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The comparison of measured mineral intakes of children in the developing world with estimated biological requirements (accretion rates + obligatory losses) which we have described in the preceding sections can be summarised as follows
1. The average intakes of P and Mg are substantially greater than biological requirements. There is evidence that absorption of these minerals and conservation of endogenous losses are likely to be high. It would appear unlikely, therefore, that inadequate dietary supply of P and Mg contributes to the poor linear growth of Third World children.
2. Zn intakes of breast-fed children are close to the biological requirement, if one assumes that there is only limited capacity to reduce losses. Children who are no longer breast-fed have intakes that are 4-5 times above the biological requirement, but Zn supply may be restricted by poor bioavailability. Supplementation studies suggest that the linear growth of vulnerable groups of children, particularly infant and adolescent boys, can be increased by raising Zn intakes. The mechanism by which this occurs is unknown, but is likely to be related to stimulation of appetite or to metabolic effects, rather than to an improved supply of Zn for bone formation per se.
3. Ca intakes at all ages are close to the biological requirement for children in many developing countries (< 1-2 times). In addition, absorption of Ca from Third World diets may be poor, because of the presence of chelating components such as phytates and oxalates. There is little data on the extent to which children can adapt to low Ca intakes in terms of enhanced absorption and decreased losses, but it must be assumed that substantial adaptation can occur. It may be that slow growth rates represent an adaptation to limited mineral supply. There are indications that marginal Ca status may be reflected in biochemical signs of hyperparathyroidism and in low bone mineral contents, and may induce or predispose children to rickets. The evidence from the small number of Ca supplementation studies on the effects of increasing Ca intakes on bone growth and development is inconclusive.
In reaching these conclusions a number of assumptions have had to be made which should be borne in mind when interpreting the findings. Firstly, no consideration has been made about the wide differences there are likely to be in requirements, intakes and ability to adapt between individual children. Secondly, estimating needs on the basis of mineral deposition rates does not take account of the intakes which may be required to maintain optimal function, especially relevant for Zn and Mg. Thirdly, the arguments are based on inadequate data, particularly with respect to the mineral content of the body, the absorption and losses of minerals from children habituated to low intakes, and the identification of marginal mineral status in Third World children. It is to be hoped that more information will become available in the future with the use of new, sensitive, non-invasive techniques, such as stable isotope technology and single photon absorptiometry, and as a result of the current search for specific markers of marginal mineral status and of bone turnover.