Vitamin D helps your body absorb calcium, keeps bones and muscles working normally and plays a part in immune function. It is also one of the few nutrients your body can make for itself, from sunlight on bare skin. Here is what vitamin D does, which of its benefits are well established, and where large trials have found less than the early headlines promised.
How was vitamin D discovered?
In 1822 the Polish physician Jędrzej Śniadecki noticed that children in inner-city Warsaw had far more rickets, a disease of soft, misshapen bones, than children in the countryside, and put the difference down to a lack of sun. About a century later, researchers showed that rickets could be prevented both by a factor in food and by ultraviolet light on the skin, and that factor was named vitamin D.
Is vitamin D a vitamin or a hormone?
A bit of both. The vitamin D you get from the sun, food or a supplement is inactive. Your liver converts it into 25-hydroxyvitamin D, the form a blood test measures, and your kidneys convert that into 1,25-dihydroxyvitamin D (calcitriol), the active hormone. Because of that two-step activation, vitamin D is often called a prohormone. Most cells and organs carry a vitamin D receptor, which is why researchers have studied its role well beyond bone.
The two forms: D2 and D3
Vitamin D2 (ergocalciferol) is made by fungi such as yeast and mushrooms when they are exposed to UV light. Vitamin D3 (cholecalciferol) is the form your skin makes, and the form in most supplements, according to the NIH Office of Dietary Supplements. Both raise blood levels, but a meta-analysis of randomized trials found D3 raised them more than D2. We compare the two in more detail in what's the difference between vitamin D2 and D3.
What are the proven benefits of vitamin D?
Strong bones and working muscles
Vitamin D's best-established job is helping your intestine absorb calcium, along with phosphorus. Without enough of it, bones cannot mineralize properly: a long-term shortfall can lead to rickets in children and to osteomalacia and osteoporosis in adults.
That benefit is about avoiding a shortfall. In the VITAL trial, 2,000 IU a day for about five years did not lower fracture rates in generally healthy adults over 50 who had not been selected for low vitamin D or low bone density.
Normal immune function
Vitamin D helps keep your muscles, nerves and immune system working normally. In one randomized trial in Japanese schoolchildren, 1,200 IU a day through the winter cut influenza A cases from 18.6% to 10.8%. The bigger picture is more modest: a 2025 meta-analysis of all the trials found that the overall effect on respiratory infections is no longer statistically clear.
What does the research say about weight, mood, cancer and blood sugar?
Many of vitamin D's headline benefits came from observational studies, which show that people with higher vitamin D levels tend to be healthier, but cannot show that vitamin D is the reason. Large randomized trials have since tested several of them.
Weight
In one study of older women, those who gained weight gained slightly less if their vitamin D levels were higher, but a meta-analysis of randomized trials found that vitamin D supplements did not reduce body weight or fat on their own.
Mood
A one-year trial in overweight adults found that high weekly doses improved depression scores. But in VITAL's depression study, 2,000 IU a day for a median of five years did not prevent depression or change mood scores in adults over 50. Anyone dealing with low mood should talk to a doctor rather than rely on a supplement.
Cancer and heart health
Cancer and heart disease rates are lower in some sunnier places, which prompted the theory that vitamin D protects against both. VITAL tested it in 25,871 adults: 2,000 IU a day did not lower rates of invasive cancer or major cardiovascular events over a median of 5.3 years.
Blood sugar
In adults with prediabetes, the large D2d trial of 4,000 IU a day did not significantly lower the risk of type 2 diabetes, though a pooled analysis of three trials found a 15% lower risk. This is a question for your doctor, not something to manage with a supplement on your own.
How common is low vitamin D?
It depends on where you draw the line. Using a cutoff of 20 ng/mL, an analysis of 2005-2006 national survey data found that about 42% of US adults fell short. A later CDC analysis of 2011-2014 data found that 5% of Americans were at risk of deficiency and another 18% at risk of inadequacy. People who get little sun, have darker skin, are older, live with obesity or have conditions that affect absorption are more likely to run low. Our guide to symptoms of vitamin D deficiency covers the signs and how testing works.
How much vitamin D do you need, and where do you get it?
The recommended dietary allowance is 600 IU (15 mcg) a day for ages 1 to 70, including during pregnancy, and 800 IU (20 mcg) after 70, with 400 IU for infants. The upper limit for adults and children 9 and over is 4,000 IU (100 mcg) a day. Our article on how much vitamin D you need goes through the numbers by age.
You can get vitamin D from sunlight, a short list of foods and supplements; we cover each in sources of vitamin D. If you supplement, Sol Food gives you 2,000 IU of vitamin D3 from lichen in one small organic, vegan tablet, which sits above the RDA and within the upper limit. If you are pregnant, have kidney or liver disease or take prescription medicines, check with your doctor before starting any vitamin D supplement.
