Vitamin D and the Michigan Winter: What West Michigan Needs to Know

There's a piece of geography most West Michigan residents have never been told, and it changes how you should think about the next six months.

Vitamin D isn't really a vitamin. It's a hormone your skin manufactures when UVB radiation hits it. And UVB has to travel through the atmosphere at a steep enough angle to reach the ground at useful intensity. When the sun sits low in the sky, that radiation gets scattered and absorbed on the way down.

Holland sits at roughly 42.8° north latitude. From about mid-October through mid-March, the sun here simply doesn't climb high enough for meaningful vitamin D production — no matter how bright the day looks, no matter how long you stand outside.

You can have a crisp, brilliant, blue-sky February afternoon on the lakeshore and make essentially zero vitamin D.

Why this matters more than it sounds

Vitamin D's best-known job is helping your body absorb calcium and maintain bone density. But receptors for it show up throughout the body, and research over the past two decades has connected vitamin D status to immune function, mood regulation, muscle strength, and inflammatory balance.

Which lines up uncomfortably well with a Michigan winter. The season when our vitamin D levels bottom out is the same season we're indoors, getting sick, and feeling flat.

Low vitamin D isn't the sole explanation for winter fatigue or low mood — light exposure, activity levels, and sleep all matter too. But it's one of the few pieces you can measure and correct directly.

Who's most affected here

Vitamin D drops further and faster for some people than others:

• Anyone with darker skin. Melanin is effective UV protection, which means significantly more sun exposure is needed to produce the same vitamin D. In a northern climate, that's a compounding disadvantage.

• Older adults. The skin's capacity to synthesize vitamin D declines with age.

• People who are mostly indoors. Office work, shift work, homebound seniors.

• People carrying more body fat. Vitamin D is fat-soluble and gets sequestered in adipose tissue, leaving less circulating.

• Anyone with fat malabsorption issues — celiac, Crohn's, or a history of certain weight-loss surgeries.

• Exclusively breastfed infants, which is why pediatricians routinely recommend drops.

How much do you actually need?

Official U.S. recommendations are 600 IU (15 mcg) per day for most adults, and 800 IU for adults over 70. Those figures were set with bone health in mind and assume minimal sun exposure.

Many clinicians working in northern latitudes suggest more than that through the winter months, often in the 1,000–2,000 IU range. The established safe upper limit for adults is 4,000 IU per day, and doses above that shouldn't be taken without medical supervision — vitamin D is fat-soluble, so it accumulates rather than flushing out.

The honest answer is that the right dose for you depends on where you're starting from, which brings us to the part most people skip.

Get tested — it's a simple blood draw

The test is called 25-hydroxyvitamin D, or 25(OH)D. It's inexpensive, commonly ordered, and usually covered when there's a reason to run it.

Interpretation varies. Many labs flag anything under 20 ng/mL as deficient, while a good number of practitioners prefer to see patients above 30 ng/mL. Your doctor can tell you where you land and what to do about it.

If you test once, make it late winter — February or March, when your levels are at their annual low. That's the number that tells you the most.

Choosing a supplement

D3 over D2. Vitamin D3 (cholecalciferol) is the form your skin produces and it's more effective at raising and holding blood levels than D2 (ergocalciferol). D3 is usually derived from lanolin; vegan D3 made from lichen is available and works the same way.

Take it with fat. Vitamin D is fat-soluble, so absorption improves substantially when it's taken alongside a meal containing some fat. With breakfast eggs or dinner works better than on an empty stomach.

Consider K2. Vitamin D increases calcium absorption; vitamin K2 helps direct that calcium into bone rather than soft tissue. Many quality D3 products now pair the two, and the combination is worth asking about — particularly if you're supplementing at higher doses.

Don't forget magnesium. The enzymes that convert vitamin D into its active form require magnesium. If your magnesium intake is low, supplementing vitamin D may not get you as far as you'd expect. (If you're not sure which magnesium to take, we wrote a whole guide to that.)

Liquid drops are an easy option. Especially for kids, for anyone who dislikes swallowing capsules, or if you'd rather add it to a morning drink.

Food helps, but only so much

Very few foods contain meaningful vitamin D naturally:

• Fatty fish — salmon, mackerel, sardines, herring

• Cod liver oil

• Egg yolks

• UV-exposed mushrooms (a genuinely interesting one — mushrooms make vitamin D from UV light much like we do)

• Fortified milk, yogurt, and plant milks

Even a solid diet built around these tends to fall short of winter requirements on its own. Food is a contributor here, not a solution.

What else helps in the dark months

Vitamin D is one lever. A few others worth pulling:

• Get outside anyway. You won't make vitamin D in January, but morning daylight still anchors your circadian rhythm and supports mood. A bundled-up walk before 10 a.m. does real work.

• Consider a light box if winter reliably flattens your mood. Talk with your provider about whether it's appropriate for you.

• Keep moving. Winter activity holds up mood, sleep, and energy in ways supplements can't replicate.

Start now, not in February

The practical takeaway: your body has been building vitamin D stores all summer, and those stores begin drawing down as soon as the sun angle drops. Starting a supplement in October means you're topping off a reserve. Starting in February means you're digging out of a hole.

Stop in and we'll walk you through the D3 options — strengths, drops versus capsules, and which ones include K2.

This article is for general educational purposes and isn't medical advice. Talk with your healthcare provider before starting a new supplement or changing your dose, especially if you're pregnant, nursing, managing a health condition, or taking prescription medication.