THE INTESTINAL TRUE ABSORPTION OF ZN-65 IN RATS IS ADVERSELY AFFECTED BY DIETS CONTAINING A FABA BEAN (VICIA-FABA L) NONSTARCH POLYSACCHARIDE FRACTION

The effects on the absorption of Zn-65 by two varieties of raw faba bean (Vicia faba L., minor) or seed components that may interfere with mineral metabolism in the gut, have been studied in growing rats. In bean diets all protein was supplied by the meals, and the fractions were tested by incorporating them in control diet at the same levels as they occur in the seeds. Absorption of Zn-65 was also measured in rats fed dephytinized bean meal produced by including phytase in the diet. Rats were pair-fed diets supplemented with amino acids and minerals to target requirements and containing 40 mg Zn/kg diet, True absorption of Zn was 50-70% lower in rats fed diets containing both cultivars of faba bean meals than in those fed the control diet. Although soluble nonstarch polysaccharides caused a significant reduction in the absorption of Zn, this effect disappeared after the removal of phytate by demineralization. In contrast, despite its negligible content of phytate, the insoluble nonstarch polysaccharides in the cell wall fraction of the cotyledon accounted for most of the reduction in Zn absorption in rats fed the faba bean diets. Addition of phytic acid to the control diet significantly reduced the absorption of Zn-65 but only from 44 to 36%. Moreover, the increase in the absorption of Zn was similarly small, from 21% to 29%, with the addition of phytase to the faba bean diet. These results suggest that, in addition to phytic acid, the insoluble nonstarch polysaccharides present in the cell walls of the cotyledon in Vicia faba seeds are mainly responsible for the lower Zn availability in rats fed faba bean diets.