Vitamin D: Why 4 in 10 Europeans Fall Short and How to Get Enough

Vitamin D behaves more like a hormone than a vitamin: the liver and kidneys convert it into its active form, which helps the gut absorb calcium and phosphorus from food. And, unlike almost every other vitamin, its main source is not food but skin exposed to the sun.

How much vitamin D you need

These figures are calculated assuming you get very little sun, and they use as a reference a blood 25(OH)D level of at least 50 nmol/L: below that, EFSA sees a higher risk to bone. The IOM sets its recommended allowance to cover 97.5% of the population.

How many people fall short (and why "almost everyone" is an exaggeration)

The best European data come from Cashman and colleagues (2016), who reanalysed 18 studies with 55,844 people from different countries using the same laboratory method:

• 13% were below 30 nmol/L on average over the year, the threshold most often used to define deficiency.
• In samples taken from October to March, 17.7% were below 30 nmol/L, compared with 8.3% in summer samples.
• 40.4% were below 50 nmol/L, the level EFSA and the IOM use as a reference.
• In groups with dark skin, the prevalence below 30 nmol/L was 3 to 71 times higher than in the white population.

In other words: 4 in 10 Europeans do not reach the reference level and just over 1 in 10 have a clear deficiency. That is a lot, but it is not "almost everyone"; in fact, the IOM warned in 2011 that the prevalence of inadequacy in North America had been overestimated. The factors that weigh most, according to EFSA, are latitude and season, time outdoors, clothing, skin colour and age.

Which foods contain vitamin D (µg per serving)

Data from USDA FoodData Central. For comparison, the EFSA reference is 15 µg a day:

Three takeaways. A serving of fatty oily fish can cover the day, but nobody eats it every day. Eggs add little. And the amount in UV-treated mushrooms and fortified foods depends on the product: the milk and soy drink values are for US fortified products, so check the label of yours. If you eat oily fish regularly, you also cover your omega-3.

The sun: what is known (without making up minutes)

Skin makes vitamin D3 when UVB radiation transforms 7-dehydrocholesterol. How much it makes depends on so many things (latitude, season, time of day, cloud, exposed skin, skin colour and age) that no number of minutes works for everyone. Three solid facts:

• In winter, far from the equator, the sun is not enough. In the classic study by Webb (1988), sunlight produced no previtamin D3 in Boston (42° N) from November to February, or in Edmonton (52° N) from October to March; further south, at 34° N, it did so even in midwinter.
• In summer it can be the main source: EFSA acknowledges this, and the NHS considers that, in the UK, most people get what they need from sunlight between late March or early April and the end of September.
• Sunscreen is not the culprit: in the lab it blocks synthesis, but in real life studies found little link with lower levels, although very high protection factors have been little studied (Neale, 2019). There is no need to stop protecting your skin for the sake of vitamin D.

What supplement trials have found

Observational studies link low levels to many diseases, but when vitamin D has been tested in trials, the results in people without deficiency have been modest:

• VITAL (2019): 25,871 US adults (men from age 50 and women from age 55) took 2,000 IU (50 µg) a day or placebo for a median of 5.3 years. There was no reduction in invasive cancer (hazard ratio 0.96) or in major cardiovascular events (0.97).
• Fractures in VITAL (2022): total, non-vertebral and hip fractures did not go down either. Participants were not selected for deficiency, low bone mass or osteoporosis.
• Bolland meta-analysis (2018): 81 trials with 53,537 people. Vitamin D did not reduce total fractures, hip fractures or falls, nor did it meaningfully improve bone density, at high or low doses.
• D3 or D2: in a meta-analysis of trials, D3 raised blood levels more than D2 when given in spaced-out doses; with daily dosing, the difference disappeared (Tripkovic, 2012).

What these trials do not say is that vitamin D does not matter: a marked deficiency causes rickets in children and osteomalacia in adults, and correcting it is a different matter. What they show is that taking a supplement without being deficient has not protected against cancer, heart attacks or fractures. That is why the 2024 Endocrine Society guideline limits untested supplementation to children and teenagers, adults over 75, pregnancy and high-risk prediabetes, and advises against routine testing in healthy people. For bones, vitamin D goes hand in hand with calcium.

Practical summary: sensible sun in spring and summer, oily fish in your week (you can log it with a photo in Renzy, which estimates calories and macros that you then correct) and, if you live where winters are long, belong to a risk group or are concerned, talk to your doctor before taking a supplement.