Winter Vitamin D: Why Most People Are Deficient

If you live north of Los Angeles, your body probably isn't making vitamin D right now. Not because you're staying inside (though that doesn't help). Because the sun isn't strong enough.

Between November and March, the angle of the sun is too low for your skin to produce vitamin D, even on a bright day. This is true for anyone living above roughly 37 degrees north latitude, which includes most of the U.S., all of Canada, and most of Europe.

The result? Winter vitamin D deficiency is extremely common. And it affects more than just your bones.

Why Sunlight Doesn't Work in Winter

Your skin makes vitamin D when UVB rays hit it. But UVB rays are easily blocked. Cloud cover blocks them. Windows block them. Sunscreen blocks them. And in winter, the atmosphere itself blocks them.

When the sun sits low in the sky, its rays have to pass through more atmosphere to reach you. The extra distance filters out most of the UVB before it gets to your skin. North of Atlanta, vitamin D production drops to zero from November through February.

You could stand outside naked at noon in January in Boston, and your vitamin D levels wouldn't budge. Not a little. Zero.

What Happens When You Run Low

Vitamin D isn't just for bones (though it does help you absorb calcium). It has receptors in nearly every tissue in your body, which means low levels affect multiple systems.

Your Immune System Gets Weaker

Vitamin D activates T-cells, which are your body's first responders to infections. Low vitamin D is linked to higher rates of respiratory infections, which explains part of why colds and flu spike in winter.

A 2017 meta-analysis in the British Medical Journal found that vitamin D supplementation reduced the risk of acute respiratory infections, especially in people who were deficient to begin with. The effect was strongest when people took daily or weekly doses, not massive monthly boluses.

Your Mood Takes a Hit

Vitamin D receptors exist in brain regions involved in mood regulation. Low levels are associated with seasonal affective disorder (SAD). Studies on supplementation for SAD show mixed results, though it seems to help some people.

A 2020 study found that people with blood levels below 20 ng/mL (which is considered deficient) were significantly more likely to report symptoms of depression. Bringing levels back to normal didn't eliminate depression, but it helped. Not a cure, but something.

Your Bones Slowly Weaken

Without enough vitamin D, your body can't absorb calcium properly. Over time, this leads to weaker bones and higher fracture risk, especially in older adults. Hip fractures peak in late winter and early spring, when vitamin D levels hit their annual low.

How Much Do You Need?

The official recommendation from the National Institutes of Health is 600-800 IU per day for most adults. But many vitamin D researchers, including Michael Holick at Boston University, argue that's too low, especially if you're starting from a deficient baseline.

The Endocrine Society's clinical practice guidelines suggest 1000-4000 IU per day for adults, depending on your starting blood level and sun exposure. For context:

• One serving of fortified milk has about 100 IU
• A serving of salmon has around 600 IU
• Your body can make 10,000-20,000 IU in 20 minutes of full-body sun exposure (in summer, at the right latitude, without sunscreen)

Food alone won't get you there in winter. You need a supplement.

Testing Your Levels

If you want to know where you stand, ask your doctor for a 25-hydroxyvitamin D blood test. This is the standard marker for vitamin D status.

The interpretation goes like this:

• Below 20 ng/mL: Deficient
• 20-30 ng/mL: Insufficient
• 30-50 ng/mL: Adequate
• Above 50 ng/mL: Probably fine, though some experts aim for this range
• Above 100 ng/mL: Too high (can cause toxicity)

Many people test in the 15-25 ng/mL range in late winter, which is suboptimal. The Endocrine Society recommends keeping levels above 30 ng/mL year-round.

How to Supplement

Vitamin D comes in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). D3 is better. It raises blood levels more effectively and keeps them elevated longer.

You can take it any time of day, but it's fat-soluble, so taking it with a meal that contains fat improves absorption.

Most over-the-counter supplements come in 1000, 2000, or 5000 IU doses. For general winter supplementation, 2000-4000 IU per day is a safe starting point for most adults. If you test low, your doctor might recommend a higher dose to bring you back up.

Vitamin D toxicity is rare and usually only happens with chronic megadoses (like 10,000+ IU daily for months). The upper tolerable limit is 4000 IU/day, though many people take more under medical supervision without issues.

Don't Forget About K2

If you're taking high doses of vitamin D (above 2000 IU), consider pairing it with vitamin K2. Vitamin D increases calcium absorption, but K2 directs that calcium into your bones instead of your arteries. The Rotterdam Study and other research on cardiovascular health suggest that taking D without K2 might contribute to arterial calcification over time, though the evidence is still debated.

The usual dose is 100-200 mcg of K2 (MK-7 form) alongside your vitamin D. Many supplements now combine the two.

What to Do

If you live in a place with real winters, assume you're low on vitamin D from November through March. You can't make it from sunlight, and you won't get enough from food. This isn't a maybe situation. It's just how the physics work.

A daily supplement is cheap and makes a measurable difference. Unlike most supplements, where deficiency is rare and benefits are questionable, vitamin D deficiency is common enough that supplementation actually helps. I take 2000 IU daily from October through April and have for years.

Test your levels if you want to dial it in. Otherwise, 2000 IU per day is a reasonable default.

Sources: Holick MF. Vitamin D Deficiency. N Engl J Med. 2007. | Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory tract infections. BMJ. 2017. | Menon V, et al. Vitamin D Status and Depression. Cureus. 2020.