A new study published in the American Journal of Clinical Nutrition suggests that substituting whole grains for refined grains increases calorie loss by reducing calories retained during digestion and speeding up metabolism.
J Philip Karl et al quantify benefits of a whole-grain diet. Image credit: Tofs.
Grains are a major food group that includes wheat, rice, oats and barley products.
Whole grains include the outer nutritious layer of grains and are found in products including whole-wheat flour, oatmeal and brown rice.
Refined grains are starches that have been processed and broken down into a finer texture, primarily to increase shelf life. This process, known as milling, drains the starch of dietary fiber, iron and many forms of the vitamin B.
Through an enrichment process, iron and B-vitamins can be added back to the refined grains, but the fiber generally is not. White flour, white bread and white rice are examples of refined grains.
Several studies have suggested health benefits of whole grains and high dietary fiber intake, including for glycemic control and insulin sensitivity.
There has been controversy, however, about whether whole grains and fiber are beneficial for weight regulation, partially because there hasnt been data from controlled metabolic studies.
The new study, led by Dr. Phil Karl of the U.S. Army Research Institute of Environmental Medicine and Tufts University, provided food to participants for eight weeks and may help explain how whole grain consumption is beneficial for weight management.
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, Dr. Karl explained.
People who ate a diet with whole grains lost close to an extra 100 calories per day due to a combination of increased resting metabolic rate and greater fecal losses. This is compared to people who ate refined grains without much fiber.
We provided all food to ensure that the composition of the diets differed only in grain source, said Dr. Susan Roberts, director of the Energy Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University and the senior author on the study.
The extra calories lost by those who ate whole grains was equivalent of a brisk 30 min walk or enjoying an extra small cookie every day in terms of its impact.
The researchers conducted an eight-week randomized single-blind comparative study with 81 men and women between the ages of 40 and 65.
In the first two weeks, all participants ate the same type of food, and individual calorie needs were determined.
After two weeks, the participants were randomly assigned to eat a diet that included either whole grains or refined grains.
The whole-grain diet and the refined-grain diet differed mostly in grain and fiber content the energy, macronutrient composition, type of food, and meal structure were similar.
The participants were asked to consume all the food provided and nothing else, return the food they had not eaten, and continue with their usual physical activity.
The purpose of this dietary control was to study the effect of whole grains compared to refined grains on resting metabolic rate and fecal energy losses, as well as feelings of hunger and fullness.
Throughout the eight weeks, the authors measured weight, metabolic rate, blood glucose, fecal calories, hunger and fullness.
At the end of the study, those who ate whole grains had an increase in resting metabolic rate and fecal energy losses compared to those who ate refined grains.
The extra fecal energy losses were not due to the extra fiber itself but from the effect the fiber had on the digestibility of other food calories.
The scientists caution that the effects of a whole-grain diet on resting metabolic rate were sensitive to dietary adherence, so cautious interpretation is warranted.
Based on previous research and current study measurements, however, they believe that the calorie loss was not due exclusively to the digestion of extra fiber intake.
They also note that commercially-available products using whole grain flour were used in the study and hypothesize that using foods with whole grain kernels might elicit a more pronounced benefit. Hunger, fullness and diet satisfaction were not statistically different between diets.
The 2015-2020 Dietary Guidelines for Americans from the United States Department of Health and Human Services and the USDA recommends that Americans replace refined grains with whole grains.
The recommended daily allowance of whole grains is a minimum of 3 ounces of whole grains for women and 4 ounces for men. This is the equivalent to consuming 1.5-2 cups of brown rice or oatmeal each day.
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J Philip Karl et al. Substituting whole grains for refined grains in a 6-wk randomized trial favorably affects energy-balance metrics in healthy men and postmenopausal women. American Journal of Clinical Nutrition, published online February 8, 2017; doi: 10.3945/ajcn.116.139683
This article is based on a press-release from Tufts University.
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Study Reveals Health Benefits of Whole-Grain Diet - Sci-News.com
