How Much Vitamin D Should You Take? The UK Evidence Guide

Type "vitamin D dosage" into Google and you'll find answers ranging from 400 IU to 10,000 IU daily. That's a 25x difference. It's like asking how fast to drive on the motorway and getting answers from "20mph" to "just floor it." Someone is very wrong. Let's look at what the actual research says — not what the marketing says.

The NHS Floor (400 IU)

Public Health England recommends 400 IU (10 micrograms) daily for all UK adults during autumn and winter, and year-round for at-risk groups. This is a population-level minimum designed to prevent outright deficiency in 97.5% of people. It's the nutritional equivalent of "this will keep you alive." It's not an optimal dose. It's a floor — and a conservative one.

The Scientific Advisory Committee on Nutrition set this based on maintaining serum 25OHD above 25 nmol/L. Many researchers now consider 50 nmol/L a more appropriate target for overall health, and getting there typically requires more than 400 IU unless you're also getting regular sun exposure (which, in the UK between October and March, you are not).

What the Biggest Studies Actually Used

VITAL — the largest vitamin D trial ever conducted — gave 25,871 US adults 2,000 IU daily for 5.3 years. Safe. Well-tolerated. Meaningful effect on autoimmune disease and cancer mortality. The ViDA trial (5,110 adults, 3.3 years) used the equivalent of about 3,300 IU daily. Also safe. The D2d trial (2,423 adults, 2.5 years) used 4,000 IU daily. Also safe.

The sweet spot in the evidence: 1,000 to 2,000 IU daily for most adults. This range consistently achieves serum 25OHD above 50 nmol/L — the threshold most researchers now consider "adequate" — without venturing into territory where the risks start appearing.

When More Is Genuinely Worse

The Calgary study is the data point everyone in the "more is better" camp wants to ignore. 311 Canadian adults received either 400, 4,000, or 10,000 IU daily for three years. Bone density was measured using the best available technology. Result: the 10,000 IU group lost more bone density than the 400 IU group. Read that again. Ten times the dose. Worse outcome.

It seems counterintuitive. But very high serum 25OHD levels (above 125 nmol/L) can cause hypercalciuria — excess calcium in the urine — and may interfere with normal bone remodelling. The Endocrine Society sets the tolerable upper limit at 4,000 IU daily. Based on the Calgary data, staying below that for long-term supplementation is the sensible call.

Who Might Need the Higher End

Not everyone responds to the same dose identically. People with darker skin produce less vitamin D from the same sun exposure. People with higher body weight have lower bioavailable vitamin D because it gets sequestered in fat tissue. Older adults have reduced skin synthesis capacity. Anyone on corticosteroids or certain anticonvulsants may need more.

If you're in any of these groups, 2,000 IU daily year-round is reasonable and evidence-based. If you want precision, a blood test from your GP (serum 25OHD) costs nothing on the NHS if you have clinical reason for it, or about £30-40 privately.

Why We Chose 2,000 IU

DailyGreens delivers D3 at 200% NRV — 2,000 IU. That's the dose used in the VITAL trial, the largest and most rigorous vitamin D study ever conducted. It sits comfortably within safe limits. It consistently achieves adequate serum levels in UK adults through winter. And we pair it with K2 (menaquinone) because directing calcium properly isn't optional — it's just that most brands skip it because it's expensive and consumers don't know to ask for it.

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