A gluten-free diet is one of the most widely adopted special diets in the world. Gluten is a major component of wheat, barley and rye, and triggers the development of coeliac disease in genetically susceptible individuals. Coeliac disease is an autoimmune disorder, seen in about 1% of the population with European ancestry. Non-coeliac gluten sensitivity is a second common disorder linked to the consumption of gluten, which results in a range of symptoms of intestinal distress. Both of these disorders respond well to the life-long removal of gluten from the diet. Recently, a gluten-free diet has been proposed as a way to relieve symptoms for IBS sufferers (1).

Outside of these medical conditions the gluten-free diet has become the dieting fad de rigeur, so wildly popular that even our beloved pets can’t escape.

A number of small-scale studies (7-30 participants per group), have looked into the effect of a gluten-free diet on the gut microbiome of coeliac patients (2-4). The most consistent finding from theses studies was the difference in the abundance and diversity of Lactobacillus and Bifidobacterium in the treated and untreated coeliac disease patients.

In a paper published in the journal Genome Medicine (5), Bonder and colleagues study the effect of a one-month gluten-free diet on the gut microbiome of twenty-one healthy individuals (9 men, 12 women), aged between 16 and 61. The study only removed gluten from the diet. Individuals’ microbiomes were sampled prior to the removal of gluten (to establish the baseline gut microbiome), once per week for the four weeks of gluten removal and then once per week for an additional four weeks after gluten was reintroduced into the diet. In addition, the research team took blood samples at two-week intervals in order to monitor biomarkers related to gut health.

Although finding no significant change in gut health biomarkers, the team did find that eight bacteria were significantly changed in abundance on the gluten-free diet: Veillonellaceae, Ruminococcus bromii, and Roseburia faecis decreased on the gluten-free diet, and Victivallaceae, Clostridiaceae, ML615J-28, Slackia, and Coriobacteriaceae increased on the gluten-free diet. The largest effect was the decrease in Veillonellaceae which is known for lactate fermentation, and the first time this bacterial family has been linked to dietary intervention. Veillonellaceae has been shown to be decreased in autistic patients (6). Interestingly, in that study 9 of the 10 autistic patients were on a gluten-free diet; suggesting that the absence of gluten was the cause of the decrease and not autism. Furthermore the Veillonellaceae bacterial family is considered to be pro-inflammatory and an increase in its abundance has consistently been seen in inflammatory bowel disease (IBD), IBS and cirrhosis patients (7-9). It is entirely plausible that the decrease in Veillonellaceae might be one of the main reasons for the beneficial effects of the gluten-free diet in IBS.