Vitamin D in the UK: Why Winter Is Different

There are very few supplements that UK public health guidance actively recommends to the entire population. Vitamin D is the one. That alone makes it worth understanding properly, because the reasoning is more interesting than "take a tablet".

Why British winter is different

Your skin makes vitamin D when UVB light hits it. UVB is a short wavelength, and short wavelengths get absorbed by the atmosphere. The further the sunlight has to travel through air to reach you, the less UVB survives the trip.

How far it travels depends on the angle of the sun. In summer it is high overhead and the path is short. In winter, at British latitudes, it sits low even at midday, so the light comes in at a shallow angle through a much thicker slice of atmosphere, and effectively no UVB gets through.

The UK runs from roughly 50 degrees north in Cornwall to 60 in Shetland. Above about 37 degrees, there is a stretch of the year where skin cannot make vitamin D at all. For Britain that window runs from around October to early March.

This is a matter of geometry, not weather. A bright crisp January afternoon produces no vitamin D whatsoever. Neither does sitting by a window in July, since glass blocks UVB.

What the NHS actually says

The NHS advises that everyone consider taking a daily vitamin D supplement during autumn and winter, and that certain groups consider taking one year round.

Those groups include people who spend little time outdoors, anyone who covers most of their skin when outside, and people with darker skin, because higher melanin means UVB is absorbed before it reaches the cells that use it. Someone with very dark skin may need several times the sun exposure of someone very pale to make the same amount, which in a country with British light levels is a meaningful difference.

D2 and D3 are not interchangeable

Vitamin D comes in two forms. D2, ergocalciferol, comes from fungi. D3, cholecalciferol, is what your skin produces and what is found in animal foods.

D3 is generally considered the more effective form at raising and maintaining levels in the blood. It is also the more expensive one, which is why cheaper products sometimes use D2.

If you are vegan, conventional D3 is usually derived from lanolin, the grease in sheep's wool. Lichen derived D3 solves that, giving genuine D3 from a plant source. Ours is listed on the label as cholecalciferol.

Micrograms and IU, and why labels confuse people

Vitamin D is measured in two units and the category uses both, often on the same shelf.

• 10µg equals 400 IU
• 20µg equals 800 IU
• 25µg equals 1,000 IU

So a product advertising 1,000 IU sounds much larger than one advertising 20µg, and is in fact only slightly more. Converting before comparing is the only way to know what you are actually looking at.

Take it with food

Vitamin D is fat soluble, meaning it is absorbed alongside dietary fat rather than in water. Taken on an empty stomach, absorption is measurably poorer. Taken with a meal containing some fat, it is considerably better.

Being fat soluble also means it is stored in body fat and the liver rather than passed out, so there is an upper limit worth respecting. This is not a nutrient where doubling the dose doubles the benefit, and very high dose supplements taken over long periods are worth a conversation with your GP rather than a guess.

Vitamin K2 usually travels with it

You will notice most vitamin D products also contain K2, and it is a fair question why.

The two are involved in overlapping processes around how the body handles calcium, and formulating them together has become standard practice in the category. K2 is menaquinone, distinct from the K1 found in leafy greens, and much less common in a normal UK diet.

One genuinely important caution: vitamin K interacts with warfarin and other anticoagulant medication. If you take a blood thinner, speak to your GP or anticoagulation clinic before taking anything containing vitamin K in either form.

Food sources, and why they rarely cover it

Oily fish is the standout: salmon, mackerel, sardines, herring. Egg yolks, red meat and liver contain smaller amounts. Some foods are fortified, including most margarines, many breakfast cereals and a good number of plant milks.

The difficulty is that very few foods contain much of it naturally. You would need to eat oily fish several times a week to approach what your skin makes on a single summer afternoon. That gap is the entire reason vitamin D is the one supplement public health guidance singles out.

The short version

• British skin makes no vitamin D from October to early March, whatever the weather
• The NHS suggests everyone consider a daily supplement over that period
• D3 is the more effective form; lichen derived D3 is the vegan route
• 20µg is 800 IU; convert before comparing products
• Take it with food, since it is fat soluble
• Speak to your GP first if you take anticoagulants, given the K2 usually alongside it

The Daily Greens+ gummy carries 20µg of D3 at 200% NRV, alongside 80µg of K2. Every dose is on the ingredients page.

This article is general information rather than medical advice. If you think you may be deficient, ask your GP for a blood test rather than guessing at a dose.