Vitamin D Dosing: Why Your IU Label Doesn't Match the RDA, and What to Actually Take

The RDA for vitamin D in the UK is 400 IU. In the US, it's 600 IU. These figures prevent deficiency disease in most of the population. They are not the doses most people need to reach blood levels that functional medicine research targets.

The gap between those two things is worth understanding before you decide what to take.

What the RDA was designed to do

The RDA for vitamin D was set to prevent rickets and severe deficiency in most adults. It is a public health floor, not a personal target.

There is also a credible argument that the current RDA was set using a statistical error that underestimated the dose needed to maintain adequate blood levels. This is contested, but the debate itself tells you that the RDA is not a settled, conservative figure that errs on the side of caution.

What we know clearly: a dose of 400 to 600 IU is not enough to raise most people's blood levels to the range that functional medicine practitioners typically target.

IU vs micrograms: the label translation

IU (international units) is a unit of biological activity, not weight. One microgram of vitamin D equals 40 IU.

In practice:

• 400 IU = 10 mcg
• 1000 IU = 25 mcg
• 2000 IU = 50 mcg
• 4000 IU = 100 mcg

UK supplement labels are required to show both IU and mcg. US products often show IU only. Multiply the IU figure by 0.025 to get mcg.

Blood test results come back in either nmol/L (used in the UK) or ng/mL (used in the US). One ng/mL equals 2.5 nmol/L. They measure the same thing in different units.

What your blood level means

Standard NHS lab ranges flag deficiency at below 25 nmol/L and insufficiency below 50 nmol/L.

Functional medicine targets are higher. Most practitioners aim for 75 to 125 nmol/L (30 to 50 ng/mL). Some target 100 to 150 nmol/L (40 to 60 ng/mL), particularly for autoimmune disease or persistently low levels.

Most UK adults fall below 50 nmol/L during winter months. One large study found that over half of UK adults were below 25 nmol/L by February.

Getting from 30 nmol/L to 100 nmol/L typically requires 2000 to 4000 IU daily, not 400 IU. The exact dose varies by starting point, body weight (vitamin D distributes into fat tissue), and how well you absorb it.

D3 vs D2

Vitamin D3 (cholecalciferol) is more effective than vitamin D2 (ergocalciferol) at raising blood levels and maintaining them. A 2011 meta-analysis found D3 was roughly 87% more effective at raising 25-OH D levels than D2.

If you are choosing between them, choose D3. Most quality supplements use D3. Vegan D3 is available, derived from lichen rather than lanolin.

The K2 question

When you supplement with vitamin D, your body absorbs more calcium from food. Vitamin K2, specifically the MK-7 form, helps direct that calcium to bones and teeth rather than leaving it to accumulate in arteries and soft tissue.

The evidence for this combination is not as strong as the evidence for vitamin D itself. But the biological mechanism is sound, the risk from K2 is low, and the potential benefit at higher vitamin D doses is real. Most people taking more than 2000 IU daily include K2 in their routine.

Standard doses used in research: 100 to 200 mcg of MK-7 daily.

The seasonal reality in the UK and northern Europe

Above roughly 50 degrees north latitude, the angle of sunlight between October and March is too low for UVB rays to produce vitamin D in the skin in meaningful amounts. Your skin cannot make vitamin D from sun exposure during those months, regardless of how much time you spend outside.

This is not a minor edge case. Public Health England recommends that everyone in the UK supplement with at least 400 IU through autumn and winter. Most vitamin D researchers consider this figure too low to move blood levels to an optimal range.

For most UK adults through winter, 1000 to 2000 IU daily is a sensible starting point if you haven't tested. If you have tested and know your levels, adjust from there.

When to test

Testing is worth doing if you are considering doses above 2000 IU daily. Also worth doing if you have been supplementing for a while and want to see where your levels actually sit. And worth doing if you have an underlying issue that may affect how you absorb or use vitamin D, such as autoimmune disease, obesity, or consistently limited sun exposure year-round.

The test to ask for is 25-hydroxyvitamin D (25-OH D). It is available from your GP, and from private labs including Medichecks and Forth.

Test in winter, when levels are lowest, to get the most useful baseline. Retest after three months on a consistent dose to see where you land.

Vitamin D and medication interactions

At doses up to 4000 IU daily, vitamin D toxicity is rare in healthy adults. The upper tolerable limit set by most bodies is 4000 IU (100 mcg) daily for long-term use without testing.

If you take thiazide diuretics, speak to your GP before supplementing with more than 1000 IU. These drugs can raise blood calcium and vitamin D supplementation adds to that.

How Stack Almanac helps

If you've logged a vitamin D blood test result in Stack Almanac, the Almanac Advisor uses your actual 25-OH D level when providing dose context. Rather than a one-size recommendation, it works from where you actually are.

Stack Almanac's mood support and over 40s protocols both dose vitamin D in this 2,000 to 4,000 IU range for exactly the reasons above, and it's the kind of dosing precision Stack Almanac's practitioner tools are built to support across a full patient panel.