The availability of iron, zinc and cadmium to rats from composite diets with different cereal grains

Many investigations have been reported in which the effects of varying the concentration of a single factor in grain diets on the availabilities of minerals for absorption have been studied. The purpose of this study in rats was to determine the effects on iron, zinc and cadmium availability of substituting whole grain wheat, oats, barley or rye for iron-supplemented endosperm wheat in a composite diet with one part grain flour and one part non-fat, dried milk with maize oil and vitamins. Iron concentrations varied by a factor of 3 among these diets, zinc by 1.5, cadmium by 5, and both fiber and phytate by a factor of 3.5. The absorption of 59Fe and 65Zn from test meals of the diets and the concentrations of iron, zinc and cadmium in selected tissues and body fluids after five weeks on the grain diets were used as independent measures of the mineral availabilities from these diets. Absorption of iron from endosperm wheat and, to a lesser extent, zinc from oats and rye appears to have been overestimated, most likely due to incomplete exchange of the radioisotopes with the supplemented or endogenous minerals in the grains. The variations in the concentrations of iron and cadmium among the diets were by far the most important factors determining the availabilities of these minerals to the rats. The fractional availabilities of iron and zinc were negatively related to the diet phytate concentration and the fractional availability of cadmium was negatively related to the iron status of the rats. The results indicate that substitution of whole grain wheat for endosperm wheat in the diet will increase cadmium accumulation considerably while substitution of oats barley or rye should result in nearly the same or lower cadmium accumulation. Due to the low availability of iron in barley and rye, substitution of either grain for iron-supplemented endosperm wheat may compromise iron status. As the diets in this study are similar to baby formulas and breakfast cereals, porridge or a sandwich with milk, the probability that these results in rats are applicable to humans strongly suggests that studies similar to this be carried out in humans.