Dietary fibre is a key nutrient for optimizing gut health through fermentation by commensal microbiota in the colon. The vast majority of studies that support the health benefits of dietary fibre are observational, whereas little evidence from interventional studies is available and mechanisms involved in their effects on gut microbiota and immune response are poorly understood.
A recent study, led by Dr. Simin Nikbin Meydani from the Jean Mayer USDA Human Nutrition Research Centre on Aging at Tufts University in Boston (USA), has found that consuming a diet rich in whole grains may lead to modest improvements in gut microbiota and immune response in healthy adults.
For the first 2 weeks (the weight maintenance run-in period), 81 metabolically healthy adults (49 men and 32 postmenopausal women, age range: 40-65 years, body mass index less than 35 kg/m2) consumed the same weight-maintaining Western-style diet high in refined grains (with high consumption of saturated fats, red meats, simple carbohydrates, and processed or refined foods and low consumption of fresh fruit and vegetables, whole grains, seafood, and poultry). For the next 6 weeks, 40 of those participants stayed on that diet (refined grains group: 8g fibre/1000 kcal), whereas 41 participants switched to a diet rich in whole grains but matched in all other ways (whole grain group: 16 g fibre/1000 kcal, which met the recommended dietary guidelines for Americans of 35 g/day). It is noteworthy to highlight that all meals were provided to the volunteers by trained staff, which aims to improve compliance and, thus, diet is not a factor of variability.
The researchers assessed the effects of the diet rich in whole grains compared with refined grains on immune and inflammatory responses: systemic and stool inflammatory cytokine concentrations and plasma lipopolysaccharide-binding protein concentrations, phenotypic and functional immune variables, and gut microbiota and microbial products. Stool, 12-h fasting blood, and saliva samples were collected, and delayed-type hypersensibility (DTH) tests were conducted at or near the end of week 2 of the run-in period and at or near the end of week 8 of the study.
Participants who consumed the whole-grain diet had increased stool weight and frequency and a modest increase in the short-chain fatty acid (SCFA) producer Lachnospira, together with decreased pro-inflammatory Enterobacteriaceae, which was associated with a higher concentration of acetate and butyrate in the stool samples. Increased SCFA production after whole grain consumption was suggested to be related to a decrease in stool pH, as lower pH favours the production of SCFAs. Adherence to the assigned diets was assessed by measuring plasma concentrations of alkylresorcinol, a biomarker of whole grain intake. The whole grain group had significantly increased plasma alkylresorcinol compared with refined grain group.
Besides this, eating whole grains resulted in a modest increase in levels of memory T cells and tumour necrosis factor-alpha production by lipopolysaccharide stimulated immune cells. There was no change in the levels of other markers of cell-mediated immunity and inflammatory cytokines.
“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 fibre 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 Dr. Simin Nikbin Meydani in a Tufts news release.
To sum up, 6 weeks of whole grain consumption over refined grain consumption resulted in a modest effect on gut microbiota composition, stool short-chain fatty acid concentrations, and certain indicators of the immune response in healthy adults. Further long-term interventional studies in humans are needed in order to explore the role of the gut microbiota in the health benefits of whole grains.
Vanegas SM, Meydani M, Barnett JB, et al. 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. Am J Clin Nutr. 2017; doi: 10.3945/ajcn.116.146928.