Vitamin D helps your body absorb calcium and keep bones strong, and it plays a role in muscle, nerve and immune function. Low vitamin D is common, particularly in people who spend little time outdoors or have darker skin. This guide covers the signs of deficiency, what your blood test means, how much you need and how to raise your level safely.
Why vitamin D matters
Your body needs vitamin D to absorb calcium and phosphorus from food. Without enough of it, bones can become thin, soft or brittle.
- In children, severe deficiency can cause rickets, which leads to soft, bent bones.
- In adults, it can cause osteomalacia, which causes bone pain and muscle weakness, and it contributes to osteoporosis and fractures.
Vitamin D also supports muscle strength, which helps prevent falls in older adults.
Where vitamin D comes from
- Sunlight: your skin makes vitamin D when exposed to UVB rays. How much depends on season, latitude, time of day, skin tone, age and sunscreen use.
- Food: few foods contain much naturally. The best sources are fatty fish such as salmon, trout, sardines and mackerel, plus cod liver oil, egg yolks and UV-exposed mushrooms.
- Fortified foods: in the United States, most milk is fortified, and many plant milks, breakfast cereals, orange juices and yogurts are too.
- Supplements: available as vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol).
Symptoms of vitamin D deficiency
Mild deficiency often causes no symptoms at all. When symptoms do appear, they can include:
- Bone pain or aching, especially in the lower back, hips or legs
- Muscle weakness, aches or cramps
- Fatigue
- Difficulty climbing stairs or getting up from a chair
- Frequent fractures or low bone density on a scan
- In children: delayed growth, bowed legs or delayed walking
These symptoms are not specific to vitamin D, so a blood test is the only way to confirm a deficiency.
Who is at higher risk
- Older adults, whose skin makes less vitamin D
- People with darker skin
- People who are housebound, live in care homes or cover most of their skin
- Breastfed infants who do not receive a vitamin D supplement
- People with obesity
- People with conditions that affect fat absorption, such as Crohn’s disease, celiac disease or after some weight-loss surgery
- People with chronic kidney or liver disease
- People taking certain medicines, such as some anti-seizure drugs and long-term glucocorticoids
Understanding your blood test
The best test measures 25-hydroxyvitamin D, often written as 25(OH)D. The NIH Office of Dietary Supplements summarizes the commonly used ranges:
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| 25(OH)D level | What it generally means |
|---|---|
| Below 12 ng/mL (30 nmol/L) | Deficient; linked to rickets and osteomalacia |
| 12–20 ng/mL (30–50 nmol/L) | Generally considered inadequate for bone health |
| 20 ng/mL (50 nmol/L) or above | Adequate for most people |
| Above 50 ng/mL (125 nmol/L) | Linked to potential adverse effects; no added benefit shown |
Some laboratories and specialist groups use a higher cut-off, such as 30 ng/mL. Your clinician will interpret your result alongside your health history. Routine screening of healthy adults without risk factors is generally not recommended.
How much vitamin D do you need?
The Recommended Dietary Allowance (RDA) in the United States is:
| Age | Daily amount |
|---|---|
| 0–12 months | 400 IU (10 mcg) |
| 1–70 years | 600 IU (15 mcg) |
| 71 years and older | 800 IU (20 mcg) |
| Pregnancy and breastfeeding | 600 IU (15 mcg) |
The tolerable upper intake level for adults and children aged 9 and over is 4,000 IU (100 mcg) a day from all sources, unless a clinician prescribes more for a diagnosed deficiency.
How deficiency is treated
If your level is low, your clinician may recommend:
- A supplement, often vitamin D3, at a dose matched to how low your level is. People with a significant deficiency may be given a higher dose for a set period, followed by a lower daily maintenance dose.
- Calcium, from diet or a supplement, if your intake is low.
- A follow-up blood test after a few months to check the response.
- Investigation of the cause, such as an absorption problem.
Vitamin D is a fat-soluble vitamin, so it may be absorbed better when taken with a meal that contains some fat.
Can you take too much?
Yes. Vitamin D toxicity is almost always caused by taking very high doses of supplements over time, not by sunlight or food. Too much vitamin D raises blood calcium, which can cause nausea, vomiting, weakness, confusion, excessive thirst, frequent urination, kidney stones and, in severe cases, heart rhythm problems. Do not take high-dose vitamin D unless a clinician advises it and monitors your levels.
Safe sun exposure
Short periods of sun on the arms and face can help your skin make vitamin D, but UV exposure also raises the risk of skin cancer. Most health organizations recommend getting vitamin D from food and supplements when needed rather than from deliberate sunbathing or tanning beds.
Frequently asked questions
Is vitamin D3 better than D2?
Both raise blood levels, but vitamin D3 tends to raise and maintain levels somewhat more effectively, so it is often preferred.
Can vitamin D prevent colds or depression?
Research on vitamin D for infections, depression, heart disease and cancer prevention has shown mixed or modest results in people who are not deficient. Its best-established role is in bone and muscle health.
Should babies take vitamin D?
Breastfed and partially breastfed infants are generally advised to receive 400 IU of vitamin D a day starting soon after birth. Ask your pediatrician for guidance.
The bottom line
Vitamin D deficiency is common and often silent. If you have risk factors or symptoms like bone pain and muscle weakness, ask about a 25(OH)D blood test. Most people can meet their needs with food, sensible sun habits and, when needed, a modest supplement.
This article is for general information and is not a substitute for advice from a qualified healthcare professional.