Vitamin D is a fat-soluble nutrient your body makes when your skin is exposed to sunlight. It also comes from certain foods and supplements. When a blood test shows low vitamin D, it means the level of this nutrient in your blood is below the range most medical organizations consider adequate for bone and muscle health. The most commonly used threshold is a blood level of 20 nanograms per milliliter (ng/mL) or higher for the form called 25-hydroxyvitamin D. Below that, your body may not absorb calcium efficiently, and over time that can affect your bones and muscles.
What Does Low Vitamin D Mean?
A low vitamin D level means your blood contains less 25-hydroxyvitamin D than the standard reference range. This is the form your doctor measures because it reflects vitamin D from both sunlight and food and stays in your blood longer than the active form.
The most widely cited cutoff is 20 ng/mL. Levels between 12 and 20 ng/mL are often called insufficient. Below 12 ng/mL is typically classified as deficient. Some organizations, including the Endocrine Society, have used a higher threshold of 30 ng/mL for certain at-risk groups, but that number is debated. A large body of research has not consistently shown that pushing levels from 20 to 30 ng/mL produces better health outcomes for most people.
One non-obvious point: the test measures a storage form, not the active hormone. Your body converts 25-hydroxyvitamin D into its active form as needed. So a low reading tells you the tank is low, not necessarily that your body is failing. It tells you the supply is short.
What Causes Low Vitamin D?
Low vitamin D almost always comes from limited sun exposure, inadequate intake, or a body that cannot absorb or process the vitamin properly. In the US, insufficient sun exposure is the most common reason.
Your skin makes vitamin D from ultraviolet B (UVB) rays. Several things reduce how much you produce:
- Limited sun exposure. Living in northern latitudes, working indoors, wearing covering clothing, or using sunscreen consistently all reduce UVB reaching the skin.
- Darker skin pigmentation. More melanin means your skin needs longer sun exposure to make the same amount of vitamin D.
- Older age. Skin becomes less efficient at producing vitamin D as you get older.
- Obesity. Vitamin D is stored in fat tissue. People with a higher body fat percentage tend to have lower blood levels, possibly because the vitamin is sequestered in fat.
- Gut absorption problems. Conditions such as celiac disease, Crohn’s disease, and cystic fibrosis reduce fat absorption, and vitamin D needs fat to be absorbed.
- Kidney or liver disease. These organs convert vitamin D into its active form. When they are impaired, levels can drop.
- Certain medications. Some drugs, including certain anticonvulsants and steroids, speed up the breakdown of vitamin D.
Breastfed infants are a special case. Breast milk does not contain enough vitamin D on its own, and infants should not be exposed to direct sunlight. This is why pediatric guidelines recommend vitamin D supplementation for all breastfed infants. This is an established recommendation, not a debated one.
What Are the Symptoms of Low Vitamin D?
Most people with mildly low vitamin D have no symptoms at all. That is the single most important thing to understand. You cannot feel a level of 18 ng/mL.
When symptoms do appear, they tend to show up with more significant or prolonged deficiency:
- Bone pain, especially in the lower back, hips, and legs
- Muscle weakness or aching
- Fatigue that does not improve with rest
- Frequent falls or balance problems, particularly in older adults
- Mood changes, though the evidence linking vitamin D to depression is mixed
In severe cases, deficiency can cause osteomalacia in adults, a softening of the bones that leads to pain and weakness. In children, severe deficiency causes rickets, a condition where bones fail to mineralize properly, leading to bowed legs and stunted growth. Rickets is now rare in the US but still occurs.
Some research also links low vitamin D to increased risk of respiratory infections, autoimmune conditions, and cardiovascular disease. These associations are real in observational studies, but observational studies cannot prove that low vitamin D causes these conditions. It is possible that illness lowers vitamin D levels rather than the reverse.
How Is Low Vitamin D Diagnosed?
Low vitamin D is diagnosed with a blood test that measures 25-hydroxyvitamin D. This is the standard test. Your doctor may order it if you have symptoms, risk factors, or a condition that affects vitamin D absorption.
The test result is reported in ng/mL or nmol/L, depending on the lab. To convert nmol/L to ng/mL, divide by 2.5. So 50 nmol/L equals 20 ng/mL.
Not everyone needs testing. For healthy adults with no symptoms and no risk factors, routine screening is not universally recommended. The US Preventive Services Task Force has stated that the evidence is insufficient to recommend screening for vitamin D deficiency in asymptomatic adults. That position is about screening the general population, not about testing people who have a reason to be tested.
How Is Low Vitamin D Treated?
Treatment depends on how low your level is and why it is low. For mild insufficiency, your doctor may recommend a daily supplement, more dietary vitamin D, or sensible sun exposure.
For significant deficiency, doctors commonly prescribe higher doses of vitamin D3 (cholecalciferol) for a period of weeks, followed by a lower maintenance dose. Vitamin D2 (ergocalciferol) is also used. Both raise blood levels, though some evidence suggests D3 raises them slightly more effectively. The exact dosing is individualized and should come from your doctor, not from a general article.
Food sources of vitamin D are limited. Fatty fish like salmon and mackerel, egg yolks, and fortified milk and cereals are the main ones. It is difficult to get enough from food alone, which is why fortified products and supplements matter so much in countries with limited sun.
What about outcomes? This is where the evidence gets more cautious. Large randomized trials have tested whether vitamin D supplements prevent fractures, cancer, or heart disease in generally healthy adults. Most of these trials have not shown clear benefit for those outcomes in people who were not deficient to begin with. The strongest evidence for supplementation is in preventing and treating deficiency itself, and in supporting bone health in people who are deficient.
Who Is Most at Risk for Low Vitamin D?
Certain groups are more likely to have low vitamin D. If you fall into one or more of these categories, it is reasonable to ask your doctor about testing.
- Older adults, especially those who are housebound or in care facilities
- People with darker skin living in northern latitudes
- People who cover their skin for religious or cultural reasons
- Exclusively breastfed infants
- People with obesity or who have had gastric bypass surgery
- People with conditions that impair fat absorption
- People taking certain medications that affect vitamin D metabolism
- People with limited sun exposure due to illness, work, or lifestyle
Living north of about 35 degrees latitude — roughly a line from North Carolina to California — means winter sunlight is too weak for your skin to make vitamin D for several months of the year. This is a well-established geographic factor.
Can You Have Low Vitamin D and Not Know It?
Yes. Most people with low vitamin D feel completely normal. There is no reliable way to detect it from how you feel. This is why testing exists and why symptoms alone are a poor guide.
That said, this does not mean everyone should rush to get tested. The evidence does not support widespread screening of healthy adults with no risk factors. The useful question is whether you have a reason to be tested — a symptom, a risk factor, or a condition that affects absorption.
If you do have low vitamin D, the good news is that it is usually correctable. The important thing is to treat the cause, not just the number. A supplement can raise your blood level, but if the underlying reason is poor absorption or a medication, that needs attention too.
Frequently Asked Questions
What vitamin D level is considered low?
A level below 20 ng/mL is generally considered low, with levels below 12 ng/mL classified as deficient. Some organizations use a higher threshold of 30 ng/mL, but that cutoff is debated.
Can low vitamin D cause fatigue?
Fatigue is sometimes reported with low vitamin D, but the evidence linking the two is mixed. Many people with low levels have no symptoms at all.
How long does it take to fix low vitamin D?
With treatment, blood levels often rise within a few weeks to a few months, depending on the dose and the cause. Your doctor may recheck your level after starting treatment to confirm it is working.
Does low vitamin D always need treatment?
Not always. Mild insufficiency may be addressed with diet and sun exposure, while significant deficiency usually needs supplementation. Your doctor can determine the right approach based on your level and health history.
