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Previous laboratory studies have shown that dietary iron during digestion forms two pools: heme and nonheme. Even though human beings absorb heme iron at a higher rate than nonheme iron, the major portion of food iron is nonheme (all grain and vegetable iron plus approximately 60 percent of the iron in meat, fish, and poultry). The absorption rate of nonheme iron can be increased markedly by concomitant consumption of ascorbic acid and/or meat/fish/poultry. These concepts have been incorporated into a model for estimating the quantity of bioavailable iron, the first trace mineral to be thus considered. This model has now been revised for easier and more refined application.

The mathematical formulation of nutrient variation in meals in presented by means of random vectors. The primary sources of nutrient variation in unit portions of menu items are identified and expressed in terms of random food-nutrient, random portion size and random ingredient composition variations. A secondary source of nutrient variation can be traced to the random selection process of combining menu items into individual meals from multiple choice menus. The separate as well as the joint effect of these sources on the total variation of the nutrient content of meals is described with the aid of variance-covariance matrices. The investigation is concluded with the formulation of multivariate probability statements concerning the adequacy of the nutrient content of meals relative to the distribution of the nutrient requirements over a given population.