Vitamin D helps the body absorb calcium and phosphate and is important for bones, teeth and muscles. It also has roles in immune function. However, describing vitamin D as an “immune booster” can be misleading: taking more than the body needs has not been shown to prevent every infection and excessive doses can be harmful.
Where vitamin D comes from
The skin produces vitamin D when it is exposed to ultraviolet B radiation from sunlight. The amount varies with season, latitude, time of day, skin pigmentation, age, clothing and time spent outdoors. Glass blocks most UVB, so sunlight through a window is not an effective source.
Food sources include oily fish, egg yolks and fortified foods. It can be difficult to obtain enough from food alone, which is why national authorities may recommend supplements during months with limited sunlight or for groups at higher risk. Follow advice for the country where you live.
Who is more likely to have low vitamin D?
- people with little outdoor exposure
- people who cover most of their skin outdoors
- people with darker skin living in regions with limited sunlight
- older adults and people living in residential care
- people with conditions that reduce fat absorption
- people taking certain medicines that affect vitamin D metabolism
Symptoms and testing
Mild deficiency may cause no obvious symptoms. Severe deficiency can lead to osteomalacia in adults and rickets in children, with bone pain, muscle weakness or skeletal problems. These symptoms have many possible causes.
Routine blood testing is not necessary for everyone. A healthcare professional may recommend a test when symptoms, risk factors or another medical condition make deficiency more likely.
Vitamin D and infections
Researchers have studied whether supplements reduce the risk or severity of respiratory infections and other illnesses. Results differ between populations, doses and baseline vitamin D levels. Vitamin D should not replace vaccination, hygiene measures, medical assessment or treatment for an infection.
Sunlight and skin safety
Sun exposure is not risk-free. Ultraviolet radiation increases the risk of sunburn, premature skin ageing and skin cancer. Do not deliberately burn or use tanning beds to raise vitamin D. Balance vitamin D advice with local sun-protection guidance.
Can supplements be harmful?
Yes. Very high intake over time can cause excessive calcium in the blood, leading to nausea, weakness, kidney stones, kidney damage or heart-rhythm problems. Supplements can also interact with medicines. Do not use high-dose products unless a clinician has recommended and monitored them.
Practical advice
- Follow your national recommendation for dose and season.
- Check labels to avoid taking vitamin D from several products unintentionally.
- Ask for advice for infants, pregnancy, kidney disease, malabsorption or prescribed medicines.
- Do not assume that fatigue alone proves vitamin D deficiency.
This article provides general educational information and does not replace individual medical advice.
Sources
- NHS: Vitamin D
- US NIH Office of Dietary Supplements: Vitamin D
- World Health Organization: Ultraviolet Radiation
Last reviewed: 28 August 2026