What Vitamin D Does in Your Body

Vitamin D is a hormone your body makes when your skin is exposed to sunlight, and it also comes from certain foods and supplements. Once in your bloodstream, it travels to your kidneys and liver, where it is converted into its active form. That active form then tells your intestines to absorb calcium and phosphorus from food — without it, your body cannot build or maintain bone strength, no matter how much calcium you eat.

Beyond bone health, vitamin D receptors sit on nearly every cell type in your body. Research shows vitamin D influences immune function, helps regulate inflammation, affects muscle strength, and plays a role in how your nervous system develops and functions. The evidence is strongest for bone health; the evidence is emerging but promising for immune support and inflammation control.

Key Takeaways

  • Vitamin D enables your intestines to absorb calcium, so deficiency leads to weak bones even if you consume enough calcium.
  • Your body makes vitamin D from sunlight exposure, but the amount depends on season, latitude, skin tone, age, and time of day.
  • Fatty fish, egg yolks, and fortified milk contain vitamin D, but most people cannot reach recommended levels from food alone.
  • Blood levels below 20 nanograms per milliliter are considered deficient; levels of 30 and above are generally considered sufficient for bone health.
  • Vitamin D toxicity is rare and requires sustained intake of 4,000 IU or more daily for months, so the risk from supplements is low.

How Vitamin D Affects Bone Health

Calcium sits in your bones as a mineral matrix that gives them hardness and strength. But calcium cannot move from your intestines into your blood without vitamin D. When vitamin D is low, your body cannot absorb the calcium you eat, so it pulls calcium from your bones to keep blood levels stable. Over time, this drains bone mineral density and raises fracture risk — a process called osteoporosis.

Human studies show that people with vitamin D levels below 20 nanograms per milliliter have significantly higher fracture rates than those with levels above 30. A large randomized trial published in the New England Journal of Medicine found that older adults who took vitamin D supplements had fewer hip fractures than those who did not, though the effect was modest. The evidence is clearest in people over 60 and in those who get little sun exposure.

Vitamin D and Immune Function

Immune cells — including T cells and macrophages — have vitamin D receptors on their surface. In laboratory studies, vitamin D activates these cells and increases production of antimicrobial proteins. Observational data shows that people with low vitamin D levels catch more respiratory infections, and some studies suggest supplementation may reduce infection risk, though the effect size is small.

A large randomized trial in the Lancet tested whether vitamin D supplements prevented respiratory infections in people with low baseline levels. Participants who took vitamin D had fewer infections than those who took placebo, but the difference was modest — about one fewer infection per person over a year. The effect was stronger in people who were severely deficient at the start. This is an area where research is ongoing and the practical benefit remains uncertain for people with adequate vitamin D levels.

Vitamin D and Muscle Strength

Vitamin D receptors appear in muscle tissue, and animal studies show that vitamin D regulates muscle protein synthesis and calcium handling inside muscle cells. Observational studies in older adults consistently find that those with low vitamin D have weaker grip strength and slower walking speed than those with adequate levels. However, randomized trials testing whether vitamin D supplements improve muscle strength in older adults have shown mixed results — some show modest gains, others show none.

The strongest evidence is in people over 70 with severe deficiency. In younger adults or those with adequate baseline vitamin D, supplementation has not consistently improved muscle function in controlled trials. This suggests vitamin D is necessary for muscle health but that supplementing someone who is already sufficient may not add benefit.

How Much Vitamin D You Need

The National Institutes of Health recommends 600 IU daily for adults aged 19 to 70, and 800 IU for those over 70. The Endocrine Society, a medical specialty group, recommends higher amounts — 1,500 to 2,000 IU daily — to maintain blood levels above 30 nanograms per milliliter. These numbers differ because the NIH based its recommendation on bone health alone, while the Endocrine Society included emerging evidence on immune and muscle function.

Your actual need depends on how much sun exposure you get, your skin tone (darker skin requires more sun time to produce vitamin D), your age, and your baseline blood level. A blood test measuring 25-hydroxyvitamin D is the standard way to check your level. If your level is below 20, most doctors recommend supplementation; if it is between 20 and 29, supplementation may help; if it is 30 or above, you are generally considered sufficient for bone health.

Food Sources and Supplementation

Fatty fish — salmon, mackerel, sardines — contain the most vitamin D naturally, with a 3-ounce serving of cooked salmon providing about 450 IU. Egg yolks contain roughly 40 IU per egg. Mushrooms exposed to sunlight contain vitamin D2. Fortified milk typically has 100 IU per cup, and fortified orange juice has similar amounts. Despite these sources, most people in northern climates cannot reach recommended levels from food alone, especially in winter.

Vitamin D supplements come in two forms: D2 (ergocalciferol, from plant sources) and D3 (cholecalciferol, from animal sources or synthesized). D3 is more potent and raises blood levels more effectively than D2 at the same dose. Supplements range from 400 IU to 4,000 IU per dose. Toxicity from vitamin D is rare and requires sustained intake well above 4,000 IU daily for months, so the risk from typical supplementation is low. Your doctor can recommend a dose based on your blood level and sun exposure.

Who Is at Higher Risk of Deficiency

People who live in northern latitudes (above 35 degrees north), work indoors, wear sunscreen consistently, or have darker skin tones are at higher risk of low vitamin D because they produce less from sun exposure. Older adults absorb vitamin D less efficiently from food and make less in their skin. People with digestive disorders like celiac disease or Crohn's disease may not absorb vitamin D well. Those taking certain medications, including some antiseizure drugs and glucocorticoids, may have lower levels.

Infants who are exclusively breastfed receive little vitamin D from breast milk, so pediatricians typically recommend supplementation. People who follow strict vegan diets may have lower intakes because most natural sources are animal-based, though fortified plant milks are available. If you fall into one of these groups, a blood test can tell you whether your level is sufficient.

Frequently Asked Questions

Can you get too much vitamin D from the sun?

No. Your body regulates vitamin D production from sunlight — once you have enough, further sun exposure does not increase your level. Sunburn and skin cancer risk do increase with excessive sun exposure, so the limit is set by skin damage, not vitamin D toxicity.

Is vitamin D2 or D3 better?

D3 raises blood levels more effectively than D2 at the same dose, so D3 is generally preferred. Both are available over the counter. If you follow a vegan diet, D2 supplements are plant-based, though some D3 supplements are now made from algae rather than animal sources.

What happens if your vitamin D level is too high?

Vitamin D toxicity causes high blood calcium, which can lead to nausea, weakness, kidney damage, and bone loss. It requires sustained intake of 4,000 IU or more daily for months. Typical supplementation at 1,000 to 2,000 IU daily does not cause toxicity. A blood test can confirm if your level is too high.

Do I need a blood test before taking vitamin D supplements?

A test is helpful if you have symptoms of deficiency (bone pain, muscle weakness), risk factors (limited sun exposure, digestive disease), or want to know your baseline level before supplementing. If you have no symptoms and live in a sunny climate with regular outdoor time, supplementing at the recommended dose is generally safe without testing first.

Can vitamin D prevent COVID-19 or other viral infections?

Observational data suggests people with low vitamin D catch more respiratory infections, and some trials show modest reduction in infection risk with supplementation. However, vitamin D is not a substitute for vaccination or other proven prevention measures. The effect is small and most pronounced in people who are severely deficient at baseline.