By February, plenty of us in the UK feel flat. The sun sits low, the afternoons are still dark early, and the stretch from October to March does more than dull your mood — it also switches off the body's main source of vitamin D. That switch is why UK health advice treats winter differently from summer. The fix is straightforward, but it helps to understand why the shortfall happens, who it hits hardest, and what a sensible daily habit looks like.

Why UK sunshine stops working in winter

Vitamin D is unusual among nutrients: your body is designed to make most of it, using UVB light from the sun on bare skin. The catch is that UVB only reaches us when the sun is high enough in the sky. At UK latitudes, that happens for roughly late March to September.

Between October and March, the sun's angle means the UVB reaching the ground is too weak for skin to synthesise vitamin D, whatever the weather. A bright, cold January midday will not do it, and neither will a long walk. Sitting indoors by a sunny window is no help either, because glass blocks UVB almost completely.

Summer is a different picture. Short spells in the sun — think ten to fifteen minutes on your forearms and face, a few times a week, without burning — are usually enough for most people. Longer exposure doesn't build a store that carries you through to spring, so by the time December arrives, most of us are relying on food and supplements.

What a shortfall can feel like

The signs are easy to dismiss. Tiredness that a good night's sleep doesn't shift. Aching bones or muscles, especially in the back, hips and legs. Feeling weaker than you'd expect. Low mood, or catching every bug going.

None of that is specific to vitamin D. Plenty of things cause the same complaints — poor sleep, stress, an underactive thyroid, anaemia — and some people with low levels notice nothing at all. That vagueness is exactly why the advice is aimed at everyone in winter, rather than only at those who feel unwell.

It's worth taking seriously over time, though. Vitamin D helps your body absorb calcium, which it needs to keep bones and muscles working properly. A prolonged shortfall is linked to bone problems later on, which is why the focus is on prevention rather than waiting for symptoms.

Who is most likely to run low

• People who spend most of the day indoors — office workers, shift workers, anyone housebound or living in a care setting.
• People with darker skin, because more melanin means more sun exposure is needed to make the same amount of vitamin D.
• Anyone who covers their skin for cultural or religious reasons, or who uses high-factor sunscreen every time they go outside.
• Older adults, whose skin makes vitamin D less efficiently.
• Pregnant and breastfeeding women.
• Babies and young children, particularly breastfed babies.
• People with conditions that affect how nutrients are absorbed, such as coeliac disease or Crohn's disease, and those taking certain medicines.

If you fall into one of these groups, the year-round version of the advice applies rather than the winter-only one.

Food sources worth knowing about

Vitamin D turns up in a handful of foods, and it's worth knowing which ones pull their weight.

Oily fish is the standout: salmon, mackerel, sardines, herring, trout. Egg yolks, red meat and liver contain smaller amounts. Then there are fortified foods — many breakfast cereals, some fat spreads, and a growing number of plant milks and yoghurts. Mushrooms exposed to UV light also contain vitamin D, and some supermarkets label them accordingly.

The honest catch is volume. Hitting 10 micrograms a day from food alone means oily fish several times a week plus fortified staples, which isn't realistic for most households. Treat food as a top-up rather than the main plan: a salmon dinner may give you a useful amount, but it won't cover the week. Check labels on fortified products, since levels vary a lot between brands.

Do you need a blood test?

Usually not. UK guidance is to consider a daily supplement through autumn and winter regardless of test results, because the dose involved is small and safe for most people.

A GP may arrange a blood test if you have symptoms that could point to deficiency, if you have a condition affecting absorption, or if you take medication that interacts with vitamin D. That's the sensible route: the result gets interpreted alongside your history, and any high-dose treatment is prescribed properly.

Private tests are easy to order online and can be useful if you're curious. But a number you don't know how to act on can create more worry than clarity. If you do test privately and the result is low, take it to your GP rather than self-prescribing large doses.

Choosing and taking a supplement

• Form: vitamin D3 (colecalciferol) is the usual choice.
• Dose: 10 micrograms a day, written as 10 µg or 400 IU, suits most adults.
• Timing: take it with a meal containing some fat, since vitamin D is fat-soluble. Breakfast is an easy anchor.
• Watch for doubling up: check your multivitamin, cod liver oil or fortified drink before adding a separate tablet.

Doses above 100 micrograms (4000 IU) a day aren't something to take on your own initiative — that's a conversation with a GP or pharmacist. If you take other medicines, particularly certain heart medicines and diuretics, it's worth a two-minute question at the pharmacy counter before you start.

Babies and children

Young children have their own guidance. Breastfed babies are usually advised to have vitamin D drops from birth, and children up to four are advised to take a daily supplement, unless they're drinking more than 500ml of fortified formula a day. Ask your health visitor, pharmacist or GP for the right product and amount rather than scaling down an adult label.

A winter routine that actually sticks

1. Start supplementing in October and keep going until the end of March.
2. Take your tablet with breakfast, or whichever meal you never skip.
3. Keep taking it year-round if you're indoors most days, have darker skin, or cover up outdoors.
4. Eat oily fish once or twice a week if you enjoy it, and keep a fortified cereal or plant milk in the cupboard.
5. Get outside in daylight when you can. It won't make vitamin D in winter, but it helps mood, sleep and activity.
6. Note anything that feels off — persistent aches, exhaustion, low mood — and raise it at your next GP appointment.

Vitamin D is one of the few areas of wellness where the advice is genuinely simple: a cheap daily tablet, a decent diet, and a bit of sun when the season allows. If you're pregnant, breastfeeding, managing a long-term condition, or you've been told your levels are low, speak to your GP or pharmacist before changing what you take — especially if a high dose is being suggested.

Photo: Lufina / Pixabay