Vitamin D: What It Actually Does, How Much to Take, and Whether It Is Worth It

Vitamin D is one of the few supplements that genuinely makes sense for most people living in northern climates — but not for the reasons you usually hear in the gym. It is not a testosterone booster and it will not add 10 kilograms to your squat. Here is what it actually does, how to know if you are short, and how much to take.

What vitamin D actually does

Technically it is not a vitamin at all, it is a prohormone. Your skin makes it from cholesterol under UVB light, your liver converts it to 25(OH)D — the form measured in blood — and your kidneys turn that into active calcitriol. Nearly every tissue in the body has a receptor for it, including muscle fibres.

Three effects are well documented:

• Calcium absorption. With adequate vitamin D you absorb roughly 30 to 40 percent of the calcium you eat. When deficient, that drops to 10 to 15 percent. If you eat 1000 mg of calcium a day, that is about a 200 mg difference — every day, for years.
• Bone health. Chronic deficiency in adults causes osteomalacia: softer bone, a dull ache in the shins, hips and ribs, and a higher stress-fracture risk if you run or do jump work.
• Muscle function. Severe deficiency causes proximal myopathy — weakness in the hips and thighs. Concretely: struggling to stand up from a chair without using your hands, the first few reps of a squat set feeling heavier than they should, feeling unsteady walking down stairs.

What it does not do

Worth being honest here. The VITAL trial followed 25,871 people taking 2000 IU daily for 5.3 years and found no reduction in cancer diagnoses or major cardiovascular events versus placebo. Strength meta-analyses tell the same story: if your level is already normal, adding vitamin D does not add weight to the bar. The strength benefit shows up almost exclusively in people who started out deficient.

Likewise: it does not burn fat, it does not fix shoulder pain, and it does not replace 7 to 8 hours of sleep. And more is not better — trials using megadoses, for example 500,000 IU once a year, recorded more falls and fractures, not fewer.

How to know if you are low

Not by how you feel. The test is called 25-hydroxyvitamin D, it is a standard venous blood draw, and you do not need to fast. Rough guide:

• below 12 ng/mL (30 nmol/L) — severe deficiency
• 12 to 20 ng/mL (30 to 50 nmol/L) — deficiency
• 20 to 30 ng/mL (50 to 75 nmol/L) — borderline
• 30 to 50 ng/mL (75 to 125 nmol/L) — the target range
• above 60 ng/mL — no additional benefit

If testing is not practical for you, 1000 to 2000 IU a day is safe for essentially anyone and very unlikely to cause trouble.

Sun and food: what you realistically get

At 44 degrees north, UVB is too weak from October through March for your skin to produce anything meaningful, no matter how bright the day looks. From April to September, 15 to 25 minutes between 11:00 and 15:00 with arms and legs uncovered, two to three times a week, is enough. Darker skin needs 2 to 3 times longer. Glass blocks UVB entirely, so sitting by a window does nothing.

Food is a weak lever:

• wild salmon, 100 g — 600 to 1000 IU
• farmed salmon, 100 g — 100 to 250 IU
• canned sardines, 100 g — about 200 IU
• egg yolk — 20 to 40 IU each

To hit 2000 IU from eggs you would need roughly 60 yolks a day. That is why a winter supplement is common sense rather than fussiness.

Dosing: actual numbers

• Maintenance (test normal or unknown): 1000 to 2000 IU, that is 25 to 50 mcg, daily from October through April.
• Correcting a deficiency (below 20 ng/mL): 4000 IU daily for 8 to 12 weeks, then retest. Anything higher or longer belongs under medical supervision.
• Body mass matters. Vitamin D is stored in fat tissue, so someone at 110 kg needs roughly 1.5 to 2 times the dose of someone at 70 kg to reach the same blood level.
• The long-term ceiling without supervision is 4000 IU per day.

Take D3 (cholecalciferol) rather than D2 — it raises blood levels noticeably more effectively. Daily dosing beats large weekly or monthly boluses.

How and when to take it

Vitamin D is fat soluble, so take it with a meal containing at least 10 to 15 g of fat — eggs, yoghurt, a handful of walnuts, a spoon of olive oil. Absorption is up to 30 percent better than on an empty stomach. Magnesium is a cofactor in vitamin D metabolism, so keep intake around 300 to 400 mg a day from food. And be patient: blood levels plateau only after 8 to 12 weeks, so retesting at week 3 tells you nothing useful.

When to see a doctor

• Before going above 4000 IU daily for longer than 3 months.
• If you have kidney or liver disease, sarcoidosis, tuberculosis or hyperparathyroidism — in these conditions vitamin D can push calcium up dangerously fast.
• If you take thiazide diuretics, digoxin or anti-epileptic medication.
• Signs of high calcium: nausea, heavy thirst, frequent urination, constipation, confusion.
• Dull bone pain, weakness in the hips and thighs, or repeated stress fractures.
• Pregnancy and breastfeeding — dosing should be set individually.

Vitamin D is cheap insurance against a deficiency, not a training tool. If your level is already fine, your progress comes down to calories, sleep, and adding weight to the bar.

The short version

• Vitamin D mainly drives calcium absorption, bone health and normal muscle function — it is not a stimulant or a booster.
• Strength benefits appear almost only in people who were deficient; if your test is normal, supplementing changes nothing.
• Test 25(OH)D. Aim for 30 to 50 ng/mL; above 60 ng/mL there is no extra benefit.
• Practical dose: 1000 to 2000 IU of D3 daily in winter, or 4000 IU for 8 to 12 weeks if deficient, taken with a meal containing 10 to 15 g of fat.
• From October to March sun exposure does nothing at this latitude, and annual megadoses are a worse option, not a shortcut.