Vitamin D deficiency means your blood levels of vitamin D have dropped low enough to affect how your body absorbs calcium, maintains bone strength, and supports other essential functions. Most experts define deficiency as a blood level below 20 ng/mL, with levels between 20 and 30 ng/mL considered insufficient. The ideal range for most people falls between 20 and 40 ng/mL, though some guidelines push the target to 30 to 50 ng/mL.
How Vitamin D Works in Your Body
Vitamin D’s most critical job is helping your intestines absorb calcium from the food you eat. Without enough of it, your gut can only passively absorb a fraction of the calcium passing through. When vitamin D levels are adequate, your body actively pulls calcium across the intestinal wall using specialized transport channels, a process that is far more efficient than passive absorption alone. Vitamin D also signals your kidneys to hold onto calcium rather than flushing it out in urine.
When both of those systems fall short, your body has a backup plan that works against you: it starts pulling calcium directly from your bones to keep blood calcium levels normal. This is why long-term deficiency weakens your skeleton. Your body will sacrifice bone density to protect the calcium levels your heart, muscles, and nerves need to function right now.
Symptoms to Recognize
Vitamin D deficiency often develops slowly, and many people have no obvious symptoms at first. When signs do appear, they tend to be vague enough that people chalk them up to aging, stress, or poor sleep. The most common symptoms in adults include fatigue, bone pain, muscle weakness, muscle aches or cramps, and mood changes like depression.
In children, mild deficiency may show up only as weak or sore muscles. Severe deficiency in children causes rickets, a condition where bones become soft enough to bend and deform during growth. In adults, the equivalent condition is called osteomalacia, where undermineralized bones become prone to fractures and painful stress injuries called pseudofractures. The underlying problem is the same in both cases: bones that can’t harden properly because they lack the calcium and phosphorus that vitamin D helps deliver.
The Link to Depression and Cognitive Health
Low vitamin D is consistently associated with higher rates of depression, particularly in older adults. Follow-up studies tracking people over time have found that deficiency is a risk factor for developing depression in the elderly, not just a marker of it. Research using a large nationally representative dataset found that depression partially mediates the relationship between low vitamin D and cognitive decline, meaning that the mood effects of deficiency may themselves contribute to thinking problems over time. This connection is especially strong in people with diabetes, where depression rates are already two to three times higher than in the general population.
Who Is Most at Risk
Your skin manufactures vitamin D when ultraviolet B rays hit a cholesterol compound sitting in the outer layers of your skin. Anything that reduces UVB exposure reduces your vitamin D production.
Skin pigmentation is one of the biggest factors. Melanin, the pigment that darkens skin, competes for the same UV light your skin needs to make vitamin D. People with dark skin may need up to ten times longer in the sun to produce the same amount of vitamin D as someone with fair skin.
Geography matters too. If you live north of about 40 degrees latitude (roughly the line from New York to Denver), the sun sits too low in the sky from November through early March to trigger meaningful vitamin D production in your skin. At higher latitudes, like Edmonton, Canada, that “vitamin D winter” stretches from October through April. People who work indoors, wear extensive sun-covering clothing, or consistently use sunscreen face similar limitations year-round.
Adults over 70 have reduced capacity to synthesize vitamin D in the skin and are more likely to spend time indoors. Obesity also plays a role, since vitamin D is fat-soluble and can become sequestered in body fat rather than circulating in the blood where it’s needed.
Digestive Conditions That Block Absorption
Because vitamin D is a fat-soluble vitamin, any condition that impairs fat digestion or absorption can cause deficiency even if your intake is adequate. Crohn’s disease damages the intestinal lining and reduces the surface area available to absorb vitamin D. Celiac disease causes similar villous damage when gluten triggers an immune response. Cystic fibrosis and chronic pancreatitis both interfere with fat digestion itself, causing vitamin D to be lost in stool rather than absorbed. People who have had portions of their small intestine surgically removed face the same challenge: less intestinal surface area means less absorption capacity.
How Much Vitamin D You Need
The recommended daily allowance assumes minimal sun exposure, so it represents what you’d need from food and supplements alone:
- Infants (0 to 12 months): 400 IU per day
- Children and adults (1 to 70 years): 600 IU per day
- Adults over 70: 800 IU per day
These are baseline recommendations for healthy people. If you’re already deficient, your provider will likely recommend a higher dose for a set period to bring your levels back up, followed by a maintenance dose.
Getting Vitamin D From Food and Sunlight
Very few foods naturally contain significant vitamin D. Fatty fish like salmon, mackerel, and sardines are the best natural sources, providing roughly 400 to 600 IU per 3-ounce serving depending on whether the fish is wild or farmed. Cod liver oil is exceptionally concentrated. Egg yolks contain small amounts, typically around 40 IU each. Beyond that, most dietary vitamin D in Western diets comes from fortified foods: milk, orange juice, breakfast cereals, and some yogurts, each typically fortified to provide about 100 to 150 IU per serving.
Sunlight remains the most efficient source for most people. Brief sun exposure on your arms and legs (roughly 10 to 30 minutes depending on skin tone, time of day, and latitude) a few times per week during warmer months can produce thousands of IU. Your body self-regulates sun-derived vitamin D, so you cannot overdose from sunlight alone.
Risks of Taking Too Much
While you can’t overdose on vitamin D from sunlight, supplements are a different story. Toxicity generally occurs at doses above 10,000 IU per day taken over time. The problem isn’t the vitamin D itself but the excess calcium it forces into your blood, a condition called hypercalcemia. Symptoms include excessive thirst, frequent urination, nausea, constipation, confusion, and muscle weakness. Over the long term, too much vitamin D can cause kidney stones, kidney damage, and calcium deposits in soft tissues. Staying below 4,000 IU daily (the tolerable upper intake for adults) is considered safe for most people without medical supervision.
How Deficiency Is Tested
A simple blood test called 25-hydroxy vitamin D measures the form of vitamin D circulating in your blood. Results come back in nanograms per milliliter (ng/mL). Below 20 ng/mL is generally considered deficient, 20 to 29 ng/mL is insufficient, and 30 ng/mL or above is sufficient for most people. The test is straightforward, widely available, and the only reliable way to know your actual vitamin D status, since symptoms alone are too nonspecific to confirm deficiency.