Effects of soybean protein and very low dietary cholesterol on serum lipids, biliary lipids, and fecal sterols in humans

Soy-base texturized vegetable protein (TVP; Archer Daniels Midland, Decatur, IL) has been used to decrease serum cholesterol and as a substitute for animal protein to achieve very low levels of dietary cholesterol. The effect of very low dietary cholesterol and of TVP on biliary lipids and fecal sterols is unclear. The study objective was to determine the effects of very low intake of dietary cholesterol, as well as TVP itself, on serum lipids, biliary lipids, and fecal sterols. We studied eight normal subjects living on a metabolic ward during three randomly ordered 6- to 7-week periods: (1) standard cholesterol diet (190 to 550 mg/d), (2) TVP-low-cholesterol diet (17 to 30 mg/d), and (3) TVP-standard cholesterol diet. By analysis of covariance (ANCOVA), reducing dietary cholesterol to these very low levels significantly decreased serum low-density lipoprotein (LDL) cholesterol (P=.048) but did not affect high-density lipoprotein (HDL) cholesterol or triglyceride. TVP resulted in a borderline significant reduction in LDL cholesterol (P=.058) with a highly significant reduction in HDL cholesterol (P=.004) and an increase in serum triglyceride (P=.010). During TVP ingestion, there was a highly significant increase in the output of fecal neutral sterols (P=.005) and a tendency for a higher output of fecal acidic sterols (P=.100). Fecal sterol balance was significantly more negative (indicating increased cholesterol synthesis) during TVP ingestion (P=.016). Neither TVP nor the very-low-cholesterol diet appreciably affected the gallbladder bile molar percent cholesterol or saturation index. The data are consistent with the hypothesis that to the extent TVP decreases serum LDL cholesterol (an effect of borderline significance in this study), the effect occurs via a reduction in the absorption of cholesterol and perhaps bile acid. However, the potential benefit of decreasing LDL cholesterol in this way seems to be at least partially offset by a concomitant reduction in HDL cholesterol and an increase in serum triglycerides.