The Protease-Resistant Fraction of Smoked, Dried Bonito Lowers Serum Cholesterol in Ovariectomized Rats Fed Cholesterol-Free Diets

The hypocholesterolemic effects of smoke-dried bonito undigested fractions remaining after microbial proteases treatment (SDBR), SDBR undigested fractions remaining after pepsin treatment (P-SDBR), SDBR undigested fractions remaining after alcalase treatment (A-SDBR), and defatted A-SDBR (A-SDBR/DF) were studied in 6-month-old, ovariectomized rats. Casein was used as the control. Smoke-dried bonito, Katsuobushi, is a traditional Japanese food. SDBR, P-SDBR, and A-SDBR contain mainly 2 components: bonito oil and protease-undigested proteins. In comparison with casein, SDBR, P-SDBR, and A-SDBR decreased plasma cholesterol concentration and increased fecal bile acid excretion and fecal output, whereas A-SDBR/DF increased fecal bile acid excretion and fecal output, but did not decrease plasma cholesterol concentration. In addition, SDBR, P-SDBR, and A-SDBR, when compared with casein, decreased plasma triglyceride (TG) concentration and increased hepatic total lipid concentration, free cholesterol, and TG concentrations and CYP7A1 mRNA levels. Yet, these effects were not observed in rats fed A-SDBR/DF. These results suggest that the hypocholesterolemic effect of SDBR, P-SDBR, and A-SDBR is mediated by an increase in bile acid excretion through a combination of promoted secretion of bile acids by bonito oil and binding of bile acids to resistant proteins. The hypotriglyceridemic effect may be mediated by reduced hepatic TG synthesis and secretion due to the intake of bonito oil.