Vitamin D deficiency is associated with cancer, cardiovascular disease and osteoporosis, but mixed results have been obtained from clinical trials of vitamin D supplements. A recent study that found ties between the vitamin and gut bacteria’s active type may help understand why.
For high immunity and preserving healthy bones and teeth, vitamin D is essential.
Despite these correlations, there has been conflicting evidence for the benefits of widespread vitamin D supplementation in otherwise healthy individuals to prevent disease.
A major clinical trial, for instance, found no proof that a vitamin D supplement in older adults reduced cardiovascular disease and cancer. No evidence was found in another study that taking a supplement improved bone health.
Health experts have been puzzled by this apparent lack of health benefits from widespread vitamin D supplementation.
A plausible explanation has now been discovered by scientists at the University of California (UC) San Diego in La Jolla. In Nature Communications, their results appear.
In order to transform inactive vitamin D into its active, health-promoting form, the paper suggests that gut bacteria can play a vital role.
Stores of inactive vitamin D
They calculate serum levels of the inactive precursor when healthcare professionals and medical researchers want to assess the vitamin D status of a person, since this represents how much vitamin D the body stores.
However, rather than how much of it is processed, the key factor could be how the vitamin is metabolized.
The UC San Diego researchers found that its levels associated with the diversity of the population of bacteria living in their gut, or gut microbiome, when calculating how much active vitamin D older males had in their blood.
Active vitamin D levels were also associated with the amount of “friendly bacteria in their gut.
By comparison, the inactive precursor form of the vitamin and the bacterial diversity or pleasant bacteria were not strongly associated.
“We were surprised to find that the diversity of microbiomes, the variety of types of bacteria in the gut of a person, was closely associated with active vitamin D but not the form of the precursor,” says senior study author Dr. Deborah Kado, director of UC San Diego Health’s Osteoporosis Clinic.
“Greater diversity of gut microbiomes is thought to be associated in general with better health,” she says.
Even after adjusting for factors known to determine microbial diversity, the association between microbial diversity and active vitamin D persisted. This included the age of the participants, where they lived, their racial origin, and their antibiotic use in the United States.
In fact, the levels of active vitamin D in the participants correlated even more strongly with the diversity of microbiomes than any of these other factors.
This is especially remarkable considering that through the action of ultraviolet light on their skin, people who live in sunnier areas, such as California, can synthesize more of their own vitamin D.
“It seems like it does not matter how much vitamin D you get through sunlight or supplementation, nor how much your body can store.”
– Dr. Deborah Kado
“It’s important how well your body can metabolize this into active vitamin D, and perhaps that’s what clinical trials need to measure in order to get a more accurate picture of the role of the vitamin in health,” explains Dr. Kado.
The researchers say further studies are needed to study the role of bacteria in the metabolism of vitamin D. They speculate that modifying the microbiota of patients could increase current bone density enhancement therapies, and probably other health outcomes.
In a study called the Osteoporotic Fractures in Men Study, the team analyzed stool and blood samples from 567 men living in six U.S. cities (MrOS).
The average age of the participants was 84 years and most indicated that they were in good or excellent health.
To classify and quantify bacteria in the stool samples, the researchers sequenced copies of a bacterial gene that delineates various species.
They also assessed the concentrations in the blood samples of three vitamin D metabolites-the precursor, the active vitamin, and a breakdown product.
Reporting their results, the authors write:
“The significant correlation between amplified vitamin D activation and greater individual stool microbial diversity supports the idea that increased microbial diversity reflects a healthy state.”
Not only did males with the highest levels of active vitamin D in their blood have the greatest microbiome diversity, but also their gut harbored more friendly bacterial species that produce butyrate.
There are a wide variety of possible health benefits to this short-chain fatty acid, which certain forms of bacteria in the colon produce as they eat fiber. Research shows that for example, butyrate helps prevent colon cancer, reduce blood cholesterol, and fight insulin resistance.
Cause or effect?
The authors emphasize that their study was unable to establish whether high levels of active vitamin D allow the production of butyrate-producing bacteria or whether these bacteria facilitate the conversion of the vitamin into its active form.
The research could not by the same way show that providing a more diverse gut microbiome contributes to improved metabolism of vitamin D. It discovered just a relation between the two.
Another limitation of the research was that the respondents were mostly older white males.
Moreover, approximately 75 percent of them reported taking some type of vitamin D supplement. The normative definition of vitamin D deficiency was met by just 7 percent of all participants.
Therefore the results of the study may not be true for other groups, for individuals who do not take the supplement, or for those identified as vitamin deficient.