Vitamin D deficiency means your blood levels of vitamin D have dropped low enough to affect your bone health and overall well-being. A simple blood test measures it, and roughly 48% of the global population has levels below the adequate threshold. The condition develops gradually, often without obvious symptoms at first, which is why many people don’t realize they have it.
How Deficiency Is Measured
The standard test measures a form of vitamin D called 25-hydroxyvitamin D in your blood. It’s the most accurate snapshot of your vitamin D status because it reflects both what you get from sunlight and what you absorb from food or supplements. Results are reported in nanograms per milliliter (ng/mL), and they fall into a few key ranges:
- Below 12 ng/mL: Deficient. At this level, you’re at risk for bone diseases like rickets (in children) or osteomalacia (in adults).
- 12 to 20 ng/mL: Inadequate. Your bones and general health may be affected, even without noticeable symptoms.
- 20 ng/mL or above: Sufficient for most people.
- Above 50 ng/mL: Potentially harmful, with risks increasing above 60 ng/mL.
A pooled analysis of 7.9 million participants across studies from 2000 to 2022 found that about 16% of people worldwide fall into the deficient category (below 12 ng/mL), while nearly half have levels below 20 ng/mL. This makes vitamin D deficiency one of the most common nutritional shortfalls in the world.
What Causes Low Vitamin D
Your body produces vitamin D when ultraviolet B rays from sunlight hit your skin. That makes geography one of the biggest factors: if you live at a higher latitude, work indoors, or spend most daylight hours inside, your skin simply doesn’t get enough UV exposure to keep up with your body’s needs. Sunscreen, while important for skin cancer prevention, also reduces vitamin D synthesis.
Skin color plays a measurable role. Melanin, the pigment that gives skin its color, acts as a natural sunscreen. Darker skin requires more sun exposure to produce the same amount of vitamin D as lighter skin. Research comparing the lightest and darkest skin types found a melanin inhibition factor of roughly 1.3 to 1.4, meaning darker skin produces vitamin D at a noticeably slower rate under the same UV conditions.
Age matters too. Your skin’s ability to manufacture vitamin D declines as you get older, putting people over 65 at higher risk. At the other end of the spectrum, breastfed infants are vulnerable because breast milk contains only small amounts of vitamin D. Several digestive conditions, including Crohn’s disease, celiac disease, and cystic fibrosis, can prevent your intestines from absorbing vitamin D properly. Obesity is another risk factor: fat cells trap vitamin D and keep it from circulating in the bloodstream. People with a BMI above 30 consistently show lower vitamin D levels. Weight loss surgeries that alter the digestive tract can compound this problem.
Symptoms You Might Notice
Mild deficiency often produces no symptoms at all, which is part of what makes it so widespread. As levels drop further, the signs tend to be vague and easy to attribute to other causes. Fatigue and a general sense of not feeling well are common early complaints. Bone pain or tenderness, particularly in the lower back, pelvis, or legs, can develop as the deficiency worsens. Muscle weakness, frequent infections, and mood changes (including low mood during winter months) are also associated with inadequate vitamin D.
In children, the symptoms can be more dramatic. Delayed growth, bowed legs, and soft skull bones are hallmarks of rickets, the childhood form of severe vitamin D deficiency. In adults, the equivalent condition is called osteomalacia, which causes widespread bone pain and muscle weakness that can make it difficult to climb stairs or get up from a chair.
Long-Term Health Risks
Left untreated, vitamin D deficiency weakens bones in a specific way. Vitamin D is essential for absorbing calcium from food. Without enough of it, your body can’t mineralize bone properly, leaving bones soft and prone to fractures. In children with rickets, this can lead to permanent bone deformities, dental defects, growth failure, and in severe cases, seizures.
In adults, prolonged deficiency contributes to osteoporosis by accelerating bone density loss. This is especially concerning for postmenopausal women and older adults who are already losing bone mass naturally. The combination of low vitamin D and low calcium absorption creates a cycle where bones become progressively more fragile.
How It’s Diagnosed
Diagnosis requires a blood draw. The test is sometimes ordered as part of routine bloodwork, but more often it’s requested when you report symptoms or have known risk factors. Your results will typically be categorized as deficient, insufficient, sufficient, or (rarely) toxic. Because vitamin D levels fluctuate with the seasons, a reading taken in late winter may look different from one taken in summer. A single test gives a useful snapshot, but your doctor may retest after a few months of treatment to see whether your levels have responded.
Recommended Daily Intake
The daily recommended amount varies by age. Children under 12 months need 400 IU. Anyone between age 1 and 70 needs 600 IU. Adults over 70 need 800 IU, reflecting the skin’s reduced ability to produce vitamin D with age. These numbers represent the amount that meets the needs of most healthy people, not the amount needed to correct a diagnosed deficiency, which is typically higher and determined on a case-by-case basis.
Getting Enough From Food and Sunlight
Very few foods naturally contain significant vitamin D. Fatty fish like salmon, mackerel, and sardines are the richest natural sources. Cod liver oil is particularly concentrated. Egg yolks, beef liver, and certain mushrooms exposed to UV light contain smaller amounts. In many countries, milk, orange juice, and breakfast cereals are fortified with vitamin D, which helps close the gap but rarely covers the full daily requirement on its own.
Sunlight remains the most efficient source. For many people with lighter skin, 10 to 30 minutes of midday sun exposure on the face, arms, and legs a few times per week can maintain adequate levels during warmer months. People with darker skin need longer exposure to produce the same amount. During winter at higher latitudes, the sun’s angle is too low for UV-B rays to trigger meaningful production, making supplementation more important during those months.
Supplements come in two forms: D2 and D3. D3 is generally preferred because it raises blood levels more effectively and maintains them longer. For people with a diagnosed deficiency, a healthcare provider will often prescribe a higher-dose regimen for several weeks to restore levels before switching to a maintenance dose. Blood levels above 50 ng/mL are associated with potential harm, including elevated calcium that can cause nausea, kidney problems, and heart rhythm issues, so more is not always better.