Vitamin D circulates in the body as 25-hydroxyvitamin D, an inactive precursor of active vitamin D, known as 1,25-dihydroxyvitamin D. Most studies simply measure the precursor, but researchers at UCSD recently discovered that levels of active vitamin D are associated with gut microbe diversity (Nature Communications, Nov. 26, 2020).
Vitamin D Is a Hormone:
When sun strikes human skin, skin cells use a cholesterol compound to manufacture previtamin D (Dermatoendocrinology, Jan. 1, 2013). The liver and the kidneys then convert this natural chemical to the circulating and then to the active forms.
Although we talk about vitamin D in the same way we discuss vitamin C or vitamin E, it is actually a hormone. When it was discovered, scientists realized that this compound is crucial for normal bone growth. Over the last half century, scientists have discovered that vitamin D receptors found in tissues throughout the body play a wide range of essential roles (Endocrine Connections, Oct. 2020). In particular, the cells lining the digestive tract are rich in these receptors, bringing us to the current research.
Studying the Microbiome:
Nearly 600 healthy older men participating in the Osteoporotic Fractures in Men Study provided data for this study. The scientists analyzed both stool and blood samples. As a result, they found that men with higher levels of the active hormone had more diverse gut microbiota. Specifically, they had higher levels of 12 types of bacteria that produce butyrate. This compound reduces inflammation and improves the function of cells lining the gut (World Journal of Gastroenterology, March 28, 2011). In addition, when bacteria produce this fatty acid, animals (and possibly humans) sleep better (Scientific Reports, May 7, 2019).
Surprisingly, there were no correlations between levels of vitamin D precursor and microbial diversity. Perhaps studies that look only at precursor levels are missing an important part of the picture.
