New research from the University of California San Diego has found that the makeup of a person’s gut microbiome is linked to the levels of active vitamin D in older men, revealing a new understanding of vitamin D and how it’s typically measured.

Vitamin D can take several forms, according to the study, although standard blood tests only detect 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.

“We were surprised to find that microbiome diversity—the variety of bacteria types in a person’s gut—was closely associated with active vitamin D, but not the precursor form,” said senior author Deborah Kado, MD, in a press release. “Greater gut microbiome diversity is thought to be associated with better health in general.”

Multiple earlier studies have suggested that individuals with low vitamin D levels are at a higher risk for cancer, heart disease, worse coronavirus disease 2019, and other diseases. However, the largest 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 bone health.

“Our study suggests that might be because these studies measured only the precursor form of vitamin D, rather than active hormone,” Kado said. “Measures of vitamin D formation and breakdown may be better indicators of underlying health issues, and who might best respond to vitamin D supplementation.”

The investigators analyzed stool and blood samples contributed by 567 men in the Osteoporotic Fractures in Men Study Research Group, living in 6 cities across the United States. The participants had a mean age of 84 years and most reported being in good or excellent health. The investigators used the 16s rRNA sequencing technique to identify and quantify the types of bacteria in each stool sample based on unique genetic identifiers, and they used a method known as LC-MSMS to quantify vitamin D metabolites in blood serum.

In addition to linking active vitamin D and overall microbiome diversity, the team 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 fatty acid that helps maintain gut lining health.

“Gut microbiomes are really complex and vary a lot from person to person,” said Serene Lingjing Jiang, a graduate student in the Biostatistics Program at Herbert Wertheim School of Public Health and Human Longevity Sciences, in a press release. “When we do find associations, they aren’t usually as distinct as we found here.”

The study authors noted that because the men live in different regions of the United States, they are exposed to different amounts of sunlight, a major 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. However, the investigators found no correlation 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,” Kado concluded. “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.”

REFERENCE
Study reveals connection between gut bacteria and vitamin D levels [news release]. EurekAlert; November 30, 2020. https://www.eurekalert.org/pub_releases/2020-11/uoc--src113020.php. Accessed December 8, 2020.