Vitamin D: Doses, Blood Levels and Who Actually Needs It

For an adult living at temperate latitudes, 800–2,000 IU of vitamin D a day from roughly October through April is usually enough — a dose that is safe and requires no blood testing. Therapeutic doses of 5,000–10,000 IU make sense only for confirmed deficiency, meaning a 25(OH)D level below 20 ng/mL, and only for a defined course. Vitamin D is not a general health amplifier: the proven benefit sits around bone, muscle and the correction of deficiency, not around the prevention of everything.

What vitamin D actually does#

Strictly speaking it is not a vitamin at all but a prohormone. It is synthesised in skin under ultraviolet B, then activated in two steps — the liver converts it to 25(OH)D, the kidney to calcitriol — after which it acts through receptors found in almost every tissue in the body. That ubiquity is the origin of most of the overpromising: receptors everywhere invites the assumption of benefits everywhere.

The core, best-established role is mineral metabolism. Vitamin D governs the absorption of calcium and phosphorus in the gut. When it is deficient, you absorb only 10–15 percent of dietary calcium instead of the usual 30–40 percent. Hence the classic consequences of shortage: rickets in children, osteomalacia and accelerated bone loss in adults.

The second solid domain is muscle. Vitamin D receptors are present in muscle tissue, and marked deficiency in older adults raises the risk of falls. Correcting that deficiency lowers the risk again — note the direction: the benefit comes from fixing a shortfall, not from topping up someone who already has enough.

Everything else — immunity, mood, metabolism, cancer risk — lives in the world of observational data and hypothesis. The association between low vitamin D and poor health has been found many times over. But in controlled trials, supplementing people who already have normal levels does not change those outcomes. Low vitamin D is frequently a marker of illness rather than its cause: someone who is unwell goes outside less and eats worse.

Reading the test#

One number matters: 25(OH)D, serum hydroxyvitamin D. The active form, 1,25(OH)2D, is not used for screening — it is hormonally regulated and can look perfectly normal in someone with profound deficiency, which makes it actively misleading in this context.

25(OH)D, ng/mL	Level, nmol/L	Interpretation	What to do
Under 10	Under 25	Severe deficiency	Treat under medical supervision
10–20	25–50	Deficiency	Loading dose, retest in 8–12 weeks
20–30	50–75	Insufficiency	1,000–2,000 IU daily, ongoing
30–50	75–125	Target range	Maintenance dose
50–100	125–250	Above target	Reduce the dose
Over 100	Over 250	Potentially toxic	Stop and see a clinician

One caveat that causes real confusion: laboratories report in different units. To convert ng/mL to nmol/L, multiply by 2.5. A result of "40" on one form and "40" on another can mean entirely different things — comfortably in range in the first case, insufficient in the second. Always check the unit before interpreting the number.

There is also a genuine expert disagreement about the lower boundary. The Institute of Medicine considers 20 ng/mL sufficient, on the grounds that it covers bone health for 97.5 percent of the population. The Endocrine Society uses 30 ng/mL. The practical compromise: 30–50 ng/mL is a comfortable range, and chasing 60–80 ng/mL is pointless — no additional benefit has been demonstrated up there, only additional cost and risk.

Who should actually be tested#

In 2021 the US Preventive Services Task Force concluded that the evidence is insufficient to recommend screening asymptomatic adults. This is not the same as "testing is useless". It means that for a healthy person it is simpler and cheaper to take a preventive dose than to draw blood.

Testing is worthwhile if you fall into one of these groups:

• osteoporosis, low-trauma fractures, bone or muscle pain;
• chronic kidney or liver disease;
• malabsorptive gut conditions — coeliac disease, Crohn's disease, or a history of bariatric surgery;
• medications that accelerate vitamin D metabolism: anticonvulsants, glucocorticoids, some antifungals;
• obesity with a BMI above 30 — vitamin D is sequestered in adipose tissue and less available;
• darker skin at northern latitudes, full body covering, or work with no daylight exposure;
• age over 70, where skin synthesis drops roughly fourfold;
• pregnancy and breastfeeding.

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Doses#

Prevention#

For a healthy adult with no test result, the working figure is 800–2,000 IU a day. It covers the requirement of the overwhelming majority of people, sits well inside the safety margin, and needs no laboratory follow-up.

Group	Preventive dose, IU per day
Infants under 1 year	400
Children 1–18 years	600–1,000
Adults 18–50	800–2,000
Adults over 50	1,000–2,000
Pregnancy and breastfeeding	1,000–2,000
Obesity (BMI over 30)	2,000–4,000

Correcting a deficiency#

With a confirmed level below 20 ng/mL, the standard approach is a loading regimen: 5,000–7,000 IU a day for 8–12 weeks, then a repeat test and a switch to a maintenance dose. An alternative is 50,000 IU weekly for eight weeks; the two schedules are comparable in effect, and the choice comes down to what you will actually remember to take.

Single megadoses of 300,000–600,000 IU, sometimes prescribed to "cover the season", are no longer recommended. In several trials in older adults they paradoxically increased the rate of falls and fractures — a useful reminder that with fat-soluble vitamins, dosing rhythm is not a matter of convenience alone.

Sun and food#

Skin synthesis is theoretically capable of covering the entire requirement: 15–20 minutes in a swimsuit under midday summer sun produces something in the order of 10,000–20,000 IU. The difficulty is that the necessary conditions rarely coincide.

UVB does not pass through window glass and barely reaches the ground when the sun sits below 45 degrees. North of the 40th parallel — a line that runs through Madrid, Istanbul, New York and Beijing — skin produces no vitamin D at all from November to March, regardless of how long you stand outside. SPF 30 sunscreen cuts synthesis by more than 95 percent, darker skin needs three to six times longer exposure, and after 65 the skin's capacity falls roughly fourfold.

Food is a simpler story, in that it plainly cannot do the job.

Food	Serving	Vitamin D, IU
Wild salmon	100 g (3.5 oz)	600–1,000
Farmed salmon	100 g (3.5 oz)	100–250
Herring	100 g (3.5 oz)	200–400
Canned sardines	100 g (3.5 oz)	200–300
Cod liver oil	1 teaspoon	400–450
Egg yolk	1 yolk	20–40
Fortified milk	250 ml (1 cup)	100–120
UV-exposed mushrooms	100 g (3.5 oz)	300–400

A typical diet delivers around 100–200 IU a day. Even someone eating oily fish three times a week rarely averages more than 400–500 IU. That is the whole argument for supplementation in one table.

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Signs of a shortfall#

There are no specific symptoms of vitamin D deficiency, and that is the central practical problem. Moderate shortfall is usually silent and shows up only on a blood test.

With severe, prolonged deficiency you may see aching bone pain — particularly in the pelvis, ribs and shins — proximal muscle weakness, which shows up as difficulty climbing stairs or rising from a chair, more frequent fractures, and in children growth delay and skeletal deformity. Chronic fatigue and low mood are routinely attributed to vitamin D, but the link is weak and non-specific: a dozen other explanations fit the same complaint at least as well.

The practical conclusion runs in both directions. Do not blame every symptom on vitamin D, and do not expect correcting it to change how you feel if your level was normal in the first place.

Overdose risk#

Specific contraindications where vitamin D should not be taken without supervision: hypercalcaemia of any cause, primary hyperparathyroidism, sarcoidosis and other granulomatous diseases (in which vitamin D activation escapes normal control), certain lymphomas, severe renal failure, and calcium-stone kidney disease.

Drug interactions are worth knowing. Thiazide diuretics combined with high-dose vitamin D raise the risk of hypercalcaemia. Digoxin becomes more dangerous against a background of elevated calcium. Orlistat and cholestyramine work the other way, reducing absorption.

Combination with high-dose calcium deserves separate attention. If you take calcium supplements, total intake including food should stay under 2,000–2,500 mg a day; going beyond that is associated with increased kidney stone risk.

What to do in practice#

1. If you are not in a risk group, start with a preventive 1,000–2,000 IU a day year-round. Testing is optional.
2. If you are in a risk group, measure 25(OH)D before starting, so you know your baseline.
3. With confirmed deficiency, take a loading course and retest at 8–12 weeks. High doses without follow-up testing are not a reasonable plan.
4. Choose D3 and take it with a meal containing fat — it is simpler and gives steadier levels.
5. Do not raise the dose "just in case". The difference between 2,000 and 10,000 IU is not speed of result; it is risk.

Vitamin D is one of the rare cases where a supplement is genuinely justified for most people living far from the equator. But that very justification is what has let so much else accumulate around it. It does not substitute for sleep, movement or decent food, and it does not treat anything that was not caused by its absence.

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