Oat flakes, seeds and bran in spoons
Swapping out refined grains for whole grains does not cause significant shifts in our gut microbes or better insulin resistance – but does result in weight loss, lower food intake, and reduced systemic inflammation, say researchers.
The findings come from a randomised crossover trial of 50 people in Denmark, which the team believes is the most comprehensive study to date of its kind, to study the effect of exchanging refined grain products in the diet – like white bread and pasta – with whole grain varieties.
Writing in the journal Gut, the team behind the trial reported that when eating the whole grain diet, participants lost weight, while blood tests showed participants had less inflammation in their bodies when eating whole grains. Furthermore, the Danish team revealed that people switched to a whole grain diet ate less food in general. The team suggested this may be due to an increase in feelings of fullness (satiety).
“Our analysis confirmed that there is a sound scientific basis for the dietary recommendation to eat whole grains. This may particularly apply to people, who are at increased risk of developing cardiovascular disease or type 2 diabetes,” commented senior author Professor Tine Rask Licht of the National Food Institute of Denmark.
“A good idea for future studies would be to examine the effect of various grain types,” she added.
Exploring the mechanisms: No significant changes in gut bacteria?
A diet rich in whole grains had previously been suggested to drive beneficial effects on health through alterations to the bacteria in our gut (the microbiota). Whole grains have also been linked to lower heart disease and diabetes risk, which may be driven by alterations to insulin sensitivity.
In the new study, the researchers used DNA sequencing to analyse stool samples from the participants in order to examine whether the different diet types affected gut bacteria composition.
Overall, this analysis did not shown major effects of the dietary grain products on the composition of the gut bacteria.
“However, even though the analysis did not reveal significant changes in the average gut microbiota after whole grain consumption, it may well be that the individual composition of our gut microbes has an impact on the individual reaction of our body to dietary whole grains, given that our bacteria help us digest the fibres in the whole grains,” added Rask Licht.
“This is something that further studies of our data may answer.”
Furthermore, the Danish team revealed that insulin sensitivity was not affected by the switch from refined grains to whole grains.
“Compared with refined grain, whole grain did not significantly alter glucose homeostasis and did not induce major changes in the faecal microbiome. Also, breath hydrogen levels, plasma short-chain fatty acids, intestinal integrity and intestinal transit time were not affected,” wrote the authors – adding that the weight loss seen in the study was ‘consistent with a reduction in energy intake’.
Rask Licht and her colleagues concluded that when compared to a diet containing refined grains, a whole grain diet reduced energy intake and body weight and the low-grade systemic inflammation markers CRP and IL-6, without significantly altering whole body insulin sensitivity, gut microbiome or gut functionality in terms of intestinal integrity and transit time.
“Thus, in contrast to our hypothesis, the health benefits of this specific diet rich in whole grains appeared to be independent of changes in the gut microbiome composition.”
Published online ahead of print, doi: 10.1136/gutjnl-2017-314786
“Whole grain-rich diet reduces body weight and systemic low-grade inflammation without inducing major changes of the gut microbiome: a randomised cross-over trial”
Authors: Henrik Munch Roager, et al