Vitamin D between October and April

It is the angle, not the cloud

Vitamin D synthesis needs UVB, and UVB is filtered out when sunlight arrives at a shallow enough angle. Above roughly 50 degrees north — which is all of Britain — the sun sits too low from around October to March for meaningful synthesis, whatever the sky is doing. A bright, cold January walk produces essentially none.

This is not a lifestyle failing. It is geometry, and it applies to everyone living here regardless of how much time they spend outdoors.

What the official advice says

Public Health England's position, carried forward by the NHS, is that everyone in the UK should consider a daily supplement of 10 micrograms — 400 IU — during autumn and winter, and that some groups should take it year round: anyone with limited sun exposure, and people with darker skin, who need substantially more UVB for the same synthesis.

Food will not cover the gap. Oily fish is the only substantial source; egg yolks and fortified spreads contribute little. Reaching 400 IU from diet alone means eating oily fish most days of the week.

Dose, and why testing changes the answer

400 IU prevents deficiency. Many people whose blood level is genuinely low need more than that to correct it, and 1,000 to 2,000 IU daily is a common, well-tolerated corrective range. Above 4,000 IU daily, supervision is sensible: vitamin D is fat-soluble, it accumulates, and toxicity — while rare — is real.

Since the right dose depends on where you are starting from, this is one of the clearest arguments for measuring rather than guessing. It is also cheap to measure and slow to change, which makes it an ideal marker to check in autumn and re-check in spring.

• 01

  Take 400 IU from October to April as a baseline

  Everyone
• 02

  Test if you want the right dose rather than a safe one

  Better
• 03

  Year round if your sun exposure is limited or your skin is darker

  Also