Effects of dietary protein levels and cholesterol on tissue lipids of rats.

The effects of the levels (10, 20, and 30%) of dietary proteins, soybean protein or casein, on various lipid parameters were examined in rats. The plasma cholesterol (CHOL) level tended to decrease with an increasing dietary protein level, in particular when diets contained CHOL. The hypocholesterolemic effect of soybean protein was evident with CHOL-enriched diets. The fecal excretion of acidic but not neutral steroids increased with an increasing protein level and it was higher in rats fed soybean protein than in those fed casein. The ratio of arachidonate to linoleate in plasma and liver phosphatidylcholine tended to increase with an increasing dietary protein level, although it was considerably higher in casein than in soybean protein diets. The results indicated that the hypocholesterolemic effect of soybean protein is influenced not only by the amount but also by the presence or absence of dietary CHOL. In addition, the degree of conversion of linoleate to arachidonate also depended on the type and level of dietary protein.