruits and tubers (West, Eilander, & van Lieshout, 2002).

There are limitations in relying on plant-based foods in efforts to prevent and address vitamin A deficiency in LMIC, and “promoting the consumption of animal-source foods if feasible” has been recommended, alongside supplementation and fortification of staple foods (Ramakrishnan

& Darnton-Hill, 2002, p. 2951S). Dietary diversification and modification focus on improving the availability, access to and utilisation of foods with high levels of bioavailable micronutrients throughout the year (Gibson, 2011).

Iron deficiency is one of the most prevalent forms of undernutrition, affecting more than two billion people across low-, middle- and high-income countries (Camaschella, 2015; Stoltzfus, 2003). Iron deficiency has been implicated as the leading cause of anaemia globally (Kassebaum et al., 2014), often co-existing alongside other risk factors in resource-poor settings, including malaria, endoparasitism and deficiencies of other micronutrients, such as vitamin B12 and folate (WHO, 2008b). Iron, a trace mineral, is required in small amounts for the formation of haemoglobin, the oxygen-carrying component of red blood cells (Lutz, Mazur, & Litch, 2014).

Haem iron, found only in animal products, is highly bioavailable: absorbed intact and less affected by inhibitory compounds within the diet which impede the uptake of non-haem iron, the sole form present in plant-source foods (Bothwell, Baynes, Macfarlane, & Macphail, 1989).

Absorption of both forms of iron varies with an individual’s iron status and requirements, but non-haem iron is particularly sensitive to the presence of inhibitors such as phytic acid, tannins and certain forms of dietary fibre (Hallberg, 1981). A significant challenge exists in many resource-poor settings, where up to 50% of iron intake is derived from cereal-based diets (Bouis, 2000) which also contain high levels of phytic acid (Gibson, 1994).

Increased iron requirements associated with growth, menstruation, pregnancy and lactation contribute to a higher prevalence of iron deficiency amongst infants, young children and women of reproductive age (Asobayire, Adou, Davidsson, Cook, & Hurrell, 2001). Despite recognition of the widespread nature of iron deficiency, there is a lack of consensus about the nature and extent of its consequences on human health (Stoltzfus, 2003). Anaemia during pregnancy is associated with a heightened risk of premature delivery, low birthweight and higher levels of perinatal mortality and maternal deaths (Kalaivani, 2009; Lister, Rossiter, & Chong, 1985). Iron status has also been linked to long-term developmental and behavioural outcomes, with severe chronic iron deficiency during infancy associated with poorer mental and motor function ten years later, as well as higher levels of anxiety, depression and social problems (Lozoff, Jimenez, Hagen, Mollen, & Wolf, 2000; Shafir, Angulo-Barroso, Calatroni, Jimenez, & Lozoff, 2006).

As for vitamin A, food-based approaches are promoted as a sustainable strategy to prevent and address iron deficiency (WHO, 2008b). It has been suggested that, in many resource-poor settings, it is almost inevitable that children 6-12 months of age (mo) and pregnant women will be unable to meet their physiological requirements through an adequate amount of absorbable iron in the diet (Stoltzfus & Dreyfuss, 1998). Studies in Cambodia, Indonesia and Myanmar have highlighted the difficulty of achieving adequate intakes of iron based on existing complementary feeding practices (Hlaing et al., 2016; Santika, Fahmida, & Ferguson, 2009; Skau et al., 2014).

The potential for plant-source foods to address iron deficiency has long been questioned (de Pee, West, Muhilal, Karyadi, & Hautvast, 1996; Yip, 1994), and the focus has instead been on programs involving fish and livestock which increase access to ASF (Roos, Wahab, Chamnan, &

Thilsted, 2007; Ruel, 2001). Recent evidence recommends the large-scale fortification of staple foods, as has been done in more affluent countries for over 80 years (Darnton-Hill et al., 2017).

The use of iron cooking pots or ingots has also been shown to be an effective and innovative form of home fortification of foods and an accessible means of improving iron intake (Charles et al., 2015; Geerligs, Brabin & Omari, 2003).

Micronutrient deficiencies in Tanzania

Information on vitamin A and iron supplementation, consumption of vitamin A- and iron-rich foods and the prevalence of anaemia (based on haemoglobin levels) was first included in the Tanzanian DHS in the 2004-05 survey (NBS Tanzania & ORC Macro, 2005). Amongst priorities outlined in the Tanzanian National Nutrition Strategy (NNS), targets to be achieved by 2015 included a reduction in the prevalence of vitamin A deficiency amongst children 6-59 mo to below 15%, and for anaemia, 55% for children 6-59 mo and 35% for pregnant women (Ministry of Health and Social Welfare [Tanzania], 2011).

No clear trends in levels of anaemia in young children and women of reproductive age are evident across the three most recent national surveys (Figure 4). World Health Organization cut-offs have been used to categorise anaemia as mild, moderate or severe – although use of the term “mild” has been suggested to be a misnomer, since in the case of iron deficiency anaemia the underlying condition is already advanced at this stage (WHO, 2011). A substantial decrease in the prevalence of anaemia in children (from 71.8% to 58.6%) can be seen between 2004-05 and 2010, but only a marginal further decrease (to 57.7%) in 2015-16. Prominent regional variation exists, with Singida Region recording the lowest levels of child anaemia nationally (36.6%) in the 2015-16 survey. Markedly lower levels of anaemia were seen amongst children whose mothers had achieved secondary education and those in the highest wealth quintile (MoHCDGEC [Tanzania Mainland] et al., 2016). In the case of women of reproductive age, reported levels of anaemia increased between the 2010 and 2015-16 DHS (from 40.1% to 44.8%, overall). Prevalence amongst pregnant women was particularly high (57.1%), and far above the national target for 2015 of 35%.

Figure 4. Prevalence of anaemia of varying levels of severity, amongst children 6-59 months of age and women of reproductive age, compiled from Tanzanian DHS data. Targets for 2015 outlined in the National Nutrition Strategy (NNS) for children and pregnant women are shown.

Opportunities to explore trends in the percentage of children consuming vitamin A- and iron-rich foods over time using DHS data have been complicated by the use of varying indicators over time. The 2004-05 survey recorded consumption of fruits and vegetables rich in vitamin A, while the two subsequent surveys also included ASFs. The 2004-05 and 2010 surveys reported on children under three years of age, while the 2015-16 survey was limited to children under two years. In a recent cross-sectional study in central Tanzania evaluating the adequacy of local foods in meeting nutrient requirements, large deficits were identified in the intake of iron by children 6-23 mo (Raymond, Agaba, Mollay, Rose, & Kassim, 2017). While children’s vitamin A intake appeared to be adequate, the authors acknowledged this finding to be based on the published average intake and composition of breast milk, which may not reflect the situation within the study site, where poor maternal nutritional status may affect breast milk composition and thus the vitamin A status of children (WHO, 1998).

In rural communities in central Tanzania, it has been shown that optimal diets for children 6-23 mo can be developed using locally-available food items, but that this requires a two-fold increase in the food budget (Raymond, Kassim, Rose, & Agaba, 2017). While acknowledging the need to consider seasonal variation and to evaluate the bioavailability of nutrients in the proposed diets, the authors upheld the value of food-based approaches to overcome identified micronutrient deficiencies in Tanzania. Increasing the availability and consumption of a nutritionally adequate diet has been suggested to be the only sustainable and long-term solution to simulataneously addressing deficiencies of iron and multiple other micronutrients (Thompson, 2011).