Your zip code decides more about vitamin D than the pill aisle

Quick answer: If you live north of roughly Memphis, Charlotte and Albuquerque, your skin makes next to nothing from the winter sun. Whether that matters for you is a blood-test question, not a guess. Ask your doctor before you buy a bottle or go above 4,000 IU a day.

A man in Minneapolis can stand outside on a bright January afternoon, squinting, feeling virtuous, and make almost no vitamin D. The sun is up. The angle is wrong. The ultraviolet B rays your skin needs get filtered out by the thick slice of atmosphere they have to cross.

That's the whole story of vitamin D and geography. Everything else is detail.

The line, and what it means

Your skin makes vitamin D when UVB hits it. Closer to the equator, that works most of the year. Farther north, there's a stretch of winter, roughly November through February or March, when the sun is too low to do much. The line runs around 35 degrees north latitude. Memphis, Charlotte and Albuquerque sit on it. Every Canadian city is above it.

Live in Phoenix or Miami and you probably make plenty from ordinary outdoor life. Live in Chicago, Denver, Boston, Seattle or Winnipeg and your body is running on whatever it stored up over summer. That reserve doesn't last forever.

Your address sets the odds. It doesn't settle your number.

Who runs low no matter where they live

Plenty of men in Texas are low. Plenty in Michigan are fine. Several things move the needle more than the map does:

• Indoor work. An office, a warehouse or a truck cab all block the sun. A roofer in Ohio often does better than an accountant in Florida.
• Darker skin. More melanin means less vitamin D made from the same sun. This is well established, and it's why men with darker skin are more likely to run low.
• Extra weight. Vitamin D is stored in body fat, which can leave less of it circulating in the blood.
• Age. Skin gets less efficient at making it as you get older.
• Covering up. Long sleeves, a ball cap and a beard all cut exposure.
• Gut and medication issues. Some digestive conditions, weight-loss surgery and certain long-term medications interfere with absorption or processing. Your doctor will know if that's you.

If three of those describe you, you aren't guessing anymore. You're a candidate for a test.

Test, don't guess

The blood test is called 25-hydroxyvitamin D, or 25(OH)D. It's usually a single draw and it can ride along with other bloodwork. If you're already getting a glucose or A1C check, ask whether it can go on the same order.

Here's the annoying part. Experts don't fully agree on what a good number is. The National Academies set 20 ng/mL as enough for most people's bone health, and a level under 12 is considered clearly deficient. The Endocrine Society points higher, toward 30. Labs print their own ranges on the report. So the number alone doesn't give you an answer. Your doctor reading it next to your history does.

And routine testing for everyone isn't recommended. The U.S. Preventive Services Task Force looked at screening adults with no symptoms and concluded there isn't enough evidence to call it either way. That's an argument for testing on purpose when you have risk factors, not for testing every man every year. Insurance may not cover it without a reason on the order, so ask what it costs first. The four numbers that set your plan's costs will tell you whether you're paying out of pocket.

The best time to raise it is your regular physical, the same visit where you'd talk through the prostate question at fifty.

If you're low, or close to it

Food helps, but it rarely carries the load. Fatty fish like salmon and trout are the best natural source. Milk is usually fortified at about 100 IU a cup. Egg yolks have a little. Some mushrooms exposed to UV light have more. Most men don't eat enough of any of those to move a low number much.

That's where supplements come in. Some facts to hold onto, with your doctor setting the actual plan:

• The recommended daily amount for adults is 600 IU, rising to 800 IU after age 70.
• The tolerable upper limit for adults is 4,000 IU a day. Going past it without a doctor watching is how men end up with problems.
• Vitamin D3 is the form most doctors reach for. It's fat-soluble, so take it with a meal that has some fat in it, not with black coffee at 6 a.m.
• More isn't better. Too much can raise calcium in your blood and lead to nausea, kidney stones and worse. That comes from supplements, not from sun.
• Check the label for IU or micrograms. One microgram equals 40 IU. Men have mixed those up.

If your level is truly low, a doctor may prescribe a higher dose for a set period and then retest. That's a medical plan, not a bottle-of-mega-dose plan.

What the pill won't do

The supplement industry loves to sell vitamin D as the fix for everything. It isn't.

A large randomized trial of more than 25,000 generally healthy adults, published in the New England Journal of Medicine, gave people 2,000 IU a day or a placebo for several years. It didn't find fewer cancers or fewer heart attacks and strokes in the vitamin D group. Men who weren't deficient didn't get a bonus from topping off.

Same with the testosterone pitch you'll see on a certain kind of label. The evidence there is thin and mixed. Don't buy a bottle for that.

Being tired, achy or down in the winter has plenty of possible causes. Low vitamin D is one. So are sleep, stress and a thyroid that's off. Fix it in the right order: see the doctor, get the labs, then decide. Skipping to the supplement just means you're guessing with your wallet. If the gloom runs deeper than a gray February, it's worth reading how stress doesn't stay in your head.

Keep the sunscreen

Don't skip sun protection to chase vitamin D. Sun damage is a proven cause of skin cancer, and a pill is a safer way to cover a shortfall than a sunburn. Any broad-spectrum SPF 30 or higher, worn every day, beats going without. Reapply every two hours, and after swimming or sweating.

If you're choosing, we'd pick mineral: zinc oxide or titanium dioxide. They sit on top of the skin, and they're the only two filters the FDA proposed in 2019 as generally recognized as safe and effective. It asked makers for more safety data on the rest. FDA studies published in JAMA in 2019 and 2020 found common chemical filters in the bloodstream after use. Approved chemical sunscreens are still considered safe and effective by the FDA and dermatologists. Mineral just has the cleaner record, and it's gentler on sensitive skin.

The trade-off is a white cast on some formulas. Tinted ones fix it. EltaMD UV Physical SPF 41 is tinted. Blue Lizard Sensitive Mineral SPF 30 and Neutrogena Sheer Zinc SPF 50 are other mineral options.

The short version of a long winter

Look at your address, your job, your skin and your waistline. If two or more point toward low, tell your doctor you'd like the test. If you're a healthy guy in Tucson who's outside half the day, leave the aisle alone.

Know your number before you pay to change it.