Why Latitude Shuts Off Vitamin D Production in Winter

Stand on a Portland sidewalk at noon in late January and notice how low the sun hangs, barely clearing the rooftops. That weak winter sun isn’t just a mood. It explains why, for a stretch of the year, your skin makes almost no vitamin D no matter how long you stand outside.

The short version: a vitamin D winter is real

Back in 1988, researchers Webb, Kline, and Holick ran a now-classic experiment. They exposed skin samples to cloudless winter sunlight in Boston (42.2 degrees north) and Edmonton (52 degrees north). The Boston skin produced no previtamin D3 from November through February. In Edmonton the dead zone ran even longer, October through March. They called it the vitamin D winter, and the name stuck.

Maine sits north of that Boston line. Every part of it.

How sunlight makes vitamin D

The process starts in your skin, not your stomach. A specific slice of ultraviolet light, UVB in the 290 to 315 nanometer band, strikes a cholesterol-like molecule called 7-dehydrocholesterol and converts it to previtamin D3. Body heat then finishes the job, turning previtamin D3 into vitamin D3. No UVB reaching the skin means no reaction. Cloud cover, glass, and sunscreen all blunt it, but the bigger variable up here is something you can’t control: the angle of the sun.

Why the sun’s angle is the whole story

In winter at northern latitudes the sun never climbs high. It tracks low across the southern sky. When the sun sits at a steep angle, its UVB rays slice through far more atmosphere on the way down, and the ozone layer absorbs almost all of them. We’re talking roughly 99 percent of UVB soaked up before it ever reaches the snow. The longer, more slanted the path, the fewer photons survive the trip.

This is also why timing matters. Even in July, very little vitamin D gets made before 10 a.m. or after 3 p.m., when the sun is low and the same physics kicks in. Season and time of day are the same lever pulled from two directions.

What it means at Maine’s latitude

One review put the cutoff plainly: above about 33 degrees latitude, very little if any vitamin D can be made in the skin from winter sun. Edmonton residents, way up at 52 north, may produce essentially none for roughly six months of the year. Portland sits at about 43.66 degrees north, and the state’s northern edge near the Canadian border reaches close to 47 degrees. The Boston study already showed zero synthesis at 42.2 north, so all of Maine falls inside that same window. For a deeper look at the season-long picture, see our guide to vitamin D in northern New England.

What researchers measured in Maine

This isn’t only lab physics. A study published in 2005 followed 23 girls, ages 9 to 11, living around Bangor (about 44 degrees north). Nearly half, 11 of the 23, showed insufficient blood vitamin D at some point over the winter. Average levels dropped 28 percent between September and March. That number is the part worth sitting with: it’s the sound of summer stores draining out while the skin has no way to top them back up.

Two other factors shift the math from person to person. After 30 minutes of summer Boston sun, about 3 percent of the skin’s 7-dehydrocholesterol converted to previtamin D3 in lighter skin versus only about 0.3 percent in darker skin. And the rise in blood vitamin D3 ran at least three times higher in adults aged 20 to 30 than in those aged 62 to 80. Skin pigment and age both change how much the same sun delivers.

The practical takeaway for the dark months

If the skin can’t make vitamin D from a low winter sun, the body leans on two other sources: what you ate over the summer and what you eat now. Researchers note that at high latitudes diet and stored summer reserves become the main supply once the vitamin D winter sets in. None of this tells you what to take or how much. That’s a conversation for your clinician and a blood test. But it does explain why so many Mainers test low by March, and why the calendar, not your effort outdoors, does most of the work. For more on where it actually comes from, our look at sun exposure at 44 degrees north covers the warm-season side of the story.

Sources

• Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. Journal of Clinical Endocrinology & Metabolism. 1988. https://pubmed.ncbi.nlm.nih.gov/2839537/
• Wacker M, Holick MF. Sunlight and Vitamin D: A global perspective for health. Dermato-Endocrinology (PMC, National Library of Medicine). 2013. https://pmc.ncbi.nlm.nih.gov/articles/PMC3897598/
• The Relationship between Ultraviolet Radiation Exposure and Vitamin D Status. Nutrients (PMC, National Library of Medicine). 2010. https://pmc.ncbi.nlm.nih.gov/articles/PMC3257661/
• Sullivan SS, et al. Adolescent girls in Maine are at risk for vitamin D insufficiency. Journal of the American Dietetic Association (PubMed). 2005. https://pubmed.ncbi.nlm.nih.gov/15942551/
• GPS coordinates of Portland, Maine (43.66 N). gps-latitude-longitude.com. 2024. https://www.gps-latitude-longitude.com/gps-coordinates-of-portland-maine

This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider before making any health decisions.