Vitamin D: what it does, and what it does not

Vitamin D behaves more like a hormone than a standard vitamin. Its best understood job is helping your gut absorb calcium from the food you eat, and calcium is the raw material your bones are built from. Your skin can make Vitamin D from sunlight, and some comes from food and fortified products.

Muscle is part of the picture too. Muscle cells respond to Vitamin D, and where someone is genuinely short of it, weakness and difficulty getting around can be part of how that shows up. Correcting a real shortfall can help. That is a different thing from topping up someone who already has enough.

Which brings us to what Vitamin D is not. It is not a muscle-building supplement, and taking more of it will not make an already well-nourished person stronger. It is not a substitute for exercise, which remains the strongest signal you can send to both muscle and bone. And more is not better: with Vitamin D the goal is having enough, not having the highest number you can reach.

One more belief worth setting aside is that a normal Vitamin D result means your bones are fine. It does not. You can have perfectly adequate Vitamin D and still have bone that has thinned with the years, because Vitamin D is one contributor among several.

A useful way to hold all this: think of bone as the house, calcium as the bricks, Vitamin D as part of what gets the bricks delivered, and exercise as the instruction to build. You need all of it, and a great deal of one will not make up for the absence of another. Whether you need testing or a supplement is a question for your doctor, who can look at your own circumstances. Sunlight helps your body make Vitamin D, but deliberately sitting out to burn is not a sensible way to go about it.