Vitamin D: What you should know

Why your body needs vitamin D

Vitamin D supports calcium absorption and contributes to healthy bones, muscles and normal immune function. The body produces it in the skin using UV-B radiation. Foods such as oily fish also provide vitamin D, but diet alone often supplies too little.

Production in the skin depends on factors including season, age, skin pigmentation and time outdoors. During winter, the body can draw on previously accumulated stores. Limited sunlight therefore does not immediately mean deficiency, but it can worsen an already low supply.

What your blood level tells you

Vitamin D status is usually assessed by measuring 25-hydroxyvitamin D, or 25(OH)D. Results are commonly reported in ng/ml or nmol/l.

The Robert Koch Institute uses the following categories, among others, in relation to bone health:

• Below 12 ng/ml or 30 nmol/l: Deficient supply
• 12 to below 20 ng/ml or 30 to below 50 nmol/l: Suboptimal supply
• From 20 ng/ml or 50 nmol/l: Generally adequate for bone health

The final row is not a recommendation to keep increasing the level. It describes a threshold generally considered adequate for bone health.

“Not deficient” and “optimally supplied” do not necessarily mean the same thing. A result above the deficiency threshold may still fall below an adequate level. Higher target ranges are also discussed in scientific literature. However, no universal optimal level has been established for every health outcome. Medical conditions, measurement methods and treatment goals all matter when interpreting an individual result.

When supplementation may be useful

Confirmed deficiency is a clear reason to improve vitamin D status. Targeted supplementation may also form part of treatment for impaired intestinal absorption or certain bone conditions.

Recent research also suggests benefits in specific groups: for people with prediabetes, supplementation may modestly reduce the risk of developing type 2 diabetes. It complements measures such as physical activity and dietary changes. The Endocrine Society guideline also supports preventive supplementation in adults aged 75 and older, because of a potential reduction in mortality. Previously confirmed deficiency is not always required in these situations.

Supplementation can therefore be useful preventively. The appropriate dose and duration depend on the reason for taking it and the individual’s circumstances.

What else recent research shows

Researchers are investigating effects beyond bones and muscles. Trials suggest fewer newly diagnosed autoimmune diseases during supplementation. Analyses of cancer mortality show a possible favourable signal particularly with daily dosing; across all dosing schedules, the overall benefit was uncertain. These findings do not establish a general recommendation to prevent these diseases.

An updated large analysis published in 2025 found no statistically significant overall protection against acute respiratory infections. Another 2025 study reported slower shortening of a cellular ageing marker. Whether this translates into fewer illnesses or a longer life remains unknown. General protection against cardiovascular disease has also not been convincingly established.

Who took part matters when interpreting these findings: results in people who already have adequate levels do not automatically answer questions about benefits in deficiency. Likewise, associations between higher achieved blood levels and better outcomes do not prove that everyone should aim for a particular target.

How to assess your own situation

A blood test may help when deficiency is suspected or certain medical conditions are present. Routine testing is not generally recommended for healthy adults without relevant indications.

If you already take vitamin D, note the product, dose and frequency. Together with previous laboratory results, this helps your doctor assess your total intake. Different supplements can add up unnoticed; persistently excessive amounts can raise calcium levels and damage the kidneys, among other effects.

The aim is a supply that fits your health needs. A useful question to discuss is: Do I need a supplement — and if so, how much and for how long?