Vitamin D
What Is Vitamin D?
Vitamin D is a fat-soluble vitamin your body makes when sunlight hits your skin. It’s also found in fatty fish, egg yolks, and fortified foods. But most people don’t get enough from food and sun alone.
About 42% of American adults are deficient. That number jumps higher if you have dark skin, live far from the equator, or spend most of your time indoors.
You’ll see vitamin D marketed for everything from bone health to cancer prevention to muscle growth. Some of those claims hold up. Most depend heavily on whether you’re deficient or not.