Digestion-resistant fraction from soybean [Glycine max (L.) Merrill] induces hepatic LDL receptor and CYP7A1 expression in apolipoprotein E-deficient mice

Soybean [Glycine max (L.) Merrill] is known to have hypocholesterolemic effects; however, the function and mechanism of its digestion-resistant fraction (RF) in cholesterol reduction is not clearly understood. In the present study, we investigated the hypocholesterolemic effects of the RF from soybean in C57BL/6J and apolipoprotein E (apoE)-deficient mice. RFs were prepared either from raw or preheated crops to measure compositional changes in RF during cooking. Preheating reduced the RF yields and the resistant starch (RS) fraction in RF. After 1 week of feeding, the raw soybean RF (5%, w/w) was the most effective in lowering plasma cholesterol concentrations by 27% (P<.05) in apoE-deficient (apoE^-/-) mice. A smaller but significant reduction was found in C57BL/6J mice. The RF from preheated soybean tended to have lower hypocholesterolemic effects than did the RF from raw soybean in apoE^-/- mice. This suggests the RS may be a key hypocholesterolemic component from soybean RF. RF consumption (5%, w/w) dramatically increased hepatic low-density lipoprotein receptor and cholesterol 7α-hydroxylase expression in both apoE^-/- and C57BL/6J mice followed by increased bile acid excretion. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase was only marginally altered. Our results show that the RF, especially from raw soybean containing high level of RS, significantly reduces plasma cholesterol concentrations under hyperlipidemic condition. The cholesterol was reduced by multiple mechanisms such as increased hepatic cholesterol uptake, cholesterol degradation into bile acids and bile acid excretion.