Vitamin D is unusual: your skin makes it when exposed to sunlight, and few foods contain much of it naturally. That is why low vitamin D is common worldwide, especially in winter, in people with darker skin and in those who spend most of their time indoors. This guide covers the symptoms of vitamin D deficiency, what causes it, how it is tested and how much you need.
Why vitamin D matters
- Bone health: vitamin D helps your gut absorb calcium and phosphorus, which are essential for strong bones and teeth.
- Muscle function: low levels are linked to weakness and a higher risk of falls in older adults.
- Immune function: immune cells have vitamin D receptors, and the vitamin helps regulate immune responses.
Symptoms of vitamin D deficiency
Many people with low vitamin D have no symptoms at all. When symptoms do appear, they are often vague and easily blamed on other things:
- Fatigue and low energy
- Bone pain or aching, often in the lower back, hips or legs
- Muscle weakness, aches or cramps
- Difficulty climbing stairs or getting up from a chair (in more severe cases)
- Low mood, which has been associated with low vitamin D in studies
- Getting sick more often
Severe, long-term deficiency softens the bones, causing rickets in children (bowed legs, delayed growth) and osteomalacia in adults (bone pain and weakness). It also contributes to osteoporosis and fractures.
Who is at risk of low vitamin D?
- People with darker skin, since melanin reduces vitamin D production from sunlight.
- People who get little sun: indoor workers, people who cover most of their skin, those living far from the equator, especially in winter.
- Older adults, whose skin makes less vitamin D.
- People with obesity, as vitamin D is stored in fat tissue and less is available in the blood.
- People with conditions affecting fat absorption, such as Crohn’s disease, celiac disease or after weight-loss surgery.
- People with kidney or liver disease, which affects vitamin D activation.
- Breastfed infants, since breast milk is low in vitamin D.
- People taking certain medicines, including some anti-seizure drugs and long-term steroids.
How vitamin D deficiency is diagnosed
A blood test for 25-hydroxyvitamin D [25(OH)D] is the standard. The US National Institutes of Health uses these ranges:
| Status | ng/mL | nmol/L |
|---|---|---|
| Deficient | Below 12 | Below 30 |
| Inadequate for some people | 12–20 | 30–50 |
| Adequate for most people | 20 and above | 50 and above |
| Potentially too high | Above 50 | Above 125 |
Some organizations use slightly different cut-offs. Routine testing is not recommended for everyone; your doctor will usually test if you have symptoms or risk factors.
How much vitamin D do you need per day?
| Age | Recommended daily amount | Upper limit |
|---|---|---|
| 0–12 months | 400 IU (10 mcg) | 1,000–1,500 IU |
| 1–70 years | 600 IU (15 mcg) | 2,500–4,000 IU (by age) |
| 71+ years | 800 IU (20 mcg) | 4,000 IU |
| Pregnancy and breastfeeding | 600 IU (15 mcg) | 4,000 IU |
If you are diagnosed with a deficiency, your doctor may prescribe a higher dose for a set period to restore your levels, then re-check. Do not take high doses long-term on your own.
Best food sources of vitamin D
| Food | Serving | Approx. vitamin D |
|---|---|---|
| Cod liver oil | 1 tbsp | 1,360 IU |
| Rainbow trout, farmed, cooked | 3 oz (85 g) | 645 IU |
| Sockeye salmon, cooked | 3 oz (85 g) | 570 IU |
| UV-exposed mushrooms | ½ cup | about 365 IU |
| Fortified milk or plant milk | 1 cup | about 100–120 IU |
| Canned sardines | 2 sardines | about 45 IU |
| Egg yolk | 1 large | about 45 IU |
Sunlight and vitamin D
Short periods of midday sun on bare arms and legs can produce a good amount of vitamin D for many people during spring and summer. How much you make depends on skin tone, latitude, season and time of day. In winter at higher latitudes, the sun is often too weak to make much at all. Balance sun exposure against skin cancer risk and avoid burning.
Choosing a vitamin D supplement
- D3 (cholecalciferol) raises blood levels more effectively than D2 (ergocalciferol) in most studies.
- Take it with a meal that contains some fat to improve absorption.
- Look for products with third-party testing seals.
- Vitamin D works closely with magnesium; see our guide to magnesium types and benefits.
Can you take too much vitamin D?
Yes. Vitamin D toxicity almost always comes from high-dose supplements, not sun or food. It raises blood calcium, causing nausea, vomiting, weakness, confusion, excessive thirst, kidney stones and heart rhythm problems. Stay within the upper limit unless your doctor advises otherwise.
Frequently asked questions
How long does it take to fix low vitamin D?
With supplements, blood levels typically rise over 2–3 months. Doctors usually re-test after about three months.
Can low vitamin D cause weight gain?
Low vitamin D and obesity are linked, but the evidence suggests excess body fat lowers vitamin D levels rather than the other way round. Supplements have not been shown to cause weight loss.
What time of day should I take vitamin D?
Any time works. Taking it with your largest meal of the day can help absorption, and a consistent routine makes it easier to remember.
Medical disclaimer: This article is for general education only and is not a substitute for personal medical advice. Talk to your doctor before taking vitamin D supplements, especially at high doses or if you have kidney disease, sarcoidosis or take other medications.
