'Low Gluten' Benefits Linked to Dietary Fibers, Not Gluten

Janis C. Kelly

December 04, 2018

Healthy adults had less bloating, increased well-being, and a small but significant weight loss on low-gluten (LG) compared with high-gluten (HG) diets, a study found. The differences appear linked to differences in dietary fiber associated with components of the two diets, not to gluten per se, the researchers report in an article published online November 13 in Nature Communications.

"[T]he changes in colonic microbial composition and fermentation suggest that the effects of a low-gluten diet in healthy middle-aged adults may to some extent be driven by qualitative changes in dietary fibres upon reduction of gluten-rich food items rather than by the reduction of gluten intake itself," Oluf Pedersen, MD, Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen, Denmark, and colleagues write.

In addition to gluten levels, the dietary change was from HG dietary fibers composed of xylose and arabinose in wheat and rye to LG dietary fibers composed of galactose and mannose from a number of vegetables, berries, and nonwheat and nonrye cereals such as oats.

Pedersen told Medscape Medical News, "Our study is a wake-up call to the food industry. Gluten-free may not necessarily be the healthy choice many people think it is. Most gluten-free food items available today are massively deprived of dietary fibers and natural nutritional ingredients. Therefore, there is an obvious need for availability of fiber-enriched, nutritionally high-quality gluten-free food items which are fresh or minimally processed. Such initiatives may turn out to be key for alleviating gastrointestinal discomfort and help facilitate weight control in the general population via modification of the gut microbiota."

"Also our study outcome is a major step forward in identifying novel prebiotics (dietary fibers composed of mannose and galactose) to boost a gut microbiota causing less bloating and a modest weight loss," he added.

The research team conducted an open-label, randomized, controlled, cross-over trial in 60 healthy middle-aged adults to compare the effects of 8 weeks on a low-gluten diet (LG: 2g gluten/day) with 8 weeks on a high gluten diet (HG: 18 g gluten/day), with a 6-week washout period between. Usual gluten intake in these subjects was 12 g gluten/day.

Participants replaced all cereal products with provided LG (derived mainly from oats) or HG (derived mainly from wheat and rye) products that had similar levels of dietary fiber and nutritional quality. Compliance was high, as assessed by food diaries and fasting plasma alkylresorcinol concentrations.

The primary endpoint was changes in gut microbiota composition and function during LG compared with HG consumption, assessed using shotgun sequencing-based metagenomics analyses of microbial DNA from fecal samples. Secondary outcomes included body weight and a broad range of metabolic and immune markers as well as patient-reported well-being and bloating.

In healthy adults, the LG diet was associated with changes in the intestinal microbiome, reduced hydrogen exhalation, improved self-reported bloating, and selectively reduced activation of the inflammasome response. Pedersen said that the researchers are confident that the microbiome changes observed are sufficient to explain the reduction in bloating because there was no evidence of leaky gut mucosa or changes in fecal transit time.

The LG diet altered intestinal fermentation, reducing both fasting and postprandial hydrogen exhalation. The authors suggest that this fermentation change was due to differences in carbohydrate composition: the LG diet had higher levels of galactose, rhamnose, mannose, and galacturonic acid and lower levels of arabinose and xylose than the HG diet.

Urinary metabolite analysis showed that the LG diet was associated with increased levels of a host-microbial metabolite of lignan degradation, which suggested altered dietary fiber degradation related to changes in the gut microbiome. Network analysis of breath hydrogen and the bacterial species and urine metabolites identified changes in 14 species of gut bacteria. This included a reduction in Bifidobacterium species associated with diminished intake of wheat and is considered to play a key role in the altered gut fermentation.

The researchers note that other studies have reported that 4 weeks of a gluten-free diet resulted in decreased Bifidobacterium populations in healthy adults and in celiac disease patients.

Pedersen explained, "It is known from previous studies that no-wheat eaters, eg, native people living a traditional lifestyle (Hadza in Africa), patients with celiac disease after switch to gluten-free dieting, and people sticking to a FODMAP diet (fermentable oligosaccharides, disaccharides, monosaccharides and polyols) are without or depleted in bifidobacteria in their gut microbiota. It is hypothesized that presence of anti-inflammatory gut bifidobacteria in most adults living a Western lifestyle is a compensatory mechanism to compensate for the immunogenic effects of various poorly digestible wheat peptides."

The LG diet also was associated with a small but statistically significant weight loss of 0.8 ± 0.3 kg. This appeared to be a result not of changes in appetite or metabolic signaling but of enhanced thermogenesis or fat oxidation, likely related to an altered microbiota tryptophan degradation pathway.

The authors warn that it is impossible to blind a study of this type; therefore, they were unable to determine the extent to which improvements in well-being and bloating following the LG intervention resulted from changes in the intestinal microbiome and fermentation rather than from placebo effects.

Clinical implications for patients with stomach complaints but not celiac disease are uncertain, but Pedersen said that if intestinal complaints persist in normal adults who have been carefully tested for known gastrointestinal disorders, it might be reasonable to consider trying an LG diet with 2 months of observation. He added, "In healthy adults there is no scientific evidence to skip gluten 100%."

The authors have disclosed no relevant financial relationships.

Nat Commun. Published online November 13, 2018. Full text

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