Vitamin D helps the body absorb calcium and plays a role in muscle function and immunity. Many people don’t get enough, especially in winter at northern latitudes. But the supplement aisle has pushed doses far beyond what most people need.
How much you need
The NIH recommends 600 IU (15 mcg) per day for adults up to 70, and 800 IU (20 mcg) over 70. The upper limit for adults is 4,000 IU (100 mcg) per day from supplements.
Where it comes from
Your skin makes vitamin D in sunlight, but production is limited in winter, with darker skin, with sunscreen and with age. Food sources are few: oily fish (salmon, sardines, mackerel), egg yolks, and fortified foods such as milk, plant milks and some cereals.
Who’s more likely to be low
- People who get little sun or live far from the equator
- People with darker skin
- Older adults
- People with obesity
- People with conditions affecting fat absorption, such as celiac or Crohn’s disease
Testing
A blood test for 25-hydroxyvitamin D measures your status. Routine testing of everyone isn’t generally recommended, but it’s reasonable if you have risk factors, osteoporosis, or symptoms such as bone pain or muscle weakness.
What supplements can and can’t do
Vitamin D supplements correct deficiency and, combined with calcium, support bone health in people who are low. But large trials, including the VITAL study of more than 25,000 adults, found that routine vitamin D supplementation in generally healthy people didn’t reduce the risk of cancer, heart disease or fractures.
Safety
Very high doses can cause dangerously high calcium levels, leading to nausea, kidney stones and heart rhythm problems. Toxicity almost always comes from supplements, not sunlight or food. Don’t take high-dose vitamin D without a clinician’s advice.
The bottom line
If you’re at risk of low levels, a daily supplement of around 600–1,000 IU is a reasonable choice for most adults. If you’re deficient, your clinician may recommend a higher dose for a period, then retesting.