A cold front rolls across the UK, daylight shrinks, and central heating hums. Energy dips, sniffles rise, and your skin spends weeks hiding under wool and waterproofs. There’s a fast way to read what that means for your vitamin D. And it takes the time it takes a lift door to close.
A man behind me rubs his neck and mutters about “this tiredness I can’t shake.” The barista jokes she gets daylight only during her smoke breaks, then we all laugh in that damp winter way.
On the bus, a woman scrolls news about colds while stretching her fingers under the heater. Tiny, ordinary moments. Little signals. We’ve all had that moment when you look down and realise your arms haven’t seen real sun in weeks.
Your skin can tell you in three seconds.
Why winter quietly empties your vitamin D tank
Stand outside at noon in January and your shadow runs long across the pavement. That’s your first clue: the sun’s angle is wrong for UVB, the slice of light your skin needs to make vitamin D. Glass blocks UVB too, so that sunny desk by the window doesn’t count.
Coats, scarves, fingerless gloves — all cosy, all UV-proof. And even if you brave the chill, you expose barely a postcard of skin. In short, winter nudges you towards a long, unintentional fast from the one nutrient nicknamed the “sunshine vitamin.”
Across the UK, surveys suggest winter levels plunge. In some datasets, roughly one in six adults dip into insufficiency as the season drags on, with higher rates in people with darker skin or those who cover up for cultural or personal reasons. Your body stores a bit from summer, but it drifts down month by month.
Vitamin D is less a classic vitamin and more a hormone messenger. It helps regulate calcium and phosphorus, supports bone strength, and backs your immune defences when coughs do the rounds. Stores fall with a half-life measured in weeks, so the tank that felt full in September can sound hollow in late January.
In the UK, October to March usually means your skin makes almost no vitamin D.
The 3‑second skin test (and what it really tells you)
Step into natural daylight, roll up a sleeve, and hold the underside of your forearm next to the outside. Take three slow seconds and just look. If both sides are equally pale, and you’ve spent weeks mostly indoors or covered at midday, that’s a quick nudge your skin hasn’t had the UV it needs to make vitamin D.
It’s not about beauty or tan; it’s about exposure. The underside of your forearm rarely sees sun, the top is the bit that would catch UV in normal life. If there’s no visible contrast at all in midwinter, it hints you’ve had little to no UVB exposure recently. *Clue*, not proof.
Dark skin won’t show a dramatic contrast even in summer, because melanin is protective. Lighter skin might show a faint summer remnant then fade. If you live north of Birmingham, the angle of the sun trims your chances further. Let’s be honest: nobody really does that every day.
Don’t use a bathroom mirror at night or fluorescent office lighting. Natural daylight is the context. Sun behind glass doesn’t count for UVB. If you’re a runner who trains at dawn or dusk, that’s kind to your joints, not your vitamin D. Keep it simple: midday daylight, three seconds, compare.
The three‑second skin test won’t diagnose you, but it flags risk fast.
This is where nuance matters. A person with darker skin can be low even if they’ve been outdoors. A person who supplements might be fine even if they’ve barely seen daylight. Think of the test as a nudge to ask better questions, not a verdict.
“Treat this as a traffic light. Red means consider a supplement or a blood test, amber means watch your habits, green means carry on — but keep an eye on the calendar.”
- You rarely get out at midday from October to March
- You cover most of your skin or wear sunscreen daily
- You have darker skin (more melanin slows vitamin D synthesis)
- You’re pregnant, breastfeeding, or over 65
- Your BMI is higher, or you have gut/absorption issues
- You eat little fish, eggs, or fortified foods
- You take medicines that affect vitamin D metabolism (e.g., some anticonvulsants)
A blood test is the only way to know for sure.
Small fixes that punch above their weight
For most people in the UK, a daily 10 microgram (400 IU) vitamin D supplement from October to March is the straightforward move. Choose D3 where possible; vegan D3 from lichen exists now too. Take it with a meal that contains some fat, because fat helps absorption. Food helps — oily fish, egg yolks, mushrooms exposed to UV, fortified milks and cereals — but food alone rarely meets winter needs.
Daylight still has a role. On bright days as spring returns, a short midday walk with face and forearms uncovered can top you up, long before you’d burn. Glass won’t help; step outside, even for five minutes. Track how you feel over a few weeks: energy, aches, mood, immune niggles. If something feels off or persists, book a test. The lab number to watch is 25‑OH vitamin D; many UK clinicians call under ~25 nmol/L deficient, over ~50 nmol/L sufficient.
One more thing: don’t go overboard. The safe upper limit for most adults is 100 micrograms (4,000 IU) a day unless guided otherwise. More isn’t more when it comes to fat‑soluble vitamins. Think steady, not spiky. A tiny, boring habit — a single tablet with breakfast — changes the season in quiet ways.
What this winter is really asking from you
Listen to the hints your skin and routine are giving you. The three-second glance at your forearm won’t replace a blood test, but it will cut through the noise and help you act sooner. Share it with a friend who’s dragging through February. Swap a couple of indoor lunches for five-minute daylight loops. Pick a supplement you’ll actually take. Tell your future self in April that you had their back. Winter takes what it’s owed; you decide how much.
| Key point | Detail | Interest for the reader |
|---|---|---|
| 3‑second skin test | Compare inner vs outer forearm in daylight to gauge recent UV exposure | Instant signal to adjust habits or consider testing |
| UK winter UVB drop | From October to March, the sun’s angle limits vitamin D synthesis | Explains why you may feel flat and why supplements help |
| Action that works | 10 µg (400 IU) daily D3, plus brief midday daylight when possible | Simple, safe steps to steady energy and immunity |
FAQ :
- How do I do the three‑second skin test?Step into natural daylight at midday, roll up a sleeve, hold the inner forearm next to the outer, and look for contrast. No contrast in midwinter often means little recent UV exposure.
- I have dark skin. Does the test still work?You may see less contrast even in summer because melanin protects against UV. Use the test as a prompt, not a verdict, and lean on supplements and, if needed, a blood test.
- How much vitamin D should I take?Most adults in the UK can take 10 µg (400 IU) daily in autumn and winter. Some people may need more or year‑round support; your GP or a blood test can guide that.
- Can I get vitamin D through a window?No. UVB doesn’t pass through standard glass. A bright desk warms you, but it won’t make vitamin D. Step outside for brief spells when you can.
- What are the warning signs of low vitamin D?Vague, overlapping things: low mood, muscle aches, bone tenderness, frequent colds, heavy fatigue. They’re nonspecific, which is why a test is helpful.
The « 3-second test » feels a bit too neat. Without a 25‑OH blood test, how do we prevent false reassurance—especialy for darker skin or supplement users? Also, any peer‑reviewed source for the “one in six” winter insufficiency stat?