The research, reported in this month's issue of Food and Chemical Toxicology, shows that total cholesterol levels in animals dropped by 42 per cent, and LDL or 'bad cholesterol' levels dropped up to 62 per cent after the rats' diets were supplemented with tocotrienol rich fraction (TRF) isolated from rice bran oil.
The findings support increasing evidence of the benefits of tocotrienols, a form of the vitamin that has been less widely researched than the more well-known tocopherol form. While corn, wheat and soybeans are rich in tocopherols, tocotrienols are found in greater quantities in barley, oats, palm and rice bran.
Researchers writing in the American Journal of Clinical Nutrition earlier this year found that rice bran oil lowered LDL levels in humans by 7 per cent, although they did not identify the compounds present in the oil that were responsible for this effect.
Dr Mohammad Minhajuddin and colleagues from the University of Rochester say that TRF inhibits the activity of HMG-CoA reductase, an enzyme involved in cholesterol biosynthesis.
They found the most effective dose in rats was 8 IU kg per day. Extrapolated to humans, a person with an average body weight of 154 pounds would get around 560 IU, which is close to the 400 IU of vitamin E normally taken, they say.
Rice bran oil, or other sources of tocotrienols, could be of interest to food makers looking to enter the growing category of cholesterol-lowering foods, currently dominated by products containing plant sterols.
Cholesterol is a major risk factor for heart disease, the disease that kills more people than any other around the world.
So far, scientists have not found any adverse effects of tocotrienols, says Minhajuddin, a research associate in the Department of Pediatrics.
Minhajuddin also has preliminary, unpublished data from a study he conducted in India, showing that TRF reduces cholesterol in humans as well as in animals.
Five healthy volunteers with total cholesterol levels in the 'normal' range of 170-230 mg/dL, who ingested TRF in capsule form at a dose of 8 IU kg/day for four weeks, saw their cholesterol levels drop by 10 per cent with a 26-per cent decline in LDL-cholesterol levels.