We previously showed that plasma cholesterol levels decreased following ingestion of a short-chain fatty acid (SCFA) mixture composed of sodium salts of acetic, propionic, and butyric acids simulating cecal fermentation products of sugar-beet fiber (SBF). In the present study, we investigated whether hepatic and small intestinal cholesterol synthesis is involved in the cholesterol-lowering effects of SCFA and SBF. In vitro (expt. 1) and in vivo (expt. 2) cholesterol synthesis rates and the diurnal pattern of SCFA concentrations in portal plasma (expt. 3) were studied in three separate experiments in rats fed diets containing the SCFA mixture, SBF (100 g/kg diet), or the fiber-free control diet. Cholesterol synthesis was measured using 3H2O as a tracer. The in vitro rate of cholesterol synthesis, measured using liver slices, was greater in the SBF group, but not in the SCFA group, than in the fiber-free control group. In contrast, the hepatic cholesterol synthesis rate in vivo was lower in the SCFA group, but not in the SBF group, than in the control group. The mucosal cholesterol synthesis rate for the whole small intestine was <50% of the hepatic rate. The rate in the proximal region was slightly but significantly lower in the SCFA group, and was significantly higher in the SBF group than in the fiber-free group. The rate in the distal small intestines was also significantly greater in the SBF group than in the fiber-free group. Plasma total cholesterol concentrations were lower in the SCFA and SBF groups than in the fiber-free group in both experiments 2 and 3. Diurnal changes in portal SCFA and cholesterol levels were studied in the experiment 3. SCFA concentrations increased rapidly after the start of feeding the SCFA diet, and changes in plasma cholesterol were the reciprocal of those observed in SCFA. These results show that a decrease in hepatic cholesterol synthesis rate mainly contributes to the lowering of plasma cholesterol in rats fed the SCFA mixture diet. Changes in portal SCFA and cholesterol concentrations support this conclusion. In SBF-fed rats, SCFA produced by cecal fermentation are possibly involved in lowering plasma cholesterol levels by negating the counteractive induction of hepatic cholesterol synthesis caused by an increase in bile acid excretion.

Rats were fed cholesterol-free purified diets containing casein, rice (RP), potato (PP) or soybean (SP) proteins having different amounts of methionine (25.9, 21.3, 16.2 and 10.9 g methionine/kg, respectively). Each protein was fed at 250 g/kg diet for 14 d. Growth rates of rats were the same in all groups. Serum total cholesterol concentrations were lower in rats fed SP, PP and RP than in those fed casein. Fecal bile acid plus neutral steroid excretion was significantly higher in rats fed the RP, PP and SP diets compared with those fed casein. There was a significant negative correlation between serum cholesterol concentration and fecal total steroid excretion (r = -0.490, P = 0.01). However, a stronger positive correlation was observed between serum cholesterol concentration and dietary methionine concentration (r = 0.674, P = 0.0003) or methionine:glycine ratios (r = 0.656, P = 0.0005). In a separate experiment in rats fed diets containing amino acid mixtures simulating the RP, PP and SP diets, serum total cholesterol concentrations were lower than in rats fed simulated casein. Fecal total steroid excretion was the same in all groups. A strong correlation was found between serum cholesterol concentration and dietary methionine concentration (r = 0.743, P = 0.0002) or the methionine:glycine ratio (r = 0.685, P = 0.0009) in rats fed the amino acid mixtures. Finally, we examined the hypocholesterolemic effects of 250 g SP or casein/kg diet with or without supplementation with 0.3 g/100 g sodium taurocholate (TC). Supplementation with TC did not alter the hypocholesterolemic effect of SP. These results support the view that RP, PP and SP lower serum cholesterol concentration in a similar manner.

The objective of this study was to examine the physiologic importance of undigested protein on cecal fermentation in rats fed a low (LAS) and high (HAS) amylose cornstarch. In Experiment 1, rats were fed diets containing LAS (655 g/kg diet) with one of four protein sources: casein, rice (RP), potato (PP) or soybean protein (SP) at 250 g/kg diet for 15 d. Apparent digestibilities of casein, RP, SP and PP were 96, 94, 93 and 92%, respectively. In rats fed the LAS diet with casein, acetate, propionate and succinate were the major cecal organic acids. The succinate pools in rats fed RP or SP were significantly lower than in those fed casein, whereas butyrate did not differ. Butyrate was significantly higher in rats fed PP, but succinate was the same as in rats fed casein. In Experiment 2, rats were fed diets containing HAS (200 g/kg diet) with one of the four protein sources at 250 g/kg diet for 10 d. HAS was substituted for the same amount of LAS. In rats fed the HAS diet, succinate was the major acid in rats fed casein; in rats fed RP or PP, however, the pools of this acid were significantly lower than in those fed casein, whereas butyrate was significantly higher in rats fed RP or PP. Fecal starch excretion was significantly lower in rats fed RP or PP than in those fed casein. In Experiment 3, rats were fed the casein-HAS diet with graded levels of PP (0, 10, 30, 50, 100 and 250 g/kg diet) for 14 d. The PP was substituted for the same amount of casein. Cecal butyrate was low in rats fed up to 100 g of PP/kg diet and then rose with 250 g of PP/kg diet. In Experiment 4, ileorectostomized rats were used and fed the same diets described in Experiment 3 for 9 d. The ileal starch/nitrogen ratio declined with increasing dietary PP, due solely to greater nitrogen excretion, whereas starch excretion was unaffected. In Experiment 5, rats were fed the casein-HAS diet with or without 60 g of artificial resistant protein/kg diet for 10 d. The resistant protein (apparent digestibility, 63%) was substituted for the same amount of casein. Rats fed the casein-HAS diet with resistant protein had significantly greater cecal butyrate and lower succinate than those fed the casein-HAS diet. These data show that large bowel fermentation of starch is altered by dietary protein. They support the hypothesis that nondigested protein, namely, resistant protein, may control fermentation efficiency as well as the fermentation profile of HAS, possibly as a result of a change in microflora through the change in the ratio of starch to nitrogen in the cecum.

The aim of this study was to clarify the effect of dietary indigestible components on small intestinal mucin secretion. We prepared polystyrene foam (PSF) with different expansion ratios (PSF-30, -60 and -90) in which powders had different settling volumes in water (SV). Rats were fed a purified diet containing 0, 10, 30, or 90 g of PSF-60/kg for 10 d. After 8 h of food deprivation, rats were refed 3 g of their respective diets within 90 min. Small intestinal mucin fractions were prepared, and periodic acid/Schiff-reactive substances and O-linked oligosaccharide chains were determined as mucin markers. Feeding of PSF-60 increased the small intestinal mucin secretion dose dependently (control vs. 30 or 60 g of PSF-60/kg, P < 0.05). When rats were fed either purified diet or diets containing PSF-30, 60, or 90 at 10 g/kg for 7 d, small intestinal mucins were greatly affected by the SV of the respective PSF tested. Rats fed the diet containing PSF-90 with the highest SV had the highest amount of mucins (vs. control, P < 0.05). In some natural dietary fibers, the small intestinal mucins and SV were correlated (r = 0.967, P = 0.002). Finally, rats were fed a purified diet or that diet containing 50 g of PSF-60/kg for 7 d. Then, each dietary group was further divided into 2 groups. After 8 h of food deprivation, rats were refed 3 g of purified or PSF diet. Greater mucins in the small intestine were manifest only in rats previously fed the PSF diet whether they were refed purified or PSF diet (control vs. PSF, P < 0.05). These results suggest that the small intestinal mucins are secreted in proportion to the SV of dietary indigestible components, and chronic ingestion of indigestible components is required for the appearance of enhanced mucin secretion.

Production of RPIWe developed a method for mass production of rice protein isolate (RPI) and evaluated its nutritional quality in rats. To obtain thick slurry, rice flour was mixed with a 0.6% Termamyl 12OL-solution (1:2, w/v) at room temperature (-23°C). The slurry was heated at 97°C for 2 hr with stirring. Gelatinization and liquefaction occurred simultaneously. RPI obtained by filtration and washing with boiling water, was more than 90% pure protein (dry matter basis). It also contained 6.4% dietary fiber, 1.3% ash, and 1.1% carbohydrate. RPI diets (40-50%) allowed the maximum growth in rats comparable to that with 25% casein diet.The general scheme for preparation of RPI is displayed in Fig. 1. To obtain thick slurry, 5 kg of rice flour were mixed with 1OL of distilled water containing 0.6% Termamyl 120L at room temperature (~23°C) and the mixture was vigorously stirred with a motor-driven propeller shaft until the rice flour completely dispersed. Then the mixture was indirectly heated in a boiling water bath and maintained at 97°C for 2 hr (usually reached 97°C within 45 min). Gelatinization and digestion of starch simultaneously occurred. After digestion, the mixture was vacuum filtered through cheesecloth. The precipitate was washed three times with 5L of boiling water to remove sugar syrup and remaining enzyme. The precipitate was washed three times with 5L ethanol (99%) and dried in air at room temperature (=23"C). The preparation was labelled RPI.