24/07/2025
"Today I learned"
Vitamin D may be called a “vitamin,” but your body treats it like a powerful steroid hormone one that regulates everything from immunity to gene expression.
Despite the name, Vitamin D isn’t technically a vitamin at all. In scientific terms, it’s now recognized as a steroid hormone, placing it in the same biochemical category as heavy-hitters like testosterone, estrogen, and cortisol. Its origin? Just like those hormones, it’s derived from cholesterol. When your skin is exposed to sunlight, it kickstarts the production of Vitamin D, which then undergoes a two-step conversion process in the liver and kidneys to become calcitriol, its active hormonal form.
Here’s where it gets interesting: once in that active form, calcitriol binds to vitamin D receptors (VDRs) inside cells and influences gene expression literally turning certain genes on or off. This means Vitamin D doesn’t just help with bones and calcium absorption; it also plays a key role in regulating immune response, inflammation, phosphate balance, and even cellular growth and repair.
So while you’ve always thought of it as something you get from sunlight and supplements, Vitamin D is much more than a simple nutrient. It’s a full-fledged hormonal player, orchestrating critical biological processes at the cellular level. Your daily sunshine might just be fueling one of the most underrated hormone systems in your body.
