Previous research has indicated that whole grains and high dietary fiber intake have several health benefits, such as for glycemic control and insulin sensitivity. However, scientists have been unable to agree whether whole grains and fiber help to regulate weight.
The grain food group includes rice, oats, wheat, and barley. While whole grains contain the whole-grain kernel and include brown rice, oatmeal, and whole-wheat flour, refined grains are starches that are processed and milled to remove the bran and germ to prolong their shelf life. Examples of refined grains include white rice, white bread, and white flour.
Milling empties the starch of dietary fiber, iron, and B vitamins. Although iron and B vitamins can be added back into the refined grains, the fiber is not often reintroduced.
In the new research, investigators conducted a study over 8 weeks that included 81 men and women aged between 40 and 65. All food was provided to the participants over the course of the study and included either whole grains or refined grains. Participants were asked only to consume the food provided, return any uneaten food, and continue with their usual levels of physical activity.
"We provided all food to ensure that the composition of the diets differed only in grain source," says senior author Susan B. Roberts, Ph.D., senior scientist and director of the Energy Metabolism Laboratory at the USDA Human Nutrition Research Center on Aging.
For the first 2 weeks, all participants ate the same type of food, and the calorie needs of each individual were determined. The participants were then randomly assigned to either a group that included whole grains or a group with refined grains.
The differences between the whole-grain diet and refined-grain diet were mostly in grain and fiber content. Type of food, meal structure, and energy and macronutrient composition were similar in both groups.
The researchers compared the effects of whole grains and refined grains on resting metabolic rate and fecal energy losses, in addition to how full or how hungry the participants felt. Measures of the study included weight, metabolic rate, blood glucose, fecal calories, hunger, and fullness.
Eating fiber in whole grains increased calories lost per day
Results showed that the group that ate whole grains had increased resting metabolic rate and greater fecal losses compared with the refined grain group. Furthermore, the increases in fecal energy losses were not because of the extra fiber, but from the effect of the fiber on the digestibility of other food calories.
Participants who consumed whole grains - an amount that matched the recommended daily allowance for fiber - lost almost an extra 100 calories per day than the participants who consumed refined grains without much fiber.
"The extra calories lost by those who ate whole grains was equivalent of a brisk 30-minute walk - or enjoying an extra small cookie every day in terms of its impact," says Roberts.
Roberts' colleagues included Phil J. Karl, Ph.D., first author of the study, an alumnus of the Friedman School of Nutrition Science and Policy at Tufts, and a nutrition scientist at the United States Army Research Institute of Environmental Medicine in Natick, MA.
"Many previous studies have suggested benefits of whole grains and dietary fiber on chronic disease risk. This study helps to quantify how whole grains and fiber work to benefit weight management, and lend credibility to previously reported associations between increased whole grains and fiber consumption, lower body weight and better health."
Phil J. Karl
The study used commercially available products that used whole-grain flour. The team hypothesizes that using foods with whole-grain kernels might affect metabolic rate and fecal loss further. Fullness, hunger, and diet satisfaction did not appear to differ significantly between the two diets.