Do You Need Vitamin D in the Winter?

Nov 1

The clocks are about to fall back. The days are getting short, and by the time most of us leave work, the sun is gone.

That's usually when clients start asking me the same thing: "Should I be taking vitamin D this winter?"

It's a fair question. Vitamin D is one of the most popular supplements out there, and the internet will tell you it fixes everything from your mood to your immune system. The real answer is more boring, but more useful. Let's walk through it.

What Vitamin D Actually Does

Vitamin D helps your body absorb calcium. That's its main, proven job. Without enough of it, your bones can get soft and weak. In kids, that's called rickets. In adults, it's called osteomalacia.

You get vitamin D in three ways:

Sunlight. Your skin makes it when UVB rays hit it.
Food. A few foods have it naturally, and many are fortified.
Supplements. Usually as vitamin D3 or D2.

Why Winter Is Different

Here's the part most people don't know. In winter, the sun sits low in the sky, and most of the UVB rays your skin needs get filtered out before they reach you.

A classic study tested this directly. In Boston, winter sunlight made no vitamin D in the skin from November through February, even on clear days. Farther north, in Edmonton, Canada, the dead zone ran from October through March. Down at lower latitudes, winter sun still worked.

We're a bit south of Boston here in Northern Virginia and DC, so it's not quite as bad. But winter production is still low. Add in cold weather, long sleeves, and indoor jobs, and most of us make very little vitamin D from about November to February.

One more thing: UVB doesn't pass through glass. Sitting by a sunny window feels nice, but it won't help your vitamin D.

What the Evidence Says

This is where I want to be honest, because the hype has gotten ahead of the science.

For bones, vitamin D clearly matters. Real deficiency is bad for your skeleton. That's well established.

For preventing big diseases, the results have been disappointing. The VITAL trial gave about 26,000 healthy U.S. adults either 2,000 IU of vitamin D a day or a placebo, for up to six years. Vitamin D did not lower the rate of cancer, heart attacks, or strokes.

Experts have adjusted their advice. In 2024, the Endocrine Society reviewed the research and concluded that healthy adults under 75 are unlikely to benefit from taking more than the standard recommended amount. They also advised against routine vitamin D blood tests for healthy people, because there's no clear evidence that chasing a certain number improves health.

They did find possible benefits from somewhat higher amounts in a few groups: kids and teens, adults 75 and older, pregnant women, and people with prediabetes. If you're in one of those groups, that's a conversation for your doctor.

So the takeaway is: getting enough vitamin D matters. Taking a lot more than enough probably doesn't do much extra.

How Much Do You Actually Need?

The National Institutes of Health recommends:

Ages 1–70: 600 IU per day
Over 70: 800 IU per day
Upper limit for adults: 4,000 IU per day from all sources

That's less than many people expect, especially when store shelves are full of 5,000 and 10,000 IU pills.

What to Actually Do This Winter

1. Start with food. A few good sources, per the NIH:

Sockeye salmon, 3 oz cooked: about 570 IU
UV-exposed mushrooms, ½ cup: about 366 IU (check the label)
Fortified milk, 1 cup: about 120 IU
Fortified soy, almond, or oat milk, 1 cup: about 100–144 IU
Fortified cereal, 1 serving: about 80 IU
One large egg: about 44 IU

Salmon twice a week plus fortified milk most days gets a lot of people close.

2. If your diet falls short, a basic supplement is reasonable. If you don't eat fish, skip dairy, or just know your diet is light on these foods, a supplement in the 600–1,000 IU range during the darker months is a simple, low-risk way to cover your bases. D3 tends to raise blood levels a bit better than D2. Take it with a meal that has some fat in it.

3. Skip the mega-doses unless your doctor prescribes them. More isn't better here, and it can cause problems.

4. Still get outside. Winter sun won't make much vitamin D, but daylight still helps your sleep rhythm, your mood, and your step count. A midday walk is never wasted.

Who Should Be Careful

People taking high doses. Too much vitamin D can raise calcium in your blood, which can cause nausea, weakness, kidney problems, and more. Stay under 4,000 IU a day unless a doctor tells you otherwise.
People on certain medications. Vitamin D can interact with steroids, some diuretics (water pills), statins, and the weight-loss drug orlistat. Check with your doctor or pharmacist.
People with kidney disease or calcium problems. Don't supplement without medical guidance.
People more likely to run low. Older adults, people with darker skin, people with obesity, people who've had gastric bypass, and people with gut conditions like celiac or Crohn's may need more attention. Talk to your doctor instead of guessing.

Coach's Take

I see two kinds of people every winter. One takes nothing and wonders why they feel run down. The other takes a handful of high-dose pills and expects them to replace sleep, real food, and movement.

Neither one is the answer. Vitamin D is a small piece of the puzzle. Cover the basics: eat some fish and fortified foods, take a modest supplement if your diet doesn't cut it, and keep training through the winter. The stuff that really keeps you strong and healthy in January is the same stuff that works in July: lifting, walking, sleeping, and eating well. Consistency beats any pill.

Don't Let Winter Stall Your Progress

Shorter days are when a lot of people fall off track. If you want help staying strong and consistent this winter, I'd love to work with you. I offer mobile 1-on-1 training across Northern Virginia and DC, plus app-based online coaching you can do from anywhere.

Book your session or start online coaching at zenithstrong.com

Sources

This post is for educational purposes and isn't medical advice. Talk with your doctor before starting a new supplement, especially if you have a health condition or take medication.

Kevin Kornieck https://www.zenithstrong.com