For about four months of the year, you can stand outside at noon on a clear day in Madison and your skin will make almost no vitamin D. A bright, freezing January day doesn't count for much, and an extra hour on the trail doesn't change it. The sun just sits too low in the sky.
That doesn't mean everyone needs a supplement. It does mean that from roughly November to February, your vitamin D has to come from food, from what your body stored over the summer, or from a bottle. Which one makes sense depends on you.
Can you get vitamin D from the sun in a Wisconsin winter?
Not much. Your skin makes vitamin D when UVB light hits it, and UVB only gets through the atmosphere well when the sun is fairly high. Madison sits at about 43° north. A well-known study done in Boston, which is at almost the same latitude, found that skin made essentially no vitamin D from November through February, even at midday. March is weak too.
A quick way to check on any given day is the shadow rule. If your shadow is longer than you are tall, the sun is too low for your skin to make much vitamin D. On a January afternoon in Wisconsin your shadow is long all day.
Your body does store some vitamin D from the summer in fat and the liver, but those stores run down over the winter. That's why blood levels in northern states tend to be lowest in late winter and early spring.
Who is most likely to run low in winter
Anyone can come out of a Wisconsin winter with low vitamin D, but some people are more likely to:
- People who work indoors or rarely get outside midday, even in summer
- Adults over 70, because skin makes less vitamin D with age
- People with darker skin, since more melanin means less vitamin D from the same sunlight
- People with larger bodies, because vitamin D gets stored in fat and less of it stays in circulation
- People who don't absorb fat well, since vitamin D is fat-soluble
- Breastfed babies. Breast milk is low in vitamin D, which is why pediatricians usually recommend drops. Ask your child's doctor.
Foods that actually help
Very few foods have much vitamin D naturally. Fatty fish is the standout. Most of what Americans get from food comes from fortified products like milk and cereal. Here's roughly what a serving gives you, based on the National Institutes of Health's figures:
| Food | Serving | Vitamin D (approx.) |
|---|---|---|
| Rainbow trout, farmed, cooked | 3 oz | 645 IU |
| Sockeye salmon, cooked | 3 oz | 570 IU |
| Mushrooms exposed to UV light | ½ cup | 366 IU |
| Fortified milk (2%) | 1 cup | 120 IU |
| Fortified soy, almond or oat milk | 1 cup | 100–144 IU |
| Fortified breakfast cereal | 1 serving | about 80 IU |
| Sardines, canned in oil | 2 sardines | 46 IU |
| Egg | 1 large | 44 IU |
Fish twice a week plus a glass of fortified milk most days gets you a good part of the way. Food alone rarely reaches the daily recommendation for most adults, though, especially if you don't eat fish. Check the label on mushrooms: only ones grown or exposed under UV light have meaningful amounts, and the package usually says so.
Should you take vitamin D in winter?
For a lot of people in the upper Midwest, a winter supplement makes sense, especially anyone on the list above. If you're not sure whether you need one, the best answer comes from a blood test. Your doctor can order a 25-hydroxyvitamin D test, which is the standard way to check your level. That's worth doing if you're thinking about taking more than the basic daily amount, or if you have a health condition or take medications.
If you live in the Madison area, The Healthy Place Clinic, our sister practice, offers vitamin D testing. Everyone else can ask their own doctor.
How much is enough, and how much is too much?
The National Institutes of Health sets these daily amounts:
- Ages 1–70: 600 IU (15 mcg) a day
- Over 70: 800 IU (20 mcg) a day
- Upper limit for adults and kids 9 and up: 4,000 IU (100 mcg) a day from all sources combined
Plenty of popular supplements are 1,000 or 2,000 IU, which sits comfortably between the daily amount and the upper limit. Labels list vitamin D in both IU and mcg; 1 mcg is 40 IU.
What about 5,000 IU a day? Shelves are full of 5,000 IU products, so it's a fair question. It's above the upper limit, though, which makes it a dose to talk over with a clinician, not one to start on your own. Some people take that much under a clinician's guidance, usually after a blood test shows a low level, and usually with follow-up testing. Vitamin D builds up in the body, and too much over time can raise calcium levels in the blood. If you think you need a high dose, get tested first.
D3 vs. D2, and why people pair D with K2
Supplements come in two forms. Vitamin D3 (cholecalciferol) is the kind your skin makes. Vitamin D2 (ergocalciferol) comes from plants and fungi. Both work, but studies have found D3 raises blood levels more and keeps them up longer, which is why most supplements use it. Vegan D3 made from lichen is available too.
You'll also see D3 sold with vitamin K2. Vitamin D helps your body absorb calcium, and vitamin K is involved in how the body uses that calcium, including in bone. That's the reason for the pairing. One caution: if you take warfarin or another blood thinner, talk to your doctor before taking anything with vitamin K, since it can interfere with how those medications work.
Picking a form that fits your routine
Honestly, the best vitamin D supplement is the one you'll remember to take. Whatever form you pick, take it with a meal that has some fat in it, since vitamin D is fat-soluble and absorbs better that way. Breakfast with eggs works. So does dinner.
- Drops are easy to dose and good if you don't like swallowing pills. Bluebonnet Liquid Vitamin D3 Drops give 2,000 IU in a single drop.
- Capsules are simplest if you already take other supplements. Our own Lively Vitamin Co. Solar Power comes in a 2,000 IU size that pairs vegan D3 with K2.
- Gummies are for people who won't take anything else. Nordic Naturals Vitamin D3 + K2 Gummies are 1,000 IU each.
You can compare more options on our vitamin D supplements page, or ask at any of our stores.
A simple winter vitamin D plan
- Get outside when you can. You won't make much vitamin D between November and February, but by late March the midday sun starts to count again.
- Eat a few vitamin D foods each week. Fatty fish, fortified milk or plant milk, and eggs are the easy ones.
- Get tested if you're unsure. A blood test tells you whether you need to do anything at all.
- If you supplement, follow the label and stay under 4,000 IU a day unless a clinician tells you otherwise.
For kids, the amounts are different. See our posts on vitamin D in kids and supporting kids through the school year.
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. This article is for general educational purposes and is not medical advice; talk with a qualified healthcare provider about your individual needs.
Written by The Healthy Place Wellness Team. The Healthy Place is a family-owned health store with locations in Madison, Sun Prairie, and Fitchburg, Wisconsin.
