Our gut microbiomes — the many bacteria, viruses and other microbes living in our digestive tracts — play important roles in our health and risk for disease in ways that are only beginning to be recognized.
University of California San Diego researchers and collaborators recently demonstrated in older men that the makeup of a person’s gut microbiome is linked to their levels of active vitamin D, a hormone important for bone health and immunity. Researchers were surprised to find that microbiome diversity — the variety of bacteria types in a person’s gut — was closely associated with active vitamin D. Greater gut microbiome diversity is thought to be associated with better health in general.
Multiple studies have suggested that people with low vitamin D levels are at higher risk for cancer, heart disease, worse COVID-19 infections and other diseases. Yet the largest randomized clinical trial to date, with more than 25,000 adults, concluded that taking vitamin D supplements has no effect on health outcomes, including heart disease, cancer or even bone health.
The study suggests that might be because these studies measured only the precursor form of vitamin D, rather than active hormone. Measures of vitamin D formation and breakdown may be better indicators of underlying health issues, and who might best respond to vitamin D supplementation.
In addition to discovering a link between active vitamin D and overall microbiome diversity, the researchers also noted that 12 particular types of bacteria appeared more often in the gut microbiomes of men with lots of active vitamin D. Most of those 12 bacteria produce butyrate, a beneficial fatty acid that helps maintain gut lining health. Gut microbiomes are really complex and vary a lot from person to person, when associations are found, they aren’t usually as distinct as we found here.”
Because they live in different regions of the U.S., the men in the study are exposed to differing amounts of sunlight, a source of vitamin D. As expected, men who lived in San Diego, California got the most sun, and they also had the most precursor form of vitamin D. But the team unexpectedly found no correlations between where men lived and their levels of active vitamin D hormone.
It seems like it doesn’t matter how much vitamin D you get through sunlight or supplementation, nor how much your body can store. It matters how well your body is able to metabolize that into active vitamin D, and maybe that’s what clinical trials need to measure in order to get a more accurate picture of the vitamin’s role in health. We often find in medicine that more is not necessarily better, so in this case, maybe it’s not how much vitamin D you supplement with, but how you encourage your body to use it.
According to the team, more studies are needed to better understand the part bacteria play in vitamin D metabolism, and to determine whether intervening at the microbiome level could be used to augment current treatments to improve bone and possibly other health outcomes.