Why are those who live in northern latitudes at risk of vitamin D deficiency?

October 20, 2025 •

4 min read

Studies indicate that at latitudes above 37 degrees north, there is insufficient UVB radiation for significant cutaneous vitamin D production during winter, creating a widespread seasonal health issue. This lack of effective sunlight is precisely why those who live in northern latitudes are at risk of vitamin D deficiency.

Quick Summary

Reduced UVB radiation in northern latitudes during winter significantly lowers the body's natural vitamin D production. This increases the risk of deficiency, requiring dietary adjustments or supplementation for residents.

Key Points

Reduced UVB Radiation: In northern latitudes, the solar zenith angle increases in winter, attenuating UVB radiation and inhibiting cutaneous vitamin D synthesis.
Seasonal Deficiency: The period of ineffective UVB exposure from autumn to spring is known as the 'vitamin D winter,' and it is a primary driver of deficiency in these regions.
Behavioral Changes: Colder weather leads to less time spent outdoors and more extensive clothing, significantly reducing skin exposure to available sunlight.
Health Consequences: Prolonged deficiency can lead to bone disorders like osteoporosis and osteomalacia in adults and rickets in children.
Mitigation Strategies: Compensating for lack of sunlight requires consuming fortified foods, dietary sources of vitamin D, and taking supplements, especially in winter.
Environmental Factors: Urban air pollution and tall buildings can further block UVB radiation, exacerbating the problem for city dwellers in northern regions.

The Science Behind Seasonal Vitamin D Deficiency

Vitamin D is often referred to as the 'sunshine vitamin' because the body can synthesize it through exposure to solar ultraviolet B (UVB) radiation. However, the efficiency of this process is heavily dependent on geographical location and season. The crucial factor is the angle of the sun, which dictates how much UVB penetrates the atmosphere to reach the earth's surface. In northern latitudes, this angle becomes too shallow during the colder months, a phenomenon known as the 'vitamin D winter'.

Solar Zenith Angle and UVB Attenuation

During winter, the sun's position is lower in the sky for those in northern latitudes. This results in a larger solar zenith angle, which increases the path length of the UVB rays through the atmosphere. This longer journey allows more of the essential UVB photons to be scattered and absorbed by the atmosphere, meaning far less of the vitamin D-producing radiation reaches the ground. Above roughly 37°N, this effect is so pronounced that from late autumn to early spring, the amount of effective UVB is negligible, making sun-based vitamin D synthesis almost impossible. This explains why reliance on sunlight is not a viable strategy during the winter in these regions.

Behavioral and Environmental Factors

Beyond the astronomical realities, human behavior and environmental conditions in northern regions compound the problem. People are naturally driven indoors by cold temperatures, resulting in significantly less outdoor time. When outside, layers of clothing cover the majority of the skin, leaving only the face and hands exposed, which is insufficient for adequate vitamin D production. Urban areas further complicate the matter. Tall buildings can create shadows that block sunlight, and air pollution can scatter and absorb UVB radiation before it reaches the ground. Even for those who do spend time outdoors, sunscreen use, which is essential for preventing skin cancer, also blocks UVB and inhibits vitamin D synthesis.

Common Factors Affecting Vitamin D Synthesis

Season and Latitude: Solar zenith angle decreases effective UVB radiation in winter.
Cloud Cover and Pollution: Atmospheric factors can attenuate UVB, reducing synthesis.
Clothing: Covering up in colder weather limits skin exposure to sun.
Aging: Older skin produces less vitamin D in response to sunlight.
Skin Pigmentation: Higher melanin content requires longer exposure for the same amount of vitamin D.
Lifestyle: Spending more time indoors reduces opportunities for sun exposure.

Health Risks and Impacts

Chronic or severe vitamin D deficiency has serious health consequences, extending beyond the well-known link to bone health. In children, a deficiency can lead to rickets, a rare but serious condition causing soft and weak bones. In adults, it can result in osteomalacia, leading to bone pain and muscle weakness, and is also linked to osteoporosis, which increases fracture risk. Furthermore, researchers have associated low vitamin D levels with a variety of other conditions, including a heightened risk of certain autoimmune diseases and poor immune function. A link to Seasonal Affective Disorder (SAD), a form of depression occurring seasonally, has also been widely discussed, as vitamin D levels often drop in winter when SAD is most prevalent.

Seasonal Differences in Vitamin D Production


Feature	Summer (Northern Latitudes)	Winter (Northern Latitudes)
UVB Radiation	High and direct. Effective for synthesis.	Low and indirect. Ineffective for synthesis.
Sun's Angle	High in the sky, more potent rays.	Low on the horizon, weaker rays.
Outdoor Exposure	Increased time spent outside for recreational activities.	Decreased time outside due to cold weather.
Clothing Coverage	Minimal clothing, maximizing skin exposure.	Heavier clothing, minimizing skin exposure.
Vitamin D Production	High potential for natural synthesis from sun.	Near-zero potential for natural synthesis from sun.

How to Mitigate Vitamin D Deficiency in Northern Latitudes

Given the limitations of sun exposure, especially in the winter months, strategies must be employed to ensure adequate vitamin D intake. The most effective approach is to combine dietary sources with supplementation.

Dietary Sources: Few foods naturally contain vitamin D, but some fatty fish like salmon, tuna, and mackerel are good sources. Other options include beef liver, egg yolks, and certain mushrooms.
Fortified Foods: Many common foods are fortified with vitamin D, including milk, breakfast cereals, yogurt, and orange juice. These can help bridge the gap in dietary intake.
Supplementation: For most people living in northern latitudes, taking a vitamin D supplement is the most reliable way to maintain adequate levels, especially from autumn to spring. It is always recommended to consult a healthcare provider to determine the correct dosage for individual needs.
Safe Sun Exposure (Summer): In the summer, moderate, unprotected sun exposure for 10-15 minutes a few times a week can boost vitamin D stores, but this must be balanced with skin cancer risk. Always follow safe sun practices.

Conclusion

The fundamental reason those who live in northern latitudes are at risk of vitamin D deficiency is the seasonal unavailability of sufficient UVB radiation. This geological reality, combined with behavioral changes during colder months, necessitates proactive measures to maintain health. A balanced approach incorporating dietary changes, consuming fortified foods, and regular supplementation under medical guidance is the most effective way to combat this widespread issue and prevent the associated health problems. A healthy vitamin D level is a critical component of overall well-being, regardless of where you live.

For more information on the health effects of vitamin D deficiency, consult this MedlinePlus resource on Vitamin D Deficiency.

Frequently Asked Questions

A 'vitamin D winter' is the period during late autumn, winter, and early spring in northern latitudes when the sun's angle is too low for effective UVB radiation to reach the earth's surface and trigger vitamin D production in the skin.

A widely cited threshold is roughly 37 degrees north (or south). Above this latitude, sunlight is generally too weak during the winter months for the body to produce sufficient vitamin D from sun exposure alone.

No, during the winter months in northern latitudes, the sun's UVB rays are too weak and indirect for the body to synthesize a meaningful amount of vitamin D, even on clear days.

Good sources include fatty fish like salmon and mackerel, fortified dairy and cereals, beef liver, egg yolks, and some mushrooms.

Yes, some studies have linked low vitamin D levels with a higher risk of Seasonal Affective Disorder (SAD), a form of depression that occurs during winter.

Health consequences include poor bone mineralization leading to rickets in children, and osteomalacia and osteoporosis in adults. It is also associated with muscle weakness and an increased risk of autoimmune diseases and infections.

For most people living in northern latitudes, supplementation is the most reliable way to maintain adequate vitamin D levels throughout the year, especially during the winter. It is advisable to discuss dosage with a healthcare provider.