Why Vitamin D Runs Low Every Winter — and What That Means for Your Immunity

Every October, without realising it, most UK adults start running out of vitamin D. By February, studies suggest that around 1 in 5 people in the UK has levels low enough to be classified as deficient. If you find yourself catching more colds in winter, feeling slower to recover, or just running on empty from November through March — this is worth understanding.

Why Winter Is the Problem

Your body makes vitamin D from sunlight — specifically UVB rays hitting the skin. The issue for anyone living above 51°N latitude (that’s most of the UK) is that between October and April, the sun sits too low in the sky to trigger vitamin D synthesis. Even on a clear winter day, you’re not making any.

Diet fills almost none of the gap. Very few foods contain meaningful amounts of vitamin D naturally. Oily fish helps a little. Fortified cereals contribute trace amounts. But you cannot eat your way to adequate vitamin D levels during a UK winter.

The NHS recommends that everyone in the UK consider a vitamin D supplement from October through March for exactly this reason. The question is what kind.

What Vitamin D Does for the Immune System

Vitamin D contributes to the normal function of the immune system — this is an EU-authorised nutrient function claim, backed by substantial research. Immune cells (including T cells and macrophages) carry vitamin D receptors; they literally need it to mount a normal response.

A large systematic review published in the British Medical Journal in 2017, covering 25 randomised controlled trials and over 11,000 participants, found that vitamin D supplementation was associated with a reduced risk of acute respiratory infection, with the strongest effect in people who were deficient at baseline (Martineau et al., BMJ 2017).

That last point matters. If your levels are already adequate, topping them up further does not give you an extra boost. But if you are running low — as most UK adults are by midwinter — restoring normal levels appears to support the immune system’s ability to function as it should.

Why the Form of Vitamin D Matters

Most vitamin D supplements on the market give you D3 alone. That is a reasonable start, but there is a problem: vitamin D increases calcium absorption, and without vitamin K2, that calcium can end up depositing in the wrong places — arteries rather than bones.

Vitamin K2 (specifically the MK-7 form) activates the proteins that direct calcium to where it belongs. MK-7 has a half-life of around 72 hours, meaning it stays active in the body throughout the day. The cheaper MK-4 form clears in 1–2 hours. If the label does not specify MK-7, it is likely MK-4.

Our Vitamin D3 K2 MK-7 combines D3 with K2 as MK-7, plus Zinc Citrate (zinc contributes to normal immune function too), Boron, and an MCT oil base. The MCT oil matters: vitamin D is fat-soluble, and most people do not take their supplements with a fatty meal. The MCT base ensures absorption regardless.

What to Expect

Vitamin D is not a fast-acting supplement. If you are deficient, it can take 4–8 weeks of consistent supplementation before levels fully normalise. The practical implication: start in September or October, not when you have already been ill twice.

Most people do not notice a dramatic change — that is the nature of maintaining a baseline rather than treating a symptom. What tends to shift over a full winter is a pattern: fewer colds, faster recovery when you do get one, and the general sense of not running below capacity for months at a time.

Getting Through Winter in Better Shape

If you are in the UK and you are not supplementing vitamin D from October to April, there is a reasonable chance your immune system is working with one hand tied behind its back. That is not a scare tactic — it is the straightforward implication of where we live.

If you want a D3 supplement that pairs the right K2 form with the nutrients that help it work properly, our Vitamin D3 K2 MK-7 with Zinc and Boron is worth a look.

Vitamin D contributes to the normal function of the immune system. Zinc contributes to the normal function of the immune system. These are EU-authorised nutrient function claims.