Vitamin D is a fat-soluble nutrient your body needs to absorb calcium, maintain strong bones, and support immune function. Despite its name, it’s technically not a vitamin at all. It’s a prohormone, meaning your body converts it into a hormone through a series of steps involving your liver and kidneys. Most people get vitamin D from a combination of sunlight, food, and supplements, but deficiency is remarkably common, with blood levels below 20 ng/mL considered deficient.
Why Vitamin D Is Really a Hormone
Vitamins, by definition, are nutrients your body can’t produce on its own. Vitamin D breaks that rule. When ultraviolet B (UVB) rays from sunlight hit your skin, they trigger a chemical reaction that converts a cholesterol compound already present in your skin cells into an early form of vitamin D. This is why vitamin D was reclassified from a vitamin to a prohormone after scientists realized your body manufactures it.
The vitamin D your skin produces (or that you swallow in food or supplements) is inactive. It can’t do anything useful until your body processes it in two steps. First, your liver converts it into a circulating form that travels through your bloodstream. This is the form doctors measure in blood tests. Then your kidneys convert that circulating form into the fully active hormone, which is the most potent version and responsible for nearly all of vitamin D’s biological effects. Immune cells, bone cells, and several other tissues can also perform this final activation step on a smaller scale.
What Vitamin D Does in Your Body
The most well-established role of vitamin D is helping your intestines absorb calcium. The active hormone opens calcium channels in the cells lining your small intestine, allowing calcium to pass through into your bloodstream. It also loosens the junctions between intestinal cells so calcium can slip through the gaps between them. Without enough vitamin D, your gut absorbs only a fraction of the calcium you eat, no matter how much dairy or calcium-rich food is in your diet.
Beyond calcium absorption, vitamin D helps regulate phosphorus levels, supports bone mineralization, and plays a role in immune cell function. Your muscles also depend on it. Deficiency is linked to proximal muscle weakness, particularly in the thighs and around the knees, which can progress to a characteristic waddling gait in severe cases.
D2 vs. D3: Which Form Is Better
Vitamin D comes in two main forms. D2 (ergocalciferol) comes from plant sources like UV-exposed mushrooms and some fortified foods. D3 (cholecalciferol) is the form your skin produces and is also found in animal-based foods like fatty fish and egg yolks.
D3 raises blood levels more effectively. In a 12-month trial comparing the two forms, participants taking D3 saw their blood levels rise by an average of 8.5 ng/mL, while those taking D2 saw only a 4.7 ng/mL increase. More notably, D2 failed to raise levels at all in people who weren’t already deficient, while D3 continued working regardless of starting levels. If you’re choosing a supplement, D3 is the more reliable option.
Best Food Sources
Very few foods naturally contain meaningful amounts of vitamin D. Fatty fish is the standout. A 3-ounce serving of cooked rainbow trout delivers 645 IU, and sockeye salmon provides about 570 IU per serving. After that, the numbers drop steeply: canned sardines have just 46 IU per two-sardine serving, and canned light tuna offers around 40 IU.
Fortified foods fill some of the gap. A cup of vitamin D-fortified milk contains about 120 IU, and fortified plant milks (soy, almond, oat) range from 100 to 144 IU per cup. White mushrooms exposed to UV light are a surprisingly strong source at 366 IU per half cup, but regular mushrooms that haven’t been UV-treated contain almost none (4 IU for the same amount of portabella). Check the label, as UV treatment is usually noted on the packaging.
How Much You Need
The recommended daily intake for most children and adults (ages 1 through 70) is 600 IU (15 mcg) per day. Adults over 70 need 800 IU (20 mcg). Pregnant and breastfeeding women fall in the 600 IU range. Infants under 1 year need 400 IU (10 mcg) daily.
The tolerable upper intake level for adults is 4,000 IU (100 mcg) per day. For younger children, the ceiling is lower: 2,500 IU for ages 1 through 3, and 3,000 IU for ages 4 through 8. Infants have the lowest limits at 1,000 to 1,500 IU depending on age.
Blood Levels and What They Mean
A simple blood test measuring your circulating vitamin D level (called a 25-hydroxyvitamin D test) is how deficiency is diagnosed. The widely accepted thresholds are:
- Below 20 ng/mL: deficient
- 21 to 29 ng/mL: insufficient
- 30 ng/mL or above: sufficient
Many experts recommend maintaining levels above 30 ng/mL to get the full range of health benefits. If your doctor orders bloodwork for vitamin D, these are the numbers you’ll see on your results.
What Happens When Levels Are Too Low
In children, severe vitamin D deficiency causes rickets, a condition where bones become soft and bend under the body’s weight because they can’t mineralize properly. This only occurs in growing children and adolescents whose growth plates haven’t yet closed.
In adults, the equivalent condition is osteomalacia, which literally means “soft bones.” Symptoms include diffuse bone pain and tenderness, particularly in the ribs, hips, and spine. Muscle weakness is extremely common, affecting anywhere from 44% to 100% of people with osteomalacia depending on how severe and prolonged the deficiency has been. The weakness tends to hit the thigh and hip muscles hardest, making it difficult to climb stairs or stand up from a chair. Fragility fractures can occur in characteristic locations: ribs, shoulder blades, pelvis, and upper thighbones.
Mild to moderate deficiency may not produce obvious symptoms for years, which is part of why it goes undetected so often. Over time, though, it quietly reduces calcium absorption and weakens bone density.
Too Much Vitamin D: Toxicity Risks
Vitamin D toxicity doesn’t happen from sunlight or food. It comes from excessive supplementation. The hallmark of toxicity is high blood calcium levels, which can cause loss of appetite, weight loss, excessive urination, and heart rhythm disturbances. If it continues, calcium deposits can form in blood vessels and organs, leading to kidney and cardiovascular damage.
Staying below the 4,000 IU daily upper limit for adults provides a wide safety margin. Some people take higher doses under medical supervision to correct a diagnosed deficiency, but routine megadosing without monitoring carries real risk.