Senior study author Simin Nikbin Meydani, Ph.D., of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston, MA, and colleagues report their findings in the American Journal of Clinical Nutrition.
There are three components that make up a grain - the bran, germ, and endosperm. A whole grain contains all three components, whereas a refined grain is processed so that the bran and germ are removed.
Whole wheat, oats, rye, barley, brown rice, and quinoa are all examples of whole-grain products, while refined-grain products include wheat flour, white rice, and enriched bread.
Whole grains are considered a key part of a healthful diet. According to the American Heart Association, they can help to improve cholesterol levels, as well as reduce the risk of obesity, heart disease, stroke, and type 2 diabetes.
Previous studies have suggested that whole grains reduce inflammation to produce such benefits. Meydani and colleagues set out to investigate this association further with their new study, noting that research comparing the effects of whole grains and refined grains on immune responses and inflammation has not controlled the diets of participants.
Whole-grain vs. refined-grain diet
The 8-week study involved 81 healthy adults, all of whom consumed a Western-style diet high in refined grains for the first 2 weeks.
For the remaining 6 weeks, 40 of the study participants continued with the Western-style diet that was rich in refined grains, while the remaining 41 participants were placed on a Western-style diet that was rich in whole grains.
Importantly, the team notes that the total energy, total fat, and total servings of fruits, vegetables, and proteins were comparable in each diet, meaning that the only difference between the two diets was the type of grains consumed.
All meals were preprepared by trained staff in line with the Dietary Guidelines for Americans, and they were designed for weight maintenance. The team explains that this was because previous studies had shown that increasing whole grain intake caused subjects to lose weight, which made it hard to determine whether reduced inflammation was a result of whole-grain consumption or weight loss.
Subjects were required to complete a food checklist with each meal, enabling the researchers to determine how much food each participant ate.
Furthermore, each participant was asked to continue with their usual physical activity, record the occurrence and severity of gastrointestinal symptoms, and refrain from using any anti-inflammatory drugs (such as aspirin) in the 72 hours prior to blood samples being taken.
Whole-grain diet led to increase in memory T cells
To evaluate how each diet influenced the gut microbiota - the population of microorganisms in the intestines - the researchers analyzed participants' stool samples. Blood samples were also collected, which the team used to assess immune responses.
Compared with participants who consumed the diet rich in refined grains, those who consumed the diet rich in whole grains showed an increase in a type of bacteria called Lachnospira, which is known to produce short-chain fatty acids.
The team explains that short-chain fatty acids are important for a healthy immune system.
Furthermore, subjects who consumed the whole-grain diet showed a reduction Enterobacteriaceae - bacteria that trigger inflammation.
The researchers hypothesize that the decrease in Enteorbacteriaceae is down to the higher concentration of acetate identified in the stool samples of subjects who consumed the diet rich in whole grains.
On assessing the blood samples of both diet groups, the team found that subjects who consumed the whole-grain diet showed an increase in memory T cells - types of white blood cells that stave off infection.
When immune cells were stimulated with foreign compounds, however, participants who consumed the diet rich in refined grains showed a reduction in the production of TNF-alpha - a cell signaling protein involved in the first phase of an immune response.
Further studies needed
The researchers stress that the differences in gut microbiota and immune responses were modest, but that their findings shed some light on how whole grains influence inflammation.
"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," says Meydani.
"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."
The researchers say that future studies should incorporate more soluble whole-grain products, which may give a clearer picture of how whole grains impact gut microbiota and immune responses.