Effects of dietary protein on turnover, oxidation, and absorption of cholesterol, and on steroid excretion in rabbits.

Rabbits fed a low fat, cholesterol-free, semipurified diet containing casein became hypercholesterolemic (congruent to 300 mg/dl) after 5 weeks on diet. Rabbits on a similar diet containing soy protein isolate had low plasma cholesterols comparable to those on commercial feed (40--60 mg/dl). Cholesterol turnover, which conformed to a two-pool model, were determined by analysis of the decay of plasma cholesterol specific activity after a single intravenous injections of [26-14C]cholesterol. Rabbits on the soy protein diet or commercial feed showed a much faster rate of cholesterol turnover and a reduced size of pool A compared to rabbits on the casein diet. They also oxidized [26-14C]cholesterol to respiratory 14CO2 at much faster rates. Analysis of fecal steroid by gas--liquid chromatography indicated that bile acid and neutral steroid excretion was increased on the soy protein and commercial diets, relative to the casein diet. Cholesterol was absorbed to a greater extent on the casein diet. Addition of 15% (w/w) butter to the semipurified diets had little effect on the above parameters of cholesterol metabolism. Comparison of cholesterol turnover measured by kinetic analysis, combined sterol balance, or analysis of fecal steroids by gas--liquid chromatography, showed that all three methods gave similar results. Measurement of bile acid production by oxidation of [26-14C]cholesterol to respiratory 14CO2 also gave results comparable to those obtained by analysis of fecal bile acids.