Consumption of buckwheat protein lowers plasma cholesterol and raises fecal neutral sterols in cholesterol-fed rats because of its low digestibility

Abstract

Buckwheat protein product (BWP) has a strong hypocholesterolemic activity in rats fed a cholesterol-enriched diet. In this study, we examined the influence of BWP on fecal excretion of sterols and nitrogen in rats fed a diet containing 5 g/kg cholesterol and 1.25 g/kg sodium cholate, and we examined whether the cholesterol-lowering activity of BWP is due to its low digestibility. In Experiment 1, rats fed BWP for 3 wk had significantly lower concentrations of plasma cholesterol and enhanced excretion of fecal total neutral sterols and nitrogen compared with rats fed casein. There was a significant correlation between fecal total neutral sterols and nitrogen (r = 0.89, P < 0.01). Fecal excretion of acidic sterols was unaffected by BWP. In Experiment 2, plasma cholesterol in rats fed trypsin-digested BWP for 2 wk was significantly higher than that in rats fed intact BWP. In Experiment 3, rats were fed BWP, low-molecular-weight fraction of the digest of BWP (LMF) or high-molecular-weight fraction of the digest of BWP (HMF) for 3 wk. Plasma cholesterol was lower in the BWP group than in the LMF group (P < 0.05), whereas that in the HMF group was intermediate. The in vitro digestibility of BWP with pepsin and pancreatin was significantly lower than that of casein. The results suggest that the cholesterol-lowering effect of BWP is mediated by higher fecal excretion of neutral sterols and that lower digestibility of BWP is at least partially responsible for the effect.