Whole-Grain Diet Increases Metabolism and Digestive Calorie Losses

Fiber has a positive effect on energy flux in the body, according to a new study.

“The combination of increased metabolism and increased fecal energy losses on the higher fiber diet resulted in a whopping 90 calories less available to the body,” said Susan B. Roberts, PhD, senior scientist and director of the Energy Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston. “That is an enormous amount, which would really add up over time.”
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Researchers conducted a randomized, controlled, parallel-arm, controlled-feeding trial with 81 men and postmenopausal women (49 men/32 woman, 40-65 years, BMI <35.0 kg/m²). Following a 2-week run-in, they randomly assigned participants to consume 1 of 2 weight-maintenance diets for 6 weeks. Diets differed in whole grain and fiber contents (whole grains: 207±39 g whole grains/day, 40±5 g dietary fiber/day vs refined grains: 0 g whole grains/day, 21±3 g dietary fiber/day), but otherwise remained similar. The investigators measured energy metabolism and body composition parameters, appetite, markers of glycemic control, and gut microbiota at 2 and 8 weeks.

By design, subjects maintained body weight in both groups. Plasma alkylresorcinols, biomarkers of whole-grain intake, increased in the whole grain-rich diet group (WG) but not the refined grain-based diet group (RG). Changes in RMR (resting metabolic rate), stool weight, and stool energy content, but not stool energy density, were higher in WG. When combined, the favorable energetic effects of WG translated into a 92 kcal/day higher net daily energy loss compared to RG. Prospective consumption and glycemia following an oral glucose tolerance test trended toward being lower in WG compared to RG. When they excluded nonadherent participants, between-group differences in stool energy content and glucose tolerance increased, and between-group differences in RMR and prospective consumption weren’t statistically significant.

“The magnitude of the fiber effect was very substantial,” Dr Roberts said. “I was impressed and delighted. It is also worth noting that we were not really overloading the fiber—this was just comparing a typical American (low-fiber) diet with one that had recommended amounts of fiber from whole grains.”

Current dietary guidelines recommend people having at least half their grains as whole grains, she said. “I think this research suggests that a 100% whole grain menu would be very healthy and helpful for weight management.”

—Mike Bederka

Reference:

Roberts SB, Karl JP, Meydani M, et al. Substituting whole grains for refined grains in 6-week randomized trial favorably affects energy balance parameters in healthy men and post-menopausal women [published online February 8, 2017]. Am J Clin Nutr. doi:10.3945/ajcn.116.139683.