From October through April, most of the northern hemisphere sits at a sun angle too low for the skin to make vitamin D — even if you walk outside daily. According to data from endocrinology centers, up to 80% of people in mid-latitudes are deficient by midwinter. Here's what the real target looks like, how much D3 to take, and why a tanning bed is not a substitute for the sun.
Vitamin D isn't really a vitamin — it's a prohormone. The body makes it in the skin under ultraviolet B (UVB) radiation at 290-315 nm. The catch: UVB only reaches the ground when the sun rises above 35° on the horizon. At London's latitude (51° N) or Moscow's (55° N), that window only opens roughly April through September, and only around midday.
In winter the sun's angle is too low — UVB scatters in the atmosphere and barely reaches the surface. Even if you spend an hour outdoors in the cold, your skin won't produce a single unit of vitamin D. This phenomenon is called "vitamin D winter" and was first described in NIH research back in the 1980s.
💡 Key fact: tanning beds are not a substitute for the sun. Most lamps emit primarily UVA (for tanning), and the UVB share is minimal. Beyond that, the WHO classifies tanning beds as Group 1 carcinogens — alongside asbestos and tobacco.
Vitamin D status is assessed by a blood test for 25-hydroxyvitamin D — abbreviated 25(OH)D or calcidiol. This is the main circulating form and what labs measure. Don't confuse it with 1,25(OH)₂D — the active form, which is only informative in specific clinical conditions.
Endocrine Society guidelines and the Russian Association of Endocrinologists set the following thresholds:
| 25(OH)D level | ng/mL | nmol/L | Action |
|---|---|---|---|
| Severe deficiency | <10 | <25 | Therapeutic doses, medical supervision |
| Deficiency | 10-20 | 25-50 | Loading course, then maintenance |
| Insufficiency | 20-30 | 50-75 | Preventive dose |
| Target range | 30-60 | 75-150 | Maintenance dose |
| Upper limit of normal | 60-100 | 150-250 | Reduce dose |
| Possible toxicity | >100 | >250 | Stop, monitor calcium |
Most international guidelines (including NIH and the US Institute of Medicine) consider levels above 20 ng/mL adequate. The Endocrine Society recommends aiming for 30-60 ng/mL — particularly for people with osteoporosis, autoimmune disease, or chronic infections.
The 25(OH)D test is widely available at commercial labs for $30-80. Public insurance generally doesn't cover screening of healthy adults — only patients with fractures, osteoporosis, myopathy, or diabetes.
Most guidelines (including USPSTF, 2021) advise against mass screening of the general healthy population — the cost outweighs the clinical benefit. If you're healthy, asymptomatic, and taking 1000-2000 IU of D3 daily, testing isn't required.
The big question: how many IU of D3 to take in winter to avoid both deficiency and overdose? Current recommendations:
| Group | Preventive | Therapeutic (deficiency) | Upper safe limit |
|---|---|---|---|
| Infants 0-12 mo | 400-1000 IU | 2000 IU | 1500 IU |
| Children 1-18 | 600-1000 IU | 3000-6000 IU | 4000 IU |
| Adults 18-50 | 800-2000 IU | 6000-10,000 IU (8 weeks) | 4000 IU long term |
| Adults 50-70 | 1000-2000 IU | 6000-10,000 IU | 4000 IU |
| 70+ | 1500-2000 IU | 8000 IU | 4000 IU |
| Pregnant/lactating | 1000-2000 IU | as prescribed | 4000 IU |
| Obesity (BMI >30) | 2000-4000 IU | 10,000 IU | 10,000 IU (monitored) |
Numbers are based on Endocrine Society, NIH ODS and Russian endocrinology guidelines. Note: with obesity the dose needs to be 2-3× higher because vitamin D is fat-soluble and gets sequestered in adipose tissue, leaving less in circulation.
⚠ Signs of overdose appear with prolonged intake above 10,000 IU/day and look like hypercalcemia: nausea, thirst, frequent urination, confusion, arrhythmia. If a clinician prescribes loading doses of 50,000 IU, that's a short, supervised course — not an everyday plan.
Drug stores carry two forms: ergocalciferol (D2) and cholecalciferol (D3). They look structurally similar but behave differently in the body:
A meta-analysis in the American Journal of Clinical Nutrition (2012, 50 studies) found D3 roughly 1.5-2× more effective than D2 at raising 25(OH)D. For most people D3 is the better choice. D2 is mainly considered for strict vegans, although vegan-friendly D3 from lichen is now available.
The delivery format doesn't matter much — what matters is taking D with a fat-containing meal (eggs with butter, avocado, oily fish). Vitamin D is fat-soluble: without lipids in the gut, absorption drops by 30-50%. Oil drops are convenient for precise dosing in children; capsules are easiest for adults.
Hitting the daily target through food alone in winter is nearly impossible, but pairing food with supplements helps. Real numbers from the USDA database:
| Food (per 100 g) | Vitamin D, IU | % of 1000 IU target |
|---|---|---|
| Cod liver | 10,000 | 1000% |
| Wild salmon | 988 | 99% |
| Farmed salmon | 250-400 | 25-40% |
| Sardines in oil | 193 | 19% |
| Mackerel | 360 | 36% |
| Canned tuna | 268 | 27% |
| Egg yolk (1) | 40 | 4% |
| Fresh shiitake | 40 | 4% |
| UV-exposed mushrooms | ~450 | 45% |
| Cheddar cheese | 24 | 2% |
To cover 1000 IU through food alone, you'd need to eat 100 g of wild salmon every day or down 5 teaspoons of cod liver oil. Realistic plan: combine 2-3 servings of oily fish per week with a daily 1000-2000 IU D3 supplement.
This claim exploded during the pandemic. Reality: people with severe deficiency do tend to have worse outcomes — but that's correlation, not causation. A Cochrane Review (2024) analyzed 78 RCTs and found preventive D supplementation cuts the risk of acute respiratory infection by about 8%. A weak effect, not a cure-all.
Hypervitaminosis D causes hypercalcemia, kidney stones, and soft-tissue calcification. The large VITAL trial (NEJM, 2019, 25,871 participants) found that 2000 IU/day for 5 years did NOT reduce cancer or cardiovascular events versus placebo in people without deficiency. "More vitamin D" doesn't equal "better health" if you're not deficient.
Not for everyone. Combined calcium + D is recommended for osteoporosis, in older adults, and for those with low dairy intake. Healthy young adults on a typical diet usually don't need extra calcium — and excess calcium may increase the risk of arterial calcification.
Vitamin D is not an innocuous "vitamin pill." There are conditions where supplementation is dangerous:
In these cases see an endocrinologist and monitor blood calcium.
💡 Simple winter plan for a healthy adult: 1000-2000 IU D3 daily with a fat-containing breakfast, 2 servings of oily fish per week, daylight walks for general health. In February, get a 25(OH)D test to fine-tune the dose. That covers 90% of people without specific risk factors.
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