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Abstract
Dietary soy protein, in comparison with casein, generally lowers the serum cholesterol concentration in rats fed on a cholesterol-enriched diet, while mixed results were observed in rats fed on a diet free of cholesterol. Soy protein also suppresses the conversion of linoleic acid to arachidonic acid in the rat liver. The present study examines whether phytate, a minor component of a soy protein isolate, is responsible for these beneficial effects of soy protein. Weanling male rats were fed for 4 weeks on a purified diet containing a 20% level of protein (either casein (CAS), soy protein (SOY), phytate-depleted SOY (PDSOY) or phytate-replenished PDSOY (PRSOY)) and cholesterol (0 or 0.5%). The dietary protein source and phytate level only affected the serum and liver cholesterol concentrations when the animals were fed on the cholesterol-enriched diet, being significantly lower in those rats fed on the SOY and PRSOY diets than in those fed on the CAS diet, while the concentrations in the rats fed on the PDSOY diet were intermediate. When the animals were fed on the cholesterol-free diet, the ratio of (20:3n-6+20:4n-6)/18:2n-6 in liver phosphatidylcholine, a Δ6 desaturation index, was significantly lower in the SOY diet group than in the CAS, PDSOY and PRSOY diet groups. Dietary cholesterol significantly depressed the ratio, but neither depletion nor replenishment of phytate affected the ratio. These results suggest that phytate in soy protein played a limited role in the cholesterol-lowering effect of soy protein and was not involved in the metabolism of linoleic acid.
