Rice bran oil found to lower cholesterol

Nutrition Research Newsletter, Feb, 2005

Rice bran, a coproduct of milled rice, and its oil may have cardiovascular health benefits. Human consumption of rice bran has been limited due to the rapid onset of rancidity in rice bran. However, methods now exist to stabilize rice bran and to extract its oil. Interest in rice bran came about because of the discovery that the inclusion of oat bran in the diet lowers serum cholesterol. Studies of rice bran supplementation in humans have found similar beneficial effects on lipoproteins.

Rice bran contains 10% to 23% oil and (unlike oat bran) negligible amounts of water-soluble alpha-glucans and larger amounts of insoluble dietary fiber. Because of these differences, it is thought that rice bran lowers cholesterol by a mechanism different from that of oat bran. However, rice bran oil typically contains 20% saturated fatty acids and approximately equal amounts of oleic and linoleic fatty acids. Research now suggests that cholesterol-lowering properties office bran oil are explained by in unsaponifiable components more than by its fatty acid composition.

To further examine the cholesterol-lowering impact of rice bran's fiber and oil apart from its fatty acid composition, investigators undertook two well-controlled feeding studies designed to evaluate the effects of using defatted rice bran and rice bran oil in an average American diet on cardiovascular disease risk factors in men and women. For the study 1, 27 healthy men and women were recruited in two cohorts. Study two included 14 healthy men and premenopausal women. All the subjects were between the ages of 18 and 50 years and had total serum cholesterol concentrations between the 25th and 90th percentiles and triacylglycerol concentrations < 90th percentile.

In study one, all subjects began with consumption of a run-in diet for 3 weeks and then were randomly assigned to either the control or intervention diet for an additional 5 weeks. Assessment of outcome measures occurred at the end of the run-in period and at the end of the study. The study 1 diets were a low-fiber control diet (control 1) and a high-fiber intervention diet containing defatted rice bran (DRB). The control 1 diet provided 13-22 grams dietary fiber per day, varying with total energy, whereas the addition of DRB (56-94 grams/day, varying with total energy level) to the intervention diet doubled the fiber content. Both diets proved 37% of total energy as fat.

Study 2 used a randomized, double-blind, crossover design with two 5-week diet periods. Assessment of outcome measures occurred at the end of each diet period. To determine whether unsaponifiable components present in rice bran oil (RBO) affect lipid metabolism, the fatty acid composition of RBO was matched with that of an oil blend that was used in the control diet. The fatty acid profile of the RBO that was obtained for the second feeding study was determined. Then other oils were combined, chemically analyzed, and adjusted until the best match of the RBO's fatty acid profile was achieved. The oil blend was composed of peanut oil, olive oil, corn oil, canola oil, palm oil, and butter. Both diets were designed to provide 37% of total energy as fat. For the study 2 control diet (control 2), one-third of the total dietary fat was in the form of the oil blend, and for the RBO intervention diet, the oil blend was replaced with RBO.

For both studies, the subjects were provided with all foods for the duration of the study. Lipids were measured following intervention, in both studies.

Defatted rice bran did not lower lipid concentrations. In study 2, total cholesterol was significantly lower with consumption of the diet containing rice bran oil than with consumption of the control diet. Furthermore, with consumption of the rice bran oil diet, LDL cholesterol decreased by 7% (P < 0.0004), whereas HDL cholesterol was unchanged.

Rice bran oil, not fiber, lowers cholesterol in healthy, moderately hypercholesterolemic individuals. Because there were no substantial differences in the fatty acid composition of the diets, the reduction of cholesterol seen was due to other component present in the rice bran oil, likely the unsaponifiable compounds.

M. Most, R. Tulley, S. Morales, et al. Rice bran oil, not fiber, lower cholesterol in humans. Am J Clin Nutr;81:64-68 (January, 2005). [Correspondence: MM Most, Pennington Biomedical Research Center, 6400 Perkins Road, Baton Rouge, LA 70808. E-mail: mostmm@pbrc.edu].