Vitamin D is unusual among nutrients: your body can make it when your skin is exposed to sunlight, which is why it's sometimes called the "sunshine vitamin." That also makes it easy to fall short — especially in darker months, at higher latitudes, or for people who spend most of their time indoors.

Why vitamin D matters

Vitamin D helps your body absorb calcium, supporting strong bones and teeth. It also plays a role in muscle function and immune health. Long-term low levels are linked to weaker bones and other health concerns, which is why it's worth paying attention to.

Possible signs of low vitamin D

Deficiency can be subtle and easy to miss, but some people notice:

  • Fatigue and low energy.
  • Aches in bones or muscles, or muscle weakness.
  • Getting sick more often.
  • Low mood, particularly in darker seasons.

These symptoms are non-specific and can have many causes — which is exactly why testing matters.

You can't reliably feel your vitamin D level. If you suspect you're low, a simple blood test is the way to know for sure.

Where vitamin D comes from

  • Sunlight: the main natural source, though the amount your skin makes depends on season, location, skin tone, and sun exposure.
  • Food: oily fish, egg yolks, and fortified foods like some milks and cereals provide modest amounts.
  • Supplements: often recommended, particularly in low-sunlight months or for those at higher risk.

Who is more likely to be low?

People who get little sun, have darker skin, are older, cover their skin for cultural reasons, or live in northern climates during winter are more likely to be low. In many regions, health authorities suggest considering a supplement during darker months.

Important: Don't self-diagnose or take high-dose supplements without guidance — too much vitamin D can be harmful. Ask your doctor about testing and the right dose for you, especially if you take other medications or have a health condition.

Vitamin D is a small thing that quietly supports big functions in your body. If you suspect you're running low — particularly in winter — a quick chat with your doctor and a simple test can set you on the right path.