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.
In a recent study at the University of California San Diego, researchers found that in older men, 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.
The findings also showed a new understanding of vitamin D and how it’s typically measured.
The study is published in Nature Communications. One author is Deborah Kado, M.D.
Vitamin D can take several different forms, but standard blood tests detect only one, an inactive precursor that can be stored by the body.
To use vitamin D, the body must metabolize the precursor into an active form.
Previous 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 current study suggests that might be because these studies measured only the precursor form of vitamin D, rather than the 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 the study, the team analyzed stool and blood samples contributed by 567 men.
The participants live in six cities around the United States, their mean age was 84, and most reported being in good or excellent health.
The researchers found a link between active vitamin D and overall microbiome diversity, and they 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.
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.
The team says 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.