Vitamin D is a nutrient your body makes when skin is exposed to sunlight, and one you can also get from food and supplements. Unlike most vitamins, your body can store it, and it affects far more than bone health—it influences immune function, mood, muscle strength, and how your cells grow and divide. Understanding what vitamin D does, how much you need, and whether you're getting enough matters because deficiency is common, especially in people who live in northern climates, spend most time indoors, or have darker skin.
These articles explain how vitamin D works in your body, what the research shows about its effects on different health conditions, which foods and supplements contain it, and how to know if you might need more. You'll find information about testing, dosage ranges, and what happens when levels are too low or too high—along with what questions scientists still haven't fully answered.