You probably know the feeling of a thinner autumn sun, the kind that still looks bright but no longer warms your skin the same way. Most of us notice the shift and move on. For your vitamin D levels, that quiet change in the sunlight means something very specific.
Blood levels of vitamin D in temperate countries follow a predictable yearly curve. They peak in late summer and reach their lowest point in late winter, often in February or early March.
A 2016 review in Nutrients mapped UV sunlight data and population blood samples across Europe. In Irish adults, the summer peak and late-winter low were about 25 nmol/L apart, roughly a third of a healthy mid-range reading.
The reason is physics. Your skin needs UVB rays strong enough to turn a cholesterol-like molecule into vitamin D. From October onward, those rays weaken fast across the northern United States.
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The farther north you live, the longer the stretch when sunlight alone cannot top you up.
A February 2026 study in Frontiers in Public Health measured vitamin D in 22,484 people and found deficiency rates rising and falling by season.
Just over 10 percent of the whole group fell below the standard deficiency cutoff of 20 ng/mL, and women and adolescents fared worse than the average.
How much your body actually needs
The NIH Office of Dietary Supplements recommends 600 IU of vitamin D a day for most people from the age of 1 to 70. For anyone over 70, the figure rises to 800 IU. Pregnant and breastfeeding women fall into the same 600 IU range.
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Few foods carry much. A three-ounce portion of cooked rainbow trout provides around 645 IU, and a similar portion of sockeye salmon around 570 IU. A cup of fortified cow's milk, soy milk or almond milk adds about 120 IU.
Egg yolks, canned tuna, and fortified yogurt and cereals fill in smaller amounts. For most people who do not eat oily fish several times a week, winter food alone does not reliably meet the daily target.
When sunlight stops counting entirely
Northern latitudes do not just dim in winter. They can switch skin vitamin D synthesis off altogether.
The Nutrients review put the "vitamin D winter" at five months for Ireland and the Netherlands, six months for the UK and Finland, seven months for Iceland, and eight months for northern Norway.
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In the United States, those latitudes correspond to much of New England, the upper Midwest and the Pacific Northwest. For someone living in Seattle, Minneapolis or Boston, the practical sun window has been closing since late September.
A smaller 2007 study of hospital inpatients in the Internal Medicine Journal measured an average vitamin D level of 35 nmol/L at the end of winter and 43 nmol/L at summer's end. Nearly three in four of the 129 patients were below 50 nmol/L in either season.
That was a hospital population and a small sample, so the exact numbers do not scale to healthy adults. The direction of travel does. In the colder months, more people slip below sufficient levels, and the shortfall deepens the longer winter runs.
Anyone who is pregnant, housebound, has darker skin, or takes medicines that reduce fat absorption should talk to their doctor about a blood test before loading up on supplements.
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US labs typically report 25-hydroxyvitamin D in ng/mL, and a reading below 20 ng/mL is the usual deficiency cutoff.
Until the UVB window reopens, food and supplements carry the load. In the northern half of the US, that window reopens around late March, and the next summer peak is nearly five months of winter away.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.