Why Vitamin D Is So Unusual (And Why You Should Care)

We throw the word “vitamin” around like it’s simple. But vitamin D is anything but. In fact, it’s so unique that calling it a vitamin is technically inaccurate.

Here’s why this essential nutrient deserves a second look—and why it might be more accurate to think of it as a hormone.

Most Vitamins Are Nutrients. Vitamin D Is a Messenger.

• Most vitamins must come from food because the body can’t make them.
• Vitamin D is different. Your body makes it when sunlight hits your skin—specifically UVB rays.
• That sunlight exposure sets off a chain reaction in the body, one that ends with the production of a powerful hormone.
• That’s right: vitamin D behaves more like a hormone than a dietary helper.

The Path from Sunshine to Hormone

Your body doesn’t just “use” vitamin D—it activates it.

• First, UVB rays help your skin produce a compound called cholecalciferol (vitamin D₃).
• The liver then turns this into calcidiol, a storage form that circulates in your blood.
• Your kidneys convert that into calcitriol—the fully active form.
• Calcitriol travels through the body like any hormone, triggering responses in cells far from where it was made.

This is what hormones do: they signal, regulate, and control body processes at a distance.

Vitamin D: The Hormone in Disguise

• Calcitriol helps regulate calcium absorption, bone strength, and even immune response.
• It acts on the nucleus of cells—changing gene expression, much like testosterone or cortisol.
• It’s fat-soluble, derived from cholesterol, and stored in body fat—just like steroid hormones.
• It doesn’t just “support health.” It regulates critical functions, including your body’s mineral balance, mood, and immunity.

Why the Name Is a Misfit

• The word “vitamin” comes from Latin: vita (life) and amine (a type of compound).
• Early scientists assumed all vitamins were nitrogen-based. They weren’t. Vitamin D has no nitrogen at all.
• So the name stuck, even though the function doesn’t match.
• If discovered today, vitamin D would probably be labeled a hormone or prohormone, not a vitamin.

The Bottom Line

Vitamin D is misnamed. It’s not just a nutrient—it’s a hormone your body makes from sunshine.

• It plays a central role in bone health, immune response, and cellular function.
• Deficiency doesn’t just mean you’re a little off—it can lead to weakened bones, poor immunity, and disrupted mood.
• If you live in a northern area like Bend, Oregon, where UVB rays drop in fall and winter, you may need to supplement or adjust your sun habits to stay in balance.

Vitamin D isn’t just good for you. It’s foundational.
And it’s not just a vitamin. It’s a hormone hiding in plain sight.

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