A recent study from Spain [Br J Nutr. 2009 May 18:1-7. (Epub ahead of print)] looked at the effects of a gluten-free diet on the microbiota (microorganisms) that live in the gut. Both helpful and harmful microorganisms live in the gut and diet is known to influence the overall composition. According to study researchers, the purpose of this study was to “analyze the impact of a gluten-free diet on the composition and immune function of the microbiota in healthy subjects to gain further insights on interactions between diet and gut microbes.”

Ten people without celiac disease or history of digestive disease participated in the study. They followed a gluten-free diet for one month by replacing the gluten-containing foods they usually ate with certified gluten-free foods (no more than 20 parts per million of gluten). Three-day food records were kept before the start of the study and again after one month. Food records were analyzed for calories and macronutrients. Gut microbiota were assessed through fecal samples collected before and after following a gluten-free diet for 1 month.

Researchers found that polysaccharide (complex carbohydrate) intake was significantly reduced while study subjects were following a gluten-free diet as compared to a gluten-containing diet. Polysaccharide intake decreased from an average intake of 117 grams while study subjects were following a normal diet to 63 grams when they were following a gluten-free diet. They also found significant differences in the composition of fecal microorganisms before and after following a gluten-free diet. In short, there was a reduction in certain helpful bacteria and an increase in certain potential pathogens. Researchers speculated that this change in composition in fecal microorganisms might be due to decreased intake of polysaccharides.

According to study researchers “these findings indicate that this dietary therapy may contribute to reduce beneficial bacteria group counts and increase enterobacterial counts, which are microbial features associated with the active phase of CD and therefore it would not favor completely the normalization of the gut ecosystem in treated CD patients.”

Dr. Yolanda Sanz, head of the research study, graciously agreed to answer some questions for us about the study.

Can you explain why a diet low in polysaccharides or complex carbohydrates might cause a decrease in good gut bacteria and an increase in potential gut pathogens?

Complex carbohydrates (polysaccharides) are only partially digested or not digested at all in the upper part of the human gastrointestinal tract, reaching the distal part of the intestine and constituting the main energy source for beneficial bacteria, such as bifidobacteria. The genome of these bacteria encodes many enzymes specialized in the utilization of non-digestible carbohydrates, which provide these bacterial groups a competitive advantage over potentially pathogenic bacteria to colonize the intestine. However, when the growth of beneficial bacteria is not supported due to a reduction in the supply of their main energy source other bacterial groups, which can be opportunistic pathogens, can overgrow leading to imbalances in the gut microbiota composition.

If persons with celiac disease received dietary counseling to increase their intake of gluten-free whole grains (e.g., quinoa, brown rice, buckwheat, etc) and products made from them might this result in gut microbiota returning to normal?

Clinical studies carried out in humans suggest that increases in the intake of complex carbohydrates, for instance by the intake of whole grains, lead to changes in numbers of some commensal bacterial groups and may increase the numbers of bifidobacteria in the gut. Therefore, in principal the answer to this question is yes. However, specific studies should be done to prove this hypothesis since we know that celiac disease is associated with alterations in the intestinal microbiota composition, which do not only depend on the diet or the phase of the disease (active or non-active under a gluten-free diet).

Are there any plans to conduct a larger study looking at different types of gluten-free diets (those based more on starch and refined grain versus those based on whole grains)?

Definitively yes, we are involved in other studies that will provide further insights into the effects of the gluten-free diet on the gut microbiota and its functions.

Thank you so much Dr. Sanz!

To read the study abstract please go to:

Copyright © by Tricia Thompson, MS, RD

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Gluten-Free Diet & Gut Microorganisms