Vitamin D in Winter: Why Grey Days Matter

Grandpa grew up somewhere the winter sun barely bothered to show up. His answer? A spoonful of cod liver oil, a brisk walk at lunchtime and a firm belief that "a bit of sun on your face sorts most things." He wasn't wrong to think about it. Here's why vitamin D deserves a little extra attention when the days get short.

Quick heads-up: This guide is general education about vitamin D. Grandpa Finn's doesn't sell cod liver oil or a standalone vitamin D, and nothing here is a claim about our products. Our range is Omega-3 Fish Oil (EPA and DHA), a Daily Multivitamin and the Daily Stack with both.

What vitamin D does

According to the NIH Office of Dietary Supplements, vitamin D helps your body absorb calcium, one of the main building blocks of bone. Your muscles need it to move, your nerves need it to carry messages, and your immune system needs it too.*

In supplement-label language: vitamin D supports normal bones, normal muscle function and normal immune system function.*

Why winter is the tricky season

Your skin makes vitamin D from sunlight

Your body makes vitamin D when bare skin is exposed to the sun's UVB rays. In winter, especially further from the equator, those rays are weaker.

The numbers

Harvard's Nutrition Source notes that people in Boston, Edmonton and Bergen can't make enough vitamin D from the sun for about 4, 5 and 6 months of the year respectively. In the southern hemisphere, people in Buenos Aires and Cape Town make far less in their winter months (June to August).

The UK's NHS says that between October and early March, people there don't make enough vitamin D from sunlight. It advises that everyone should consider a daily 10 mcg vitamin D supplement during autumn and winter.

Other things that reduce vitamin D from the sun

The NIH and Harvard list:

• Clouds and smog
• Spending most of your time indoors
• Sunlight through a window (glass blocks UVB)
• Sunscreen and clothing that covers most of your skin
• Darker skin tones
• Older age

Don't ditch the sunscreen, though. Too much sun is harmful to your skin, and health experts still recommend sun protection.

Who's more likely to run low?

The NIH lists these groups:

• Older adults, because the skin makes less vitamin D with age
• People who are rarely outdoors or keep their skin covered
• People with darker skin
• People with conditions that affect fat absorption
• Breastfed infants (speak to your doctor or child health nurse)
• People with obesity or who've had gastric bypass surgery

If you think you might be low, the reliable way to know is a blood test. Ask your doctor.

How much vitamin D do you need?

Recommendations differ by country, so here are two respected ones:

Check your own country's health authority for local advice.

Don't overdo it

Vitamin D is stored in the body, so more is not better. The NIH and NHS both set an adult upper limit of 100 mcg (4,000 IU) a day from all sources. The NIH says too much can cause nausea, vomiting, muscle weakness and kidney stones, and very high levels can be dangerous. You can't overdose from sunshine; it's almost always from supplements.

Winter vitamin D sources

Food

Very few foods contain much vitamin D naturally. The NIH and Harvard list:

• Fatty fish: salmon, trout, tuna, mackerel, sardines
• Fish liver oils, including cod liver oil, which Harvard lists among the best natural sources
• Egg yolks, beef liver and cheese (smaller amounts)
• UV-exposed mushrooms
• Fortified foods, such as some milks, cereals and spreads

Supplements

Vitamin D comes in standalone supplements, multivitamins and cod liver oil. The NIH says both D2 and D3 raise blood levels, though D3 might raise them higher and for longer. Because vitamin D is fat-soluble, take it with a meal or snack containing some fat.

A note on cod liver oil

Cod liver oil gives you vitamin D plus omega-3s and vitamin A. Just remember the vitamin A: if you also take a multivitamin or eat liver, add up your totals, and avoid cod liver oil in pregnancy unless your doctor says otherwise. More in cod liver oil vs fish oil.

Medicines that interact with vitamin D

The NIH lists several, including orlistat, some steroids such as prednisone, statins (at high vitamin D doses) and thiazide diuretics. If you take any regular medicine, check with your doctor or pharmacist.

Frequently asked questions

Do I need vitamin D in winter?

Many people make less vitamin D in winter. The NHS advises everyone in the UK to consider a daily supplement in autumn and winter. Check your local guidelines and ask your doctor.

Can I get vitamin D through a window?

No. Glass blocks the UVB rays your skin needs to make vitamin D.

Is cod liver oil a good source of vitamin D?

Yes. Harvard and the NIH both list fish liver oils among the best natural sources.

Is vitamin D a winter cure-all?

We can't claim that. Vitamin D supports normal immune system function,* but supplements aren't intended to diagnose, treat, cure or prevent any disease.

How do I know if I'm low in vitamin D?

A blood test from your doctor is the only reliable way.

Grandpa's verdict

Short days, long coats, grey skies: winter makes vitamin D harder to come by. Get outside when you can, eat your oily fish, and if you top up, stick to sensible amounts.

Want a top-up? Grandpa Finn's makes an Omega-3 Fish Oil and a Daily Multivitamin, or grab both together as the Daily Stack. Shop the range.

Sources

• NIH Office of Dietary Supplements, Vitamin D – Consumer Fact Sheet
• Harvard T.H. Chan School of Public Health, The Nutrition Source, Vitamin D
• NHS, Vitamin D

Grandpa's fine print: This article is general information only, not medical advice. Supplements don't replace a varied, balanced diet. Talk to your doctor or pharmacist before starting any supplement, especially if you're pregnant, planning a pregnancy or breastfeeding, taking blood thinners (anticoagulants such as warfarin) or other medicines, have a health condition, or are buying for a child. Always read the label and use only as directed.

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.