What is vitamin D for?

Vitamin D's main job is helping the gut absorb calcium to keep bones strong; it's also involved in muscle function and immunity. Your skin makes it from sunlight, but an indoor lifestyle or a low-sun season can leave you short, so some supplement it. Being fat-soluble, it absorbs better with food, and steady intake should stay under the adult upper limit (about 4,000 IU/day).

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