On the one hand, numerous studies show that low levels of vitamin D in the body are associated with an increased risk of cancer, cardiovascular and other diseases, as well as a high risk of severe disease and death from COVID-19.
However, published in January 2019, the results of the largest clinical study on the subject conducted so far, which involved more than 25,000 people, show that taking vitamin D supplements has no effect on health.
The discovery, made by scientists at the University of California (San Diego, USA), whose article was published in the journal Nature Communications, may explain this contradiction.
The fact is that the standard blood test for vitamin D concentration shows the level of inactive precursor, of this substance stored in the body. In order for the compound to start working, the precursor must be metabolized to the active form of vitamin D.
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To understand how intestinal bacteria play a role in this transformation, the researchers analyzed blood and stool samples from 567 healthy adult men living in six different American cities.
The researchers determined the types of bacteria found in the participants’ guts and the levels of the precursor and the active form of vitamin D in their blood serum.
It turned out that the more diverse the composition of the intestinal microflora, the higher the concentration not of the precursor but of the active form of vitamin D in the blood of the participants.
The researchers also identified 12 types of bacteria, most common in the gut flora of participants with high levels of active vitamin D in their blood. Most of these bacteria produce butyric, a butyric acid that is beneficial to the intestinal mucosa.
Butyric acid is obtained by fermentation from intestinal microorganisms of dietary fiber found in foods rich in fiber and complex carbohydrates.
“The conflicting results from previous studies appear to be due to the fact that they only took into account the concentration of the precursor and not the active form of vitamin D in the blood,” the scientists explained.
As the study participants lived in different regions of the United States, they were exposed to various levels of sunlight, a natural source of vitamin D.
For example, participants who lived in sunny San Diego, California, received more sun during the year than others, and the highest concentrations of vitamin D precursor in their blood.