Scientists from the Medical University of Graz report that high daily doses of vitamin D3 for five weeks led to significant reductions in bacteria such as Pseudomonas​ spp. and Escherichia/Shigella​ spp.

“The marked reduction in Gammaproteobacteria, which include typical opportunistic pathogens and the increase in phylotype richness, supports the beneficial effect of a high-dose vitamin D3 supplementation on the human gut microbiome,” ​wrote the researchers.

“This might in part explain the effects of a vitamin D-rich diet on IBD or bacterial infections and encourage studying the effects of vitamin D in these patients.”​

Study details​

The researchers recruited 16 healthy people to participate in their open-label, pilot study. Participants had their GI tracts sampled at four different places, starting with the stomach and ending with a stool sample, before and after receiving high dose vitamin D3 for eight weeks. The dose for the first four weeks was 980 IU/kg bodyweight of vitamin D3 per week, and the dose for the second four weeks was 490 IU/kg bodyweight per week.

Results showed that the gut microbiome in the upper GI tract was changed as a result of D3 supplementation.

The vitamin D3 supplements were also associated with significantly higher number of CD8+ T immune cells in the upper GI tract, which may have driven the decrease in Gammaproteobacteria observed, said the researchers.

“CD8+ T cells have the highest expression level of [vitamin D receptors] compared with other immune cells, which suggests that CD8+ T cell function is also regulated by vitamin D,” ​they wrote. “Calcitriol [also known as 1,25-dihydroxyvitamin D (1,25(OH)2D), the hormonally active metabolite of vitamin D] influences cytokine production in human CD8+ T cells and is known to act as a stop signal in inflammatory processes. CD8+ T cells are also capable of directly destroying harmed or infected host cells, thus lowering the number of proinflammatory cells.​

“Reducing such an inflammatory environment by vitD3 could diminish the competitive advantage of opportunistic pathogens, such as ​Escherichia/Shigella spp. or ​Pseudomonas spp. which are evolutionary better adapted to inflammation and can out-compete commensal bacteria. In return, a low inflammatory environment allows beneficial bacteria such as ​Bacteroidetes to outcompete opportunistic pathogens, resulting in increased phylotype richness which we found in this study.”​

Source: European Journal of Nutrition​​

June 2016, Volume 55, Issue 4, pp 1479-1489, doi: 10.1007/s00394-015-0966-2

“Effects of high doses of vitamin D3 on mucosa-associated gut microbiome vary between regions of the human gastrointestinal tract”​

Authors: M. Bashir, et al.