Bioavailability algorithms in setting Recommended Dietary Allowances: lessons from iron, applications to zinc

In addition to determining the amounts of isolated nutrients required for physiological function and homeostatic control, setting Recommended Dietary Allowances requires a consideration of nutrient bioavailability from foods in common diets. Bioavailability algorithms, or mathematical models to estimate nutrient bioavailability from different diets, have great appeal to help establish recommended intakes and identify beneficial dietary modifications. Accurate algorithms are difficult to develop because of the chemical complexity of the food supply and the numerous interactions that affect the bioavailability of mineral elements. A combination of reductionist and holistic approaches is necessary to identify the effects of isolated dietary components, and then to evaluate these effects in the complicated matrix of a whole diet. The most extensive development of bioavailability algorithms for mineral nutrients has been for iron, and understanding the strengths and weaknesses of the iron algorithms may help in development of algorithms for other inorganic nutrients such as zinc. Examples are provided to indicate the possible revision of iron bioavailability algorithms, the difficulty of verifying the iron bioavailability algorithms with iron status indices, and the possible development of zinc bioavailability algorithms. Bioavailability must be considered in the context of the biological control of mineral absorption, utilization and excretion, which may outweigh the importance of mineral bioavailability from diets in affluent countries with a varied and abundant food supply. While bioavailability algorithms may be helpful in estimating the effects of dietary changes or recommendations, and in improving diets of populations with a high incidence of deficiency of the mineral, they are not as yet sufficiently developed and validated for setting quantitative recommendations for "bioavailable" mineral intake.