Research into the importance of vitamin D is increasing, as is research into the importance of a healthy gut microbiome. Those two worlds collided recently when researchers from the University of California-San Diego found a link between gut health and levels of active vitamin D in older men.
Scientists were careful to distinguish between what they called active vitamin D, that which has been metabolized by the body into an active form for use, and vitamin D in its precursor form that is being stored in the body. A simple blood test can determine how much vitamin D is stored in the body but these researchers were able to better understand how much vitamin D the body was actively using.
"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, director of the Osteoporosis Clinic at UC San Diego Health. "Greater gut microbiome diversity is thought to be associated with better health in general."
Recent studies have linked low vitamin D levels to poor outcomes from COVID-19 as well as an increased risk for cancer and heart disease. Yet Kado was puzzled by other studies that seemed to show vitamin D supplementation did not positively affect health outcomes. That’s what led her to look further into the body’s use of that vitamin.
"Our study suggests that might be because these studies measured only the precursor form of vitamin D, rather than active hormone," said Kado. "Measures of vitamin D formation and breakdown may be better indicators of underlying health issues, and who might best respond to vitamin D supplementation."
A total of 567 men with a mean age of 84 participated in the study. They lived in six different cities around the U.S., which gave scientists the opportunity to see if geographic regions possibly played a role in the levels of vitamin D. Those living in southern California had the highest levels of the precursor form but there was no correlation between geography and levels of active vitamin D.
Stool and blood samples were analyzed to determine the composition of the gut microbiome as well as the vitamin D metabolites (precursor, active hormone and breakdown product). Researchers found a link between the level of active vitamin D and the overall microbiome diversity, which is a measure of its health. They also found those with the highest levels of active vitamin D also had the highest levels of 12 particular strains of gut bacteria that product butyrate, a beneficial fatty acid that helps maintain the health of the gut lining.
"Gut microbiomes are really complex and vary a lot from person to person," researcher Serene Lingjing Jiang said. "When we do find associations, they aren't usually as distinct as we found here."
"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 said. "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."