Consuming a diet rich in whole grains rather than refined grains has been shown to bring modest improvements to the gut microbiota and to reduce inflammation — and could potentially be a management or preventative tool for certain diseases like inflammatory bowel disease (IBD).
Researchers at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston showed that adults who consumed whole grains also reduced their risk of heart disease, type 2 diabetes, and certain types of cancer, as well as improving some immune responses.
The study, titled “Substituting whole grains for refined grains in a 6-wk randomized trial has a modest effect on gut microbiota and immune and inflammatory markers of healthy adults,” was published in the American Journal of Clinical Nutrition.
Grains mostly include wheat, rice, oats and barley. Whole grains include the outer nutritious layer of grains and are found in whole-wheat flour, oatmeal and brown rice.
Refined grains, on the other hand, are starches that have been processed and broken down, mainly to increase shelf life, through a process known as milling.
Milling drains the starch of dietary fiber, iron and B-vitamins, although artificially enriching the grains can add iron and forms of B vitamins, but not the fiber lost in milling. Refined grains include white flour, white bread and white rice, among others.
The 2015-2020 Dietary Guidelines for Americans from the U.S. Department of Health and Human Services recommends that people replace refined grains with whole grains for a daily minimum of three ounces of whole grains for women and four ounces for men. This translates, for example, to 1½ to 2 cups of brown rice or oatmeal a day, for example.
For this study, the team from Tufts evaluated the results of an eight-week clinical trial with 81 healthy participants. The trial aimed to investigate the effects of a whole grain-rich diet, compared to refined grains, on immune and inflammatory responses, gut microbiota and stool frequency.
During the first two weeks, participants were given the same diet, rich in refined grains, they needed to maintain their weight. For the following six weeks, 40 trial participants maintained that diet while 41 switched to a whole grain-rich diet.
Both groups received the same levels of energy, including fat, and the number of fruit, vegetable and protein servings. The only difference was the source of grain. All meals were prepared according to the new Dietary Guidelines for Americans and were designed in a way that would maintain the weight of trial participants.
Regarding gut microbiota, a key element of a healthy immune and inflammatory function, researchers found that those who ate the whole-grain diet had an increase in Lachnospira, a bacteria that produces short-chain fatty acids.
This increase could be a result of a more favorable stool pH, a result of the whole-grain diet. Whole grains were also associated with a decrease in the pro-inflammatory bacteria Enterbacteriaceae.
Regarding immune response, eating whole grains resulted in an increased level of memory T-cells. There was no difference in the level of inflammatory cytokines, which are involved in the development of IBDs.
The authors caution that results were modest, and further research would be important to help identify the role of whole grains on gut microbiota and immune response. One of this study’s limitations was that all participants were healthy and so it wasn’t possible to generalize the results to people with compromised immune systems.
“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,” Simin Nikbin Meydani, DVM, PhD, senior author of the study and director of the immunology laboratory conducting the study at Tufts University, said in a press release.
“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,” Meydani added.