Daily wellness
Vitamin D in a Michigan winter: what to know
From November into early spring, the sun over Michigan sits too low for your skin to make much vitamin D. Here is what that means, how much you need, and when it is worth asking about your levels.
Key points
- Your skin makes vitamin D from UVB sunlight, and winter sun at Michigan's latitude is weak.
- A classic study in Boston, at about the same latitude as Detroit, found no vitamin D made in skin from November through February.
- Adults through age 70 need 600 IU a day, and adults over 70 need 800 IU, from all sources combined.
- Darker skin, a higher body weight, older age and some medications can all make low vitamin D more likely.
- A licensed provider can help you decide whether testing or a supplement makes sense for you.
Why winter sun makes less vitamin D
Vitamin D is unusual because your body can make it. When bare skin is exposed to the UVB part of sunlight, it starts the process that becomes vitamin D. How much you make depends on how strong that UVB is, and that depends on the season and how far north you live.
In a well-known 1988 study in the Journal of Clinical Endocrinology and Metabolism, researchers exposed skin samples to sunlight in Boston, at 42.2 degrees north. On cloudless days from November through February, the samples made no previtamin D3, the first step toward vitamin D. Detroit sits at about the same latitude as Boston, and much of Michigan is farther north.
Harvard's Nutrition Source lists Boston among the cities where people can't make enough vitamin D from sunlight for part of the year. Gray skies, heavy coats and more time indoors only add to that. The NIH also notes that sunlight through a window does not count.
How much vitamin D adults need
The NIH Office of Dietary Supplements sets these daily amounts, which include food, drinks and supplements together:
- Adults 19 to 70: 600 IU (15 mcg) a day
- Adults over 70: 800 IU (20 mcg) a day
- Upper limit for adults: 4,000 IU (100 mcg) a day, unless a licensed provider is treating a deficiency
Where vitamin D comes from in food
Very few foods contain much vitamin D on their own. Fatty fish such as salmon, trout and sardines, along with fish liver oils, are the best natural sources. Egg yolks, cheese and beef liver have smaller amounts, and some mushrooms, especially ones exposed to UV light, add a little.
In the US, most people get the bulk of their vitamin D from fortified foods. Nearly all cow's milk is fortified, and many plant milks, cereals and juices are too. Checking labels in the winter months is an easy first step.
Who is more likely to run low
According to the NIH and Harvard, some groups tend to have lower vitamin D levels:
- People with darker skin, because melanin reduces how much vitamin D skin makes from sunlight
- People with obesity, because vitamin D is stored in body fat and released less easily
- Older adults, whose skin makes vitamin D less efficiently
- People who have had gastric bypass surgery or have conditions that limit fat absorption, such as Crohn's or celiac disease
- People taking certain medications, including orlistat, cholestyramine or steroids such as prednisone
Testing, supplements and your licensed provider
If you are in one of these groups, or you are working on weight care and eating less overall, it can help to raise vitamin D at your next visit. A blood test called 25-hydroxyvitamin D measures your level, and your licensed provider can tell you whether it is worth checking and what your result means.
More is not always better. Vitamin D is stored in the body, and very high intakes over time can raise blood calcium. It can also interact with some medications, including thiazide water pills and statins at high doses. That is why we suggest sharing any supplement you take with your licensed provider, who decides what fits your health history.
Our daily wellness programs are built around ongoing care with a licensed provider. Vitamin D is one small part of the picture, alongside sleep, movement, protein and the basics that carry you through a long Michigan winter.
Sources
- Vitamin D: Fact Sheet for Consumers. NIH Office of Dietary Supplements
- Vitamin D. The Nutrition Source, Harvard T.H. Chan School of Public Health
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab. 1988;67(2):373-378 (Scilit record)
This article is educational and is not medical advice. Rx programs require an online visit with a licensed provider, who decides whether a treatment is right for you. Results vary. Compounded medications are not FDA-approved.