Vitamin D is a fat-soluble nutrient that your body needs to absorb calcium, maintain strong bones, and support your immune system. Unlike most vitamins, it functions more like a hormone, and your skin can manufacture it from sunlight. Most adults need 600 IU (15 mcg) per day, though many people fall short of that target.
How Your Body Makes Vitamin D
Your skin produces vitamin D when exposed to ultraviolet B (UVB) rays from sunlight, specifically wavelengths between 290 and 320 nanometers. A cholesterol compound already present in the outer layers of your skin converts into a precursor form of vitamin D3 when UVB light hits it. This happens primarily in the two deepest layers of the epidermis.
But the vitamin D your skin produces (or that you get from food) isn’t ready to use yet. It needs two chemical conversions before it becomes active. First, your liver transforms it into a circulating storage form that doctors measure in blood tests. Then your kidneys convert that storage form into the fully active version that your cells actually respond to. Some immune cells, intestinal cells, and other tissues can also perform that second conversion locally for their own use.
The Two Forms: D2 and D3
Vitamin D comes in two main forms. Vitamin D3 (cholecalciferol) is the type your skin synthesizes from sunlight, and it’s also found in animal-based foods like fatty fish and egg yolks. Vitamin D2 (ergocalciferol) comes from plants and fungi and can be manufactured synthetically. Both forms are inactive until your body processes them through the liver and kidneys. Most research suggests D3 is more effective at raising and maintaining blood levels, which is why it’s the more commonly recommended supplement form.
What Vitamin D Does in Your Body
Vitamin D’s best-known job is helping your intestines absorb calcium and phosphorus from food. Without enough of it, your body can only absorb a fraction of the calcium you eat, no matter how much dairy or other calcium-rich food you consume. The active form of vitamin D triggers the production of transport proteins in your gut, bones, and kidneys that move calcium and phosphorus into your bloodstream where they’re needed.
This makes vitamin D essential for bone health. Children who are severely deficient develop rickets, a condition where bones become soft and bend. In adults, prolonged deficiency leads to osteomalacia (soft bones) and contributes to osteoporosis over time.
Immune Function
Vitamin D also plays a significant role in how your immune system operates. Your T cells, the white blood cells that identify and attack infections, have vitamin D receptors on their surface. When a T cell first encounters a pathogen, it increases its number of these receptors as part of the activation process. Vitamin D signaling then helps the T cell complete its activation and develop the ability to migrate toward sites of infection.
At the same time, vitamin D acts as a brake on the immune system. It can suppress production of certain inflammatory signals, which helps prevent the immune response from overreacting. This dual role, supporting immune activation while limiting excess inflammation, is why vitamin D status has been linked to autoimmune conditions and susceptibility to respiratory infections. Vitamin D also influences B cells, the immune cells responsible for producing antibodies, by dampening some of their inflammatory signaling pathways.
How Much You Need
The recommended daily amounts set by the National Institutes of Health are:
- Infants (0 to 12 months): 400 IU (10 mcg)
- Children and teens (1 to 18 years): 600 IU (15 mcg)
- Adults (19 to 70 years): 600 IU (15 mcg)
- Adults over 70: 800 IU (20 mcg)
- Pregnant or breastfeeding women: 600 IU (15 mcg)
Some researchers argue these targets are too low to maintain the blood levels needed for optimal calcium absorption and other functions. The goal for blood levels is generally a minimum of 30 ng/mL (75 nmol/L) of the storage form.
Testing and Blood Levels
Vitamin D status is measured with a blood test that checks your level of 25-hydroxyvitamin D, the storage form produced by your liver. The standard thresholds are:
- Deficient: below 20 ng/mL (50 nmol/L)
- Insufficient: 20 to 30 ng/mL (50 to 75 nmol/L)
- Sufficient: 30 ng/mL (75 nmol/L) or above
Deficiency is common worldwide. If your levels are low, a doctor will typically recommend a higher supplemental dose for several weeks to rebuild your stores, then a maintenance dose to keep levels above 30 ng/mL.
Food Sources
Few foods naturally contain significant amounts of vitamin D, which is one reason deficiency is so widespread. Fatty fish like salmon, mackerel, and sardines are among the richest natural sources, providing several hundred IU per serving. Egg yolks contain a modest amount. Cod liver oil is an exceptionally concentrated source. UV-exposed mushrooms provide vitamin D2, the plant-based form.
In many countries, common staples are fortified with vitamin D. Milk, orange juice, and breakfast cereals often have added vitamin D, typically around 100 to 150 IU per serving. Even with fortified foods, most people find it difficult to reach 600 IU through diet alone, especially if they eat little fish.
Factors That Limit Production
Several things reduce how much vitamin D your skin can make. Melanin, the pigment that gives skin its color, absorbs UVB radiation. People with darker skin need considerably more sun exposure to produce the same amount of vitamin D as someone with lighter skin. Aging also matters: older adults have less of the precursor compound in their skin, so they produce vitamin D less efficiently even with the same sun exposure.
Geography plays a large role. If you live above roughly 37 degrees latitude (approximately the level of San Francisco or Athens), the sun’s angle during winter months is too low for UVB rays to reach your skin effectively. Sunscreen with SPF 15 or higher blocks most UVB absorption as well. Cloud cover, air pollution, and spending most of the day indoors all further reduce your skin’s vitamin D output. For many people, supplementation during fall and winter is the most practical way to maintain adequate levels.
Too Much Vitamin D
You cannot overdose on vitamin D from sunlight, because your skin has a self-limiting mechanism that breaks down excess precursor. Toxicity comes from supplements. When blood levels climb too high, the excess vitamin D causes your body to absorb too much calcium, a condition called hypercalcemia. Symptoms include nausea, vomiting, weakness, frequent urination, and in severe cases, kidney damage or abnormal heart rhythms.
The tolerable upper intake level for adults is 4,000 IU (100 mcg) per day. Toxicity typically occurs at sustained daily doses well above that threshold, often 10,000 IU or more taken over months. Standard supplemental doses of 600 to 2,000 IU per day are safe for most people.