Introduction

Nowadays the use of a gluten-free diet (GFD) is erroneously considered a healthy life choice worldwide, despite evidence supports its beneficial effect only in the treatment of celiac disease and, possibly, gluten sensitivity. GFD can modify microbiota composition, decreasing specifically bacterial species involved in polysaccharides metabolism and short chain fatty acids (SCFAs) production. In fact, a GFD usually show a lower carbohydrate composition with an increase of dietary fats, and a fiber intake significantly below the recommended range. In this study, we investigated whether the amount of daily intake complex carbohydrates during GFD was associated with the reduction of microbial richness and species’ relative abundance.

Aims & Methods

We enrolled 36 healthy subjects and randomly assigned to a self-made GFD (SGFD) regimen (n=23, yrs=35±14) or to a GFD under nutritional supervision (NGFD) that ensured 60% of daily dietary carbohydrates intake (n=13; 29±12 yrs). A total of 144 fecal samples were collected at day 0, and 3, 7, 30 days during GFD, sequenced via shotgun metagenomic and profiled via bioBAkery 3. Data were analyzed on MetaPhlAn 4.0 and HUMAnN 3.0 for taxonomic and functional profiling. Alpha-diversity (Shannon index, SI) and relative abundances were computed. Only species detectable in least 5 subjects were considered (600 species). Statistical analysis was performed using a linear mixed model and Friedman’s ANOVA. P-values were combined and corrected for multiple hypothesis testing (q-value).

Results

Alpha-diversity did not significantly change during 30 days of GFD in both groups.
However, 14 and 10 species significantly changed in their relative abundance in the SGFD and NGFD groups,respectively. We observed a decrease of Bifidobacterium longum (SGB17248) and Candidatus Cibionibacter quicibialis (SGB15286) (q<0.05) with no intergroup differences. SGFD showed a decrease of butyrate producing species, i.e. Dorea longicatena and Anaerobutyricum hallii.

Conclusion

During GFD, microbial alpha-diversity preservation is independent from the daily carbohydrate intake.However, we identified consistent modifications in microbial relative abundances that partially differ in the two groups. The NGFD group showed a significant decrease in five Bifidobacterium species’ bacteria. Bifidobacteria are functionally linked to complex carbohydrate metabolism and associated with immunomodulatory properties. Despite we cannot point out specific role for gluten or carbohydrate intake in the microbiota’s richness variation, healthy subjects should not be encouraged to follow a GFD for its negative effects on gut health.

References

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