In a clinical trial, adults who consumed a diet rich in whole grains rather than refined grains had modest improvements in healthy gut microbiota and certain immune responses. The research was conducted in tandem with a study that looked at the effects of a whole–grain diet on energy metabolism.

Both studies were published online in the American Journal of Clinical Nutrition.

Whole grain consumption has been associated with reduced risk of heart disease, type 2 diabetes, and some cancers. Researchers have speculated that whole grains lessen risk for diseases through reducing inflammation, but studies comparing the effects of whole grains versus refined grains consumption have not controlled the diets of study participants and have not evaluated cell–mediated immune responses to uncover the impact of whole grains on immune and inflammatory responses.

The research team analyzed the results from an eight–week randomized, controlled trial with 81 participants 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. 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. The meals were prepared by trained staff according to the Dietary Guidelines for Americans. The meals were designed so that the participant would maintain their weight. In many of the previous studies, subjects lost weight when they increased their whole grain consumption so it was not clear if the anti–inflammatory effect observed was a secondary effect due to weight loss or consumption of fiber itself.

“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. 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,” said corresponding and senior author Simin Nikbin Meydani, DVM, PhD, director of the nutritional immunology laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston (HNRCA).