Vitamin D deficiency is so common in Central and Northern Europe that calling it an epidemic is barely an exaggeration. Across Europe, about 40% of people have levels below 20 ng/mL (50 nmol/L), and about 13% below 12 ng/mL (30 nmol/L). In Germany, the national health survey found about 60% of adults below 20 ng/mL and about 30% below 12 ng/mL, with the lowest levels in winter and spring.

Despite this, vitamin D is one of the most misunderstood nutrients. The wellness industry has inflated its importance beyond what the evidence supports, while conventional medicine has often been too dismissive. Here's what is actually known.

What vitamin D does — and doesn't do

The evidence for vitamin D is clearest for bone health, with a smaller but real signal for respiratory infections. Severe deficiency causes rickets in children and osteomalacia in adults. Low levels are associated with low bone density and fractures. Whether supplements prevent fractures, however, is less clear than often claimed: in the VITAL trial, 2,000 IU a day did not reduce fractures in generally healthy adults who had not been selected for deficiency or osteoporosis. Many trials in older adults combined vitamin D with calcium, which makes the effect of vitamin D alone hard to isolate.

For immune function, the evidence is more modest. Vitamin D plays a documented role in innate immune regulation. Deficiency is associated with increased susceptibility to respiratory infections. A large 2017 meta-analysis of individual patient data found that supplementation reduced acute respiratory infections by about 12% overall, and by about 70% in people with severe deficiency (below 10 ng/mL) who took daily or weekly doses. The 2024 Endocrine Society guideline, however, rates the evidence for preventing respiratory infections in children as low certainty.

Where the evidence gets weaker is the long list of other conditions vitamin D has been linked to: cardiovascular disease, cancer prevention, depression, autoimmune diseases, cognitive decline. For most of these, the associations are real but the causation is unproven. Large randomized trials (particularly VITAL, n=25,871) did not show that vitamin D supplementation prevents cancer or cardiovascular disease in people who aren't deficient.

The honest summary

Vitamin D deficiency is common, easy to detect, cheap to treat, and has clear consequences for bone health. But current guidelines advise against routine testing in healthy adults, and vitamin D is not a miracle supplement, and the mega-dosing trend in the wellness community is not supported by evidence.

What levels actually matter

Vitamin D levels are measured as 25-hydroxyvitamin D (25-OH-D) in ng/mL. The categories most labs still use come from the 2011 Endocrine Society guideline:

  • Below 20 ng/mL: Deficient. Supplementation is usually advised; a recheck after about 3 months shows whether the dose is working.
  • 21–29 ng/mL: Insufficient.
  • 30–100 ng/mL: Sufficient. No supplementation needed for this reason alone.
  • Above 100 ng/mL: Above the range the guideline considered sufficient. Stop supplementing and discuss with your doctor.

Some practitioners aim for 40–60 ng/mL, but no target level has been established in clinical trials, and the 2024 Endocrine Society guideline deliberately no longer endorses any specific target or cut-offs. Dosing should be agreed with your doctor, particularly if you take other medications or have kidney disease.

Why latitude matters

Much of Central Europe lies at high latitude — Munich, for example, sits at 48°N, roughly the same as Vancouver or Paris. Roughly from October to March, the sun angle at these latitudes is too low for meaningful vitamin D synthesis through the skin. If you live this far north, or spend the winter months there, there's a good chance your vitamin D is lower in winter, however much time you spend outdoors.

Even so, current guidelines advise against routine testing in healthy adults, because trials have not shown that screening improves outcomes. Testing is reasonable where there is a specific medical reason (for example, abnormal calcium levels) — discuss this with your doctor. For some groups, such as adults over 75, pregnant women and children, the 2024 guideline instead suggests supplementation without prior testing. If a level is measured and it's low, the treatment is simple and cheap — oral supplementation. No infusion, no injection, no exotic formulation. Just a pill.

References

  1. Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency. J Clin Endocrinol Metab. 2011;96(7):1911-1930.
  2. Demay MB, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024;109(8):1907-1947.
  3. Cashman KD, et al. Vitamin D deficiency in Europe: pandemic? Am J Clin Nutr. 2016;103(4):1033-1044.
  4. Rabenberg M, et al. Vitamin D status among adults in Germany: results from DEGS1. BMC Public Health. 2015;15:641.
  5. LeBoff MS, et al. Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med. 2022;387(4):299-309.
  6. Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory tract infections (IPD meta-analysis). BMJ. 2017;356:i6583.
  7. Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease (VITAL). N Engl J Med. 2019;380:33-44.
Prof. Dr. med. Markus Guba

Head of Liver Surgery and Transplantation at LMU University Hospital Munich. Edits this independent journal in a private capacity; views are his own. About →

This article is general information and not individual medical advice. Decisions about tests and treatment belong in a conversation with your own physician.