Studies Suggest Whole Grains Benefit Metabolism and Improve Gut Microbiota
09 Feb 2017 --- Brand new clinical trials have shown that adults who consume a diet rich in whole grains as opposed to refined grains, show modest improvements in healthy gut microbiota and certain immune responses. Additional research conducted in tandem also found that substituting whole grains for refined grains reduces calories retained during digestion and speeds up metabolism.
Both studies are published online in the American Journal of Clinical Nutrition.
Studies comparing the effects of whole grains versus refined grains consumption have not controlled the diets of study participants - until now.
The study looking into whole grains effect on gut microbiota was an eight-week randomized, controlled trial with 81 participants. The aim was to see what effect a diet rich in whole grains, as opposed to a diet rich in refined grains, would have on immune and inflammatory responses, gut microbiota, and stool frequency in healthy adults.
Whole grain consumption has been associated with reduced risk of heart disease, type 2 diabetes, and some cancers, and previous research has speculated that whole grains lessen the risk for diseases through reducing inflammation.
For the first two weeks, participants consumed the same weight-maintaining Western-style diet rich in refined grains. For the next six weeks, 40 of those participants stayed on that diet, while 41 participants consumed a diet rich in whole grains.
The diets were similar in total energy, total fat, and number of fruit, vegetable, and protein servings. The only difference was in the source of grain.
Based on how refined grains are processed, those who consumed the refined-grained diet inherently consumed less fiber and certain micronutrients.
To discover how whole-grain diet influences gut microbiota, the bacterial content and concentration of short-chain fatty acids in the stool was measured.
Whole grains have been previously shown to increase the variety and wealth of gut microbiota and increase production of short-chain fatty acids, a crucial source of energy for cells found in the colon.
Both gut microbiota and short-chain fatty acids are considered vital contributors to healthy immune and inflammatory functions.
Those who ate the whole-grain diet had an increase in Lachnospira, the bacteria that produces short-chain fatty acids.
The authors speculate this increase to be a result of a more favorable stool pH that occurs from consuming a diet rich in whole grains. Additionally, it was found that those who ate whole grains had a decrease in the pro-inflammatory bacteria, Enterbacteriaceae.
The authors speculate this decrease in bacteria that causes inflammation to be due to the higher concentration of acetate in the stool samples of those who ate the diet with whole grains.
“The strength of the study is that we found modest effects of whole grain on gut microbiota and measures of immune function in the context of a controlled energy and macronutrient diet where all food was provided to participants, allowing them to maintain their body weight constant, thus eliminating the confounding effect of weight loss associated with increasing fiber consumption on immune and inflammatory markers,” said corresponding and senior author Simin Nikbin Meydani, D.V.M., Ph.D., director of the nutritional immunology laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston (HNRCA).
“Additionally, our study incorporated markers of diet adherence and whole grain consumption, allowing us to more confidently determine the effect whole grains have on the gut microbiota and inflammatory responses.”
Whole Grains and Metabolism
A further new study provided food to participants for eight weeks, in at attempt to understand how whole grain consumption is beneficial for weight management.
“Many previous studies have suggested benefits of whole grains and dietary fiber on chronic disease risk,” says Phil J. Karl, PhD, first author of the study, alumnus of the Friedman School of Nutrition Science and Policy at Tufts, and current nutrition scientist at the U.S. Army Research Institute of Environmental Medicine in Natick, MA.
“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.”
There has been controversy, however, about whether whole grains and fiber are beneficial for weight regulation, partially because there hasn't been data from controlled metabolic studies.
The study showed that People who ate a diet with whole grains, which matched the recommended dietary allowance (RDA) for fiber, 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. 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,” said senior author Susan B. Roberts, Ph.D., senior scientist and director of the Energy Metabolism Laboratory at USDA HNRCA.
The research team 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 of the study all participants ate the same type of food, and individual calorie needs were determined.
After two weeks, participants were randomly assigned to eat a diet that included either whole grains or refined grains.
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.
Researchers 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.
They found that the extra fecal energy losses were not due to the extra fiber itself (which was accounted for in calculations) but from the effect the fiber had on the digestibility of other food calories. However, the researchers 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.
