Sun Exposure and Vitamin D Production at 44 Degrees North Latitude

Portland, Maine sits at 43.7 degrees north latitude. Concord, New Hampshire is at 43.2. At these latitudes, the angle of the sun from October through April is too low for UVB rays to penetrate the atmosphere at the intensity needed to trigger vitamin D synthesis in the skin. This isn’t a theory. It’s measurable photobiology, and it has real consequences for anyone who spends winter in northern New England.

Why the Sun’s Angle Matters

Vitamin D3 (cholecalciferol) is made in the skin when UVB radiation at wavelengths between 290 and 315 nanometers reaches a compound called 7-dehydrocholesterol in the outer layer of the epidermis. That’s a narrow UVB window, and it only opens when the sun is high enough in the sky.

At latitudes above 37 degrees north, the solar zenith angle during winter months prevents sufficient UVB from reaching ground level. Researchers call this the “vitamin D winter.” For Maine and New Hampshire, it runs roughly from late September to mid-April — about five to six months when the body can’t produce meaningful amounts of vitamin D from sun exposure, no matter how much time you spend outside at midday.

Cloudy days make no difference. Neither does sitting near a window. Standard glass blocks UVB entirely.

What Happens to Blood Levels Over the Season

Population studies in northern New England show that serum 25-hydroxyvitamin D levels (the standard blood marker for vitamin D status) tend to drop 10 to 20 ng/mL between October and March. Someone who reaches a healthy level by the end of August may fall well below that range by February.

Several factors push the winter trough lower. Darker skin produces less vitamin D per minute of sun exposure because melanin competes with 7-dehydrocholesterol for UVB. Regular sunscreen use, which is advisable for skin cancer prevention, also reduces synthesis. And many people in Maine and New Hampshire simply don’t spend much time outdoors with skin exposed during the warm months when synthesis is possible.

Age matters too. The skin’s ability to synthesize vitamin D3 from UVB declines with age, so older residents start autumn at a disadvantage.

What the Research Shows About Vitamin D and Health

Vitamin D functions more like a hormone than a typical vitamin. It acts on receptors found in nearly every cell type, and it plays a role in immune regulation, bone metabolism, muscle function, and mood. Deficiency (typically defined as serum 25-hydroxyvitamin D below 20 ng/mL) has been associated in observational research with increased rates of respiratory infections in Maine’s shoulder seasons, bone loss, depression, and fatigue.

Observational associations don’t prove causation, and the research on vitamin D supplementation for various outcomes has produced mixed results in clinical trials. What’s clearer is that severe deficiency causes real harm (rickets in children, osteomalacia in adults), and that people in northern latitudes are at elevated risk of falling into deficient ranges during winter. One area of active investigation is vitamin D’s relationship to cognitive health over time, with recent midlife studies adding to the research picture.

The Endocrine Society defines vitamin D sufficiency as 25-hydroxyvitamin D at or above 20 ng/mL, with some researchers and clinicians arguing for higher thresholds (30 ng/mL or above). Testing is the only reliable way to know where you stand.

How Mainers and New Hampshirites Typically Get Vitamin D in Winter

With outdoor UVB synthesis off the table for half the year, dietary and supplemental sources become the primary options. Very few foods contain meaningful amounts of vitamin D naturally. Fatty fish (salmon, mackerel, sardines), egg yolks, and beef liver are among the better dietary sources. Many foods, including most commercial dairy products and some orange juices, are fortified with vitamin D2 or D3, though the amounts added vary by brand.

For most people in northern New England, diet alone doesn’t close the winter gap. Published research from the Endocrine Society and other bodies supports the use of supplemental vitamin D3 (cholecalciferol), which is the same form the skin produces, as more effective at raising blood levels than D2 (ergocalciferol). Dose, form, and what to look for in a D3 supplement are worth understanding before you buy, since products vary considerably. What the right amount is for any individual depends on their baseline blood level, body composition, age, and other factors — something a healthcare provider can assess with a simple blood test.

Vitamin D is fat-soluble, meaning it accumulates in body fat and can reach harmful levels with prolonged high-dose supplementation. This is uncommon but real, and it underscores why testing before supplementing is worth the small cost.

The Practical Picture for Northern New England Residents

Maine and New Hampshire winters are long. If you don’t get tested, you’re guessing. A 25-hydroxyvitamin D blood test is widely available through primary care providers, and many labs offer it directly. Late winter or early spring (February through April) is a reasonable time to check, since that’s when levels are typically at their lowest after months of no solar synthesis. Low vitamin D is also one of several factors that may compound seasonal affective disorder in northern New England, making the overlap worth understanding.

Spending time outdoors during summer remains valuable, both for vitamin D synthesis and for the many other health benefits of outdoor activity. But relying on summer sun to carry you through six months of northern winter is an unreliable strategy. The math doesn’t work at 44 degrees north.

This is one piece of a bigger picture. See our complete guide to vitamin D in Maine for the full season-by-season rundown.

Sources

• Holick MF. Vitamin D: Evolutionary, Physiological and Health Perspectives. Current Drug Targets. 2011;12(1):4-18.
• 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 and Metabolism. 1988;67(2):373-378.
• Holick MF, et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology and Metabolism. 2011;96(7):1911-1930.
• Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. National Academies Press, 2011.

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