Vitamin D supports bone health, immune function, and mood, and plays a role in preventing autoimmune diseases. Most adults need 600 IU daily, yet deficiency remains common, especially among people who spend limited time outdoors or live in northern climates. Here’s what the evidence actually shows about each benefit and how much difference adequate vitamin D makes.
Stronger Bones and Better Calcium Absorption
Vitamin D’s most established role is helping your body absorb calcium and phosphorus, the two minerals that make up most of your bone tissue. Without enough vitamin D, your intestines absorb only a fraction of the calcium you eat, no matter how much dairy or leafy greens you consume. This is why severe deficiency causes rickets in children (soft, bowed bones) and osteomalacia in adults (weak, painful bones that fracture easily).
Vitamin D also regulates how calcium moves in and out of bone. It signals your body to deposit minerals into the bone matrix during growth and repair, and it helps maintain the balance between building new bone and breaking down old bone. This makes it essential not just in childhood but throughout life, particularly after menopause and in older age when bone density naturally declines.
One important caveat: the U.S. Preventive Services Task Force found that taking vitamin D supplements (with or without calcium) does not prevent fractures in postmenopausal women or men over 60 who are not already deficient. The benefit for bones is primarily about correcting or avoiding deficiency, not about mega-dosing on top of already adequate levels.
Immune Defense Against Respiratory Infections
Vitamin D helps activate immune cells that fight off bacteria and viruses, and multiple large analyses have measured exactly how much that matters for everyday illness. A pooled analysis reviewed by the WHO found that vitamin D supplementation reduced the risk of acute respiratory tract infections by about 12 to 42%, depending on the study and population. The strongest protection showed up in children aged 1 to 16, who were 40% less likely to develop a respiratory infection when supplemented.
How you take it matters more than you might expect. Daily low doses consistently outperformed large monthly or quarterly doses. In one analysis, people taking a small daily amount saw a 49% reduction in respiratory infection risk, while those taking a large dose every few months saw no significant benefit. The body seems to use vitamin D best when it arrives in a steady stream rather than a periodic flood.
Reduced Risk of Autoimmune Disease
The VITAL trial, one of the largest randomized controlled trials of vitamin D to date, followed more than 25,000 people for five years. Participants who took 2,000 IU of vitamin D daily had a 22% lower rate of confirmed autoimmune diseases, including rheumatoid arthritis, autoimmune thyroid disease, and psoriasis, compared to those on a placebo. The effect grew stronger over time. When researchers looked at only the last three years of the trial, the vitamin D group had 39% fewer new autoimmune diagnoses.
People with a lower body mass index appeared to benefit more, possibly because vitamin D is fat-soluble and gets sequestered in fat tissue, leaving less available for the immune system in people carrying extra weight. This trial provided some of the first strong experimental evidence that vitamin D does more than correlate with immune health; it actively shapes it.
Mood and Depression
A meta-analysis pooling results from multiple controlled trials found that vitamin D supplementation produced a modest but statistically significant improvement in depressive symptoms. On standardized depression scales, people taking vitamin D scored meaningfully lower (better) than those on placebo, with the effect holding up across interventions lasting more than a year.
The benefit was clearest in people who already had a medical condition alongside their depression. In that group, vitamin D supplementation was associated with a substantial reduction in depression scores. For otherwise healthy individuals with depression, the effect was not statistically significant. This suggests vitamin D may help most when the body is under additional physiological stress, and that it works as a supporting factor rather than a standalone treatment for mood disorders.
The Link Between Vitamin D and Inflammation
Blood levels of vitamin D and C-reactive protein (a marker your liver produces when inflammation is present) move in opposite directions. When vitamin D drops, inflammatory markers tend to rise. For every 10 ng/mL decrease in vitamin D below the median level, C-reactive protein rises by a measurable amount. This inverse relationship is consistent across large population studies.
There is an important chicken-or-egg problem here. Vitamin D acts as what researchers call a negative acute-phase reactant, meaning that inflammation itself drives vitamin D levels down. A surgical procedure, for example, can reduce circulating vitamin D by about 40% compared to pre-surgery levels. So a low vitamin D reading on a blood test doesn’t always mean you’re deficient in the traditional sense. It can also be a signal that your body is fighting inflammation and consuming vitamin D in the process. If your inflammatory markers are normal and your vitamin D is still low, true deficiency is more likely.
How Much You Need
The NIH lists these recommended daily amounts:
- Infants (0 to 12 months): 400 IU
- Children and teens (1 to 18 years): 600 IU
- Adults (19 to 70 years): 600 IU
- Adults over 71: 800 IU
- Pregnant or breastfeeding women: 600 IU
These amounts are designed to maintain a blood level of at least 20 ng/mL, which the National Academies considers adequate for bone and overall health. Below 12 ng/mL is classified as deficient, and levels between 12 and 20 ng/mL are considered inadequate. On the other end, levels above 50 ng/mL are linked to potential harm, particularly above 60 ng/mL.
Best Sources of Vitamin D
Your skin produces vitamin D when exposed to UVB sunlight, and roughly 10 to 30 minutes of midday sun on your face and arms a few times per week is often enough for people with lighter skin. Darker skin tones, sunscreen use, northern latitudes, and winter months all reduce production significantly.
Food sources are limited but worth knowing. Fatty fish like salmon, mackerel, and sardines are the richest natural sources. Cod liver oil delivers a concentrated dose. Egg yolks contain a small amount. In the U.S., most milk, many orange juices, and many breakfast cereals are fortified with vitamin D. UV-exposed mushrooms are one of the few plant-based sources. For people who get little sun and don’t eat fish regularly, a daily supplement is the most reliable way to maintain adequate levels.