Vitamin D is a fat-soluble nutrient that your body uses to absorb calcium, build bones, and regulate your immune system. Despite its name, it functions more like a hormone than a traditional vitamin. Your skin produces it when exposed to sunlight, and you can also get it from certain foods and supplements. About 42% of American adults have levels considered insufficient, making it one of the most common nutritional gaps worldwide.
Why It’s Called a Vitamin (but Acts Like a Hormone)
Most vitamins work as helpers in chemical reactions or act as antioxidants. Vitamin D does neither. Its active form is a fat-soluble hormone that binds to receptors found in nearly every tissue in your body, from your intestines to your brain. It was labeled a “vitamin” in the early 1900s because researchers discovered it in food before they understood it could be made by the body. The name stuck, but biologically, vitamin D is a prohormone: a substance your body converts into an active hormone through a two-step process in the liver and kidneys.
How Your Body Makes It From Sunlight
Your skin contains a cholesterol compound that absorbs ultraviolet B (UVB) rays from sunlight. When UVB hits this compound, it breaks apart the molecule’s ring structure, creating a precursor that slowly converts into vitamin D3 through body heat. This process is surprisingly efficient. A short period of sun exposure can trigger vitamin D production in the skin that continues for hours afterward, even after you’ve gone inside.
Several factors determine how much vitamin D your skin actually produces. Melanin, the pigment that gives skin its color, competes with the vitamin D precursor for UV absorption. People with darker skin may need up to ten times longer in the sun to produce the same amount as those with lighter skin. Geography matters too: if you live above about 40 degrees north latitude (roughly the line from New York to Madrid), UVB rays are too weak for meaningful vitamin D production from November through early March. Move another ten degrees north, to places like Edmonton, Canada, and that “vitamin D winter” stretches from October to April. Age, sunscreen use, and how much skin you expose all play a role as well.
D2 vs. D3: Two Forms, Different Potency
Vitamin D comes in two main forms. D3 (cholecalciferol) is the type your skin makes and the type found in animal foods like fatty fish, egg yolks, and fortified milk. D2 (ergocalciferol) comes from plant sources, mainly mushrooms and yeast. Both forms can correct a deficiency, but they aren’t equally effective at raising and maintaining your blood levels.
A meta-analysis comparing the two found that D3 is consistently better at boosting blood levels of vitamin D regardless of the dose, the delivery method, or the age of the person taking it. D2 has a shorter half-life in the bloodstream (about 14 days versus 15 for D3) and binds less efficiently to the proteins that carry vitamin D through your blood. If you’re choosing a supplement, D3 is the stronger option. D2 supplements are still useful, particularly for people who avoid animal products, but you may need a higher dose to get the same effect.
Food Sources
Few foods naturally contain significant vitamin D. Fatty fish like salmon, mackerel, and sardines are the richest natural sources. Cod liver oil is exceptionally high. Egg yolks contain small amounts, and UV-exposed mushrooms provide D2. In many countries, milk, orange juice, and breakfast cereals are fortified with vitamin D to help fill the gap. Still, most people find it difficult to meet their daily needs through food alone, which is why sunlight and supplements play such a large role.
What It Does in Your Body
Vitamin D’s best-known job is managing calcium. When your vitamin D levels drop, your intestines absorb less calcium from food. Your body responds by releasing parathyroid hormone, which pulls calcium from your bones to keep blood calcium levels stable. Over time, this borrowing weakens your skeleton. Adequate vitamin D keeps this system in balance, allowing your bones to hold onto their mineral stores.
Beyond bones, vitamin D plays a significant role in immune function. It helps your immune cells produce natural antimicrobial compounds (called cathelicidins and defensins) that destroy bacteria and viruses. At the same time, it dials down overly aggressive immune responses by shifting certain white blood cells toward a more tolerant state. This dual action, boosting pathogen defense while calming inflammation, is why vitamin D status has been linked to susceptibility to respiratory infections and to the severity of autoimmune conditions.
How Much You Need Each Day
The recommended daily amount depends on your age:
- Infants (0 to 12 months): 10 mcg (400 IU)
- Children and teens (1 to 18 years): 15 mcg (600 IU)
- Adults (19 to 70 years): 15 mcg (600 IU)
- Adults over 70: 20 mcg (800 IU)
- Pregnant or breastfeeding women: 15 mcg (600 IU)
These amounts assume minimal sun exposure. The tolerable upper limit, the most you should take without medical supervision, is 4,000 IU per day for anyone older than 8.
Blood Levels: What the Numbers Mean
A simple blood test measuring 25-hydroxyvitamin D tells you where you stand. The results are reported in either ng/mL or nmol/L:
- Below 12 ng/mL (30 nmol/L): Deficient. This level raises the risk of bone disease in both children and adults.
- 12 to 20 ng/mL (30 to 50 nmol/L): Inadequate for bone and overall health.
- 20 ng/mL (50 nmol/L) or above: Sufficient for most people, according to the National Academies of Sciences.
- Above 50 ng/mL (125 nmol/L): Potentially harmful. Levels in this range have been associated with increased rates of all-cause mortality, certain cancers, and cardiovascular events.
Many vitamin D researchers advocate maintaining levels above 30 ng/mL year-round to capture the full range of health benefits, though the official sufficiency threshold remains 20 ng/mL.
Signs of Deficiency
Mild vitamin D deficiency often has no obvious symptoms. You might feel more tired than usual or notice vague bone and muscle aches, but these are easy to dismiss. As deficiency worsens, symptoms become more distinct. Adults can develop osteomalacia, a softening of the bones that causes deep bone pain (especially in the pelvis, lower back, and legs) and muscle weakness that makes it harder to climb stairs or get up from a chair.
In children, severe deficiency causes rickets. The growing ends of bones soften, leading to bowed legs or knock-knees, thickened wrists and ankles, a protruding breastbone, delayed growth, and delayed motor development. Infants with very low blood calcium from vitamin D deficiency may develop tight muscle tone and irregular breathing patterns. Rickets is now rare in developed countries thanks to fortified foods, but it still occurs in children with limited sun exposure and inadequate dietary intake.
Too Much Vitamin D: Toxicity Risk
You cannot overdose on vitamin D from sunlight because your skin self-regulates production. Toxicity comes from supplements, typically after taking very high doses (well above 4,000 IU daily) for extended periods. The danger isn’t the vitamin D itself but the flood of calcium it causes your body to absorb.
Early symptoms of toxicity include weakness, fatigue, loss of appetite, and nausea. As calcium levels climb, you may experience confusion, excessive thirst, frequent urination, constipation, abdominal pain, and kidney stones. In severe cases, dangerously high calcium can cause heart rhythm disturbances. These problems resolve when supplementation stops and calcium levels return to normal, but kidney damage from prolonged toxicity can sometimes be lasting.
Who’s Most at Risk for Low Levels
People with darker skin, those living at higher latitudes, older adults (whose skin becomes less efficient at producing vitamin D with age), and anyone who spends most of their time indoors are all more likely to run low. Obesity also plays a role: vitamin D is fat-soluble, so it gets sequestered in fat tissue and is less available in the bloodstream. People with conditions that impair fat absorption, such as celiac disease, Crohn’s disease, or a history of gastric bypass surgery, are also at higher risk. If any of these apply to you, a blood test is the most reliable way to know whether your levels need attention.