Nutrient-nutrient interaction in the process of digestion and absorption is more important for child than adult nutrition. The requirements for micronutrients are high in childhood because of the need to form new tissues in growth. However, phytates from cereal and vegetable foods bind minerals, particularly calcium, vitamin D and iron, in the bowel and reduce absorption. Asian children in northern latitudes on high-phytate traditional diets are at risk of developing vitamin D deficiency rickets. This is partly due to poor absorption of both calcium and vitamin D because they are bound with phytates in the small intestine and absorption is reduced. Inadequate synthesis of vitamin D from precursors in the skin because of low sunlight exposure will exacerbate poor vitamin D nutrition and further impair calcium absorption. Thus, whilst nonstarch polysaccharides (NSP) from unrefined cereals, whole fruits, and vegetables should be an increasing proportion of the diets of children as they grow, NSP intakes should only be gradually increased to perhaps 15 g day"1 by 10 years of age.
Nutrient-nutrient interactions are not necessarily disadvantageous. Vitamin C, through its reducing power, maintains iron in the ferrous state in the gastrointestinal tract and thus facilitates absorption of this important micronutrient. Vitamin C also facilitates absorption of a number of other micronutrients. Each meal should contain a good source of vitamin C to optimize utilization of other micronutrients.
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