Despite the widely known health benefits of whole grains in the diet, the mechanisms responsible for these benefits have remained elusive. The gut microbiome has been of particular interest, but previous studies have been inconclusive when it comes to identifying a consistent signature of whole grains on the gut microbiota.
A new cross-over dietary intervention study has found that a whole grain diet had positive effects on body weight and inflammation—but perhaps surprisingly, that it did not impact gut microbiota in a substantial manner.
Study subjects participated in two 8-week dietary intervention periods: (1) the whole grain diet (179 plus or minus 50 g/day of whole grains), and (2) the refined grain diet (13 plus or minus 10 g/day), with a washout period of at least 6 weeks in between. 50 adults at risk of developing metabolic syndrome completed both of these interventions.
Although several studies to date have examined composition of the gut microbiota in the context of dietary whole grains, this comprehensive study reported on gut microbiota composition and function as well as colonic transit time, short-chain fatty acids, urine metabolomics, and intestinal permeability.
Compared to the refined grain diet, the whole grain diet decreased body weight and systemic low-grade inflammation (i.e. serum inflammatory markers, interleukin (IL)-6, and C-reactive protein)—but other measures, including gut microbiota composition and function, did not differ significantly between the two dietary interventions.
GMFH editors interviewed first author Dr. Henrik Munch Roager, of the National Food Institute, Technical University of Denmark, on the significance of these results and the direction of future research on whole grains, gut microbiota, and health.
What led you to undertake a dietary intervention study?
If you attend conferences these days, a lot of people will say that we need to do dietary interventions to find out how we can change the gut microbiome and to find the link between diet and the gut microbiome.
Everyone agrees that the diet has a huge influence on the gut microbiome and we can see that. But really understanding how we can change the gut microbiome by changing our diet—that is not really clear, not if you talk about small dietary changes.
There is really a need to do these studies in humans, although these studies are very expensive and complex.
Did you expect the results you got?
I think we would have expected that if you feed the bacteria ten times more whole grain that it should somehow be reflected in the gut microbiome. But that was not the case in this population. However, in line with epidemiologic studies, we did find that a whole grain diet in comparison to a refined grain diet reduced body weight and reduced levels of low grade systemic inflammation in the blood.
Do you think your study indicates that the gut microbiota is irrelevant to the health effects of whole grains?
Definitely not. I think this proves that, as we already know, that people’s gut microbiota is responding very individually to dietary changes, and that more and more focus should be on understanding personalized response – also to a diet like this.
It also shows that it’s a little naive to think we can find bacteria that respond in the same way in all individuals. For example, in this study where we had 50 completers, we did not find very strong changes in any species across all these 50 individuals.
Therefore, the next step for us is to go deeper in the data and see how each individual actually responded in terms of the gut microbiome: can we find some people that go in one direction, and other that go in another direction?
What’s the take-home message from your study?
It’s important to emphasize that, although we do not see changes in the gut microbiome that are alike in all individuals, we still see clear health benefits of changing to a whole grain diet in terms of body weight and reduced inflammation.
In the future, how do you think the gut microbiota will be relevant to dietary advice about whole grains?
I definitely think that in the future, dietary advice will include assessing the gut microbiome because I think the gut microbiome is playing a vital role in determining how you will respond to a given diet.
It’s clear that you do not find bacteria that respond the same way in all individuals. And that’s probably why we don’t see the ‘signal’ when we study the response globally in this cohort.
It might be that people responded but they responded differently.
Roager HM, Vogt JK, Kristensen M, et al. 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. Gut. 2017. doi: 10.1136/gutjnl-2017-314786.
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