Vitamin D and northern winters

Why latitude decides this

Your skin makes vitamin D from UVB radiation. UVB only reaches the ground when the sun is high enough in the sky — below roughly 45 degrees of elevation, the atmosphere absorbs essentially all of it. Which means that for much of the year at northern latitudes, you could sit outside naked all day and produce nothing.

Tallinn sits at 59°N. Between October and March, vitamin D synthesis is effectively zero regardless of how much time you spend outdoors — and those are precisely the months when people are indoors, covered up, and training inside.

What deficiency actually affects

Being precise matters here, because vitamin D has been claimed to fix almost everything and the well-supported list is shorter than the marketed one.

Well established:

• Bone health. Calcium absorption depends on it. Severe deficiency causes rickets and osteomalacia.
• Muscle function. Severe deficiency causes measurable weakness, particularly proximal, and correcting it restores function.
• Stress fracture risk in athletes with low status.

Reasonable evidence:

• Correcting a deficiency improves strength. Meta-analyses of D3 supplementation in athletes show favourable effects on serum levels, with strength benefits appearing mainly in those who were deficient to begin with.
• Respiratory infection frequency — modest reductions with supplementation, particularly in deficient people.
• Mood in winter — plausible and not firmly established.

Not established:

• Supplementing above sufficiency improves performance. If your levels are already adequate, more does nothing.
• High-dose vitamin D as a performance enhancer. No.
• Testosterone. Weak and inconsistent evidence.

The pattern is consistent: correcting deficiency helps; exceeding sufficiency does not. That makes this a repletion question rather than an optimisation one.

Dosing

• 1,000–2,000 IU (25–50 µg) daily is a sensible maintenance dose for adults at northern latitudes through winter. This will keep most people sufficient.
• Take D3, not D2. D3 (cholecalciferol) raises blood levels more effectively. Vegan D3 from lichen is available.
• Take it with a meal containing fat. It is fat-soluble, and absorption is meaningfully better with food.
• Daily beats weekly or monthly bolus dosing for stable levels, though weekly is acceptable if adherence is the problem.
• If deficient, higher short-term doses correct faster. One study used 5,000 IU for four weeks to move people from deficiency to sufficiency. Do this with a blood test and medical input rather than by guessing.
• Upper safe limit is generally 4,000 IU/day for adults without medical supervision.
• Summer: most people at these latitudes can stop from April to September if they get regular midday sun exposure. Continuing at a low dose is also fine.

Testing, and whether you need to

A 25(OH)D blood test tells you your actual status, and it is inexpensive.

• Below 25–30 nmol/L (10–12 ng/mL): deficient. Needs correction, ideally with medical input.
• 30–50 nmol/L: insufficient. Supplement.
• 50–125 nmol/L (20–50 ng/mL): sufficient. This is the target.
• Above 250 nmol/L: potentially harmful. Only reachable through high-dose supplementation, never through sun.

Do you need to test? Not necessarily. At 1,000–2,000 IU daily the risk of overshooting is negligible and the probability of benefit at northern latitudes in winter is high, so testing is optional for a healthy adult. Test if you have symptoms, a malabsorption condition, very dark skin at high latitude, cover your skin for cultural or medical reasons, are pregnant, or want to use a higher dose.

What about sun and food?

Sun

In summer at these latitudes, around 15–30 minutes of midday sun on arms and legs a few times a week is generally sufficient for lighter skin — considerably longer for darker skin. Through glass does not work, and sunscreen reduces synthesis substantially, though the skin cancer trade-off favours the sunscreen.

Food

Dietary vitamin D is limited, which is why this is a supplement question rather than a diet one.

• Oily fish is the best source: a portion of salmon has roughly 400–600 IU.
• Egg yolks: around 20–40 IU each.
• Fortified foods — some milks, cereals, spreads — vary by country and are modest.
• UV-exposed mushrooms: a genuine vegan source, and variable.
• Cod liver oil: high in vitamin D and also very high in vitamin A, which has its own upper limit. Do not use it as a daily high-dose vitamin D source.

Reaching 1,000 IU daily from food alone means eating oily fish most days. Realistic for some people; a supplement is simpler.

Common questions

How much should I take?

1,000–2,000 IU of D3 daily through winter, with a meal containing fat.

Do I need it in summer?

Probably not at 40°N or below if you get regular midday sun. Above 55°N, continuing at a low dose is reasonable.

Will it improve my lifting?

Only if you are deficient. Correcting deficiency helps; exceeding sufficiency does not.

Should I take vitamin K2 with it?

Commonly recommended and not strongly supported at these doses. Not harmful.

Is 10,000 IU a day safe?

Above the usual upper limit and unnecessary. Do not use doses like that without a blood test and medical guidance.

Can I get enough from a multivitamin?

Often only 200–400 IU, which is below what a northern winter requires. Check the label.

Does a sunbed count?

Some emit UVB and do raise levels. The skin cancer risk makes this a poor trade for a cheap capsule.

Is deficiency common even in sunny countries?

Yes — indoor lifestyles, covering up and sunscreen all reduce synthesis regardless of latitude.

The practical version

• Above about 40°N there is no useful UVB for two to six months of the year.
• 40–70% of athletes are deficient in winter; risk is higher above 40°N and in indoor sports.
• 1,000–2,000 IU of D3 daily, October to March, with a fat-containing meal.
• Correcting deficiency improves strength; exceeding sufficiency does not.
• Target 50–125 nmol/L; test if you have symptoms or risk factors, otherwise optional.
• Darker skin, older age and covered skin all increase requirements at high latitude.