Vitamin D is best known for bones, but it has been studied for weight, mood, heart health and immunity too. Some of those links looked exciting in early research. Here is what larger trials have found since, and what vitamin D reliably does.
Does vitamin D help with weight loss?
Not on its own. 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 body fat without calorie restriction. Low vitamin D is more common in people with obesity, but correcting it is not a weight-loss strategy.
Does vitamin D improve mood?
The evidence is mixed. A one-year trial in overweight adults found that high weekly doses improved depression scores. But in the much larger VITAL trial, 2,000 IU a day for about five years did not prevent depression or improve mood scores in adults over 50. If your mood is low, talk to a doctor rather than relying on a supplement.
Does vitamin D support heart and immune health?
Vitamin D helps keep your muscles, nerves and immune system working normally. Beyond that, the research is more nuanced than early headlines suggested:
- Infections. One trial in schoolchildren found 1,200 IU a day in winter reduced influenza A, but a 2025 meta-analysis of all the trials found the overall effect on respiratory infections is no longer statistically clear.
- Multiple sclerosis. In US military personnel, higher vitamin D levels were associated with a lower risk of multiple sclerosis. That is an association from blood samples, not proof that supplements prevent it.
- Heart disease. In VITAL, 2,000 IU a day did not lower the rate of major cardiovascular events compared with placebo.
What does vitamin D reliably do?
Its best-established role is helping your intestine absorb calcium, which bones need to stay strong. A long-term shortfall can lead to soft bones, bone pain and muscle weakness; our guide to vitamin D deficiency symptoms covers the signs, and what vitamin D does covers its role in full.
Most adults need 600 IU a day, or 800 IU after 70, and the adult upper limit is 4,000 IU a day; see how much vitamin D you need. If sun and food do not get you there, our Sol Food tablets provide 2,000 IU of plant-based vitamin D3 from lichen in one small tablet. Check with your doctor first if you are pregnant, have kidney disease or take prescription medicines.
