The finding came out of the D-Health trial – the world’s second largest trial of high-dose vitamin D supplementation led by Professor Rachel Neale at QIMR Berghofer Medical Research Institute located in Brisbane.
Writing in Gut Microbes, the researchers said that little difference was seen between the vitamin D intervention and placebo group in terms of the abundance of different gut bacteria genus, as well as the firmicutes-to-bacteroidetes ratio.
The firmicutes-to-bacteroidetes ratio is widely accepted as having an influence in normal intestinal homeostasis. An increased or decreased ratio is regarded as dysbiosis. For instance, an increase is associated with obesity and the latter with inflammatory bowel disease.
A total of 21,315 Australians aged 60 to 84 years old were involved in the randomised, double-blind, and placebo-controlled D-Health Trial.
They were randomised take either 60,000 IU of vitamin D3 or placebo monthly for five years.
Stool samples were collected from 835 participants – 417 from the placebo and 418 from the intervention group approximately five years after randomisation to analyse the effects of large dose vitamin D intake on the gut microbiome.
A key finding was that there was no significant difference in the Firmicutes-to-Bacteroidetes ratio between the two groups – 1.5 for the placebo group and 1.6 for the intervention group.
There was also negligible change in the Shannon diversity index between the two groups.
Shannon diversity index measures the diversity of species, which means that the intervention group did not see a more diverse gut microbiota by the end of the trial.
Shannon diversity index was 3.51 and 3.52 in the placebo and intervention group respectively – which was similarly not significantly different.
The researchers also did not observe clustering of bacterial communities in the intervention group.
“We did not find an effect of vitamin D supplementation on the beta diversity of the gut microbiome as measured by Bray-Curtis and UniFrac distances (p values 0.10 and 0.61, respectively), or on the abundance of different genera.
“There was a lower abundance of genus Bacteroides in the vitamin D group, although this was not statistically significant after adjustment for multiple testing,” the researchers added.
They highlighted that the gut microbiome profile of the trial participants was in line with those from previous studies.
Findings from existing studies
Previous studies analysing the effects of large dose vitamin D supplementation on gut microbiome have yielded mixed results.
A systematic review published in the European Journal of Nutrition in 2019 reported a link between vitamin D and the composition of the gut microbiome as measured by diversity and the abundance of different bacteria.
Subsequently, a non-randomised study involving 80 healthy women found significant increases in gut microbial diversity and the abundance of health-promoting probiotic taxa, namely Akkermansia and Bifidobacterium, after weekly supplementation of vitamin D at 50,000 IU for 12 weeks.
On the other hand, there was a 16-week RCT which showed that large dose vitamin D supplementation between 4,000 IU and 100,000 IU did not affect alpha or beta diversity in overweight or obese adults.
Source: Gut Microbes
The effect of vitamin D supplementation on the gut microbiome in older Australians - Results from analyses of the D-Health Trial
Authors: Pham H et al