Does an onion-enriched diet beneficially affect the microbiotal composition in healthy human subjects?

Regular onion consumption may have many beneficial effects on human health due mainly to well documented probiotic and antioxidant effects. Health effects comprise e.g. anti-inflammatory, anti-tumorigenic, cardiovascular, and gastrointestinal properties. However little is known of the specific mechanisms involved.

Onions are rich in fructooligosaccharides (FOS), which are well acknowledged prebiotic substances. FOS consumption have previoulsy been associated with an increased level of fermenting bacterial genera e.g. Lactobacillus and Bifidobacterium. Generally, these groups of bacteria are considered to have beneficial effects on the intestinal environment.

The aim of the present study was to analyze the effects of onion consumption on the gut microbiotal profile. In this project, five male and five female subjects were randomized to two 14 days intervention periods including one onion enriched diet and one non-enriched supplemented diet in a double-blinded crossover design with a 25 days wash-out period in between.

Six of the subjects delivered fecal samples on the last two days before starting on the diet and on the two last days of the 14 day diet. Total DNA was isolated from these samples using the Qiagen Stool Kit and subsequently quantitative PCR was performed with primers representing the Genera: Lactobacillus, Bifidobacterium, Bacteroidetes, and Clostridium to analyze effects of onion consumption on the gut microbial composition.

Moreover, principal component analysis of profiles of the faecal microbiota obtained by denaturing gradient gel electrophoresis of PCR amplified universal bacterial 16S rRNA genes was done to analyze for differences in the phylogenetic profiles as a consequence of the onion consumption. Results from these experiments will be presented at the LMC symposium.