Which type of climate is most likely to cause vitamin D deficiency?

January 8, 2026 •

4 min read

According to a study published by the WHO, many regions with abundant sunshine still report high rates of vitamin D insufficiency. This global phenomenon is tied to various factors, with certain climates being most likely to cause vitamin D deficiency due to insufficient UVB exposure, regardless of perceived sun availability.

Quick Summary

High-latitude, cold, or high-humidity climates are most likely to cause vitamin D deficiency due to lower-intensity UVB radiation and reduced skin exposure, especially in winter. Atmospheric conditions and behavioral patterns also play a significant role.

Key Points

High-Latitude Risk: Climates further from the equator (high latitudes) are most susceptible to vitamin D deficiency, especially during winter months.
Low UVB Exposure: High latitudes experience insufficient UVB radiation during winter, leading to little to no vitamin D synthesis from sunlight for several months.
Seasonality is Key: Even in sunny regions, vitamin D levels typically peak in late summer and are at their lowest by the end of winter.
Beyond Latitude: Atmospheric factors like cloud cover, humidity, and pollution can also reduce UVB exposure and contribute to deficiency in any climate.
Behavioral Factors: Indoor lifestyles, sun-protective clothing, and diligent sunscreen use all limit sun exposure and can lead to vitamin D deficiency regardless of the climate.
Mitigation Strategies: In climates with limited sun, dietary intake and supplementation become crucial for maintaining healthy vitamin D levels year-round.

The Core Connection: UVB Radiation and Vitamin D

Vitamin D is often called the 'sunshine vitamin' for a crucial reason: the primary and most natural source of vitamin D synthesis is solar ultraviolet-B (UVB) radiation. When UVB photons from sunlight strike the skin, they convert a cholesterol precursor, 7-dehydrocholesterol, into previtamin D3, which is then thermally isomerized into vitamin D3. This process is the most efficient way for the human body to produce this vital nutrient. However, the intensity and availability of UVB radiation vary dramatically depending on several environmental factors, most notably climate and location.

Latitude: The Primary Climate Determinant

The Earth's tilt means that the angle at which sunlight hits the surface is a major factor in how much UVB radiation penetrates the atmosphere. The closer a region is to the equator (lower latitudes), the more direct and powerful the UVB rays are throughout the year. Conversely, at higher latitudes (further from the equator), the sun's rays travel through more of the atmosphere, which filters out much of the UVB. This leads to a distinct phenomenon known as 'vitamin D winter,' where there is insufficient solar UVB to synthesize vitamin D for several months of the year. In locations like Edmonton, Canada (52°N), effective vitamin D production from the sun is negligible from mid-October to mid-April. By contrast, in tropical areas like San Juan (18°N), production is possible all year long.

Season and Time of Day

Seasonal shifts directly impact vitamin D production, especially in higher-latitude climates. Even in sunny, high-latitude regions, people can have higher rates of vitamin D deficiency during winter compared to summer. The seasonal variation is so pronounced that serum vitamin D levels can drop by as much as one-third to one-half by the end of winter. Furthermore, the time of day matters. UVB radiation is most intense when the sun is highest in the sky, typically between 10 a.m. and 4 p.m.. Cloud cover also plays a role, with complete overcast conditions blocking up to 99% of UVB rays. However, broken clouds can sometimes enhance UVB radiation through reflection.

Atmospheric and Geographical Influences

Beyond latitude and season, other atmospheric and geographical conditions can reduce the amount of UVB reaching the skin, impacting vitamin D synthesis.

Air Pollution: Atmospheric aerosols and pollution can scatter and absorb UVB radiation, significantly reducing the amount that reaches the ground. This has been noted in studies of the 'Asian Brown Cloud' and its impact on vitamin D production in India.
Cloudiness and Humidity: Even in warm climates, persistent cloud cover or high humidity can limit effective sun exposure. A study in Sari, a high-humidity city, found high rates of vitamin D deficiency even in summer.
Altitude: At higher elevations, there is less atmosphere to absorb UVB, meaning sun exposure can be more effective for vitamin D synthesis, even in winter.

Behavioral Factors Affecting Sun Exposure

It's not just the climate that matters; human behavior is a powerful modifier of vitamin D status.

Indoor Lifestyle: The modern trend towards indoor work and leisure means less time spent outdoors, directly reducing natural vitamin D synthesis.
Cultural Clothing: Wearing sun-blocking clothing, common in some cultures, can severely limit the skin's exposure to UVB and increase the risk of deficiency.
Sunscreen Use: While vital for preventing skin cancer, sunscreen with an SPF of 8 or more can effectively block UVB rays. Many people apply sunscreen incorrectly, but consistent use can impact vitamin D levels.

Comparison of Climate Zones for Vitamin D Production


Climate Zone	Latitude Range	UVB Availability	Annual Variation	Deficiency Risk Factors
Tropical	Near the Equator	High year-round	Low	Often more related to cultural attire, air pollution, or indoor lifestyle than climate itself.
Subtropical	23.5° to 40° North/South	High in summer, moderate to low in winter	Moderate	Clear seasonal changes; risk higher in winter. Studies show sunlight is more important than diet here.
Temperate	40° to 60° North/South	Strong seasonal fluctuations	High	Significant 'vitamin D winter'; supplements often needed for half the year or more.
Polar	Near the Poles	Extremely low or absent for several months	Extreme	Vitamin D production is seasonal; heavy reliance on diet or supplements during long winters.

Supplementation and Dietary Solutions

For those living in climates where natural vitamin D production is limited, especially during the colder months, dietary intake and supplementation become critical. Vitamin D can be obtained from foods such as oily fish (salmon, tuna), cod liver oil, fortified milk, and certain wild mushrooms. Regular use of supplements is a common strategy in high-latitude countries to maintain adequate vitamin D status year-round. In certain regions, public health policies promote food fortification to combat widespread deficiency.

Conclusion

The climate most likely to cause vitamin D deficiency is a high-latitude one, where the seasonal and low solar angle significantly reduce UVB radiation for extended periods. However, it's a mistake to assume all sunny climates are protective. Factors like persistent cloud cover, high humidity, atmospheric pollution, and personal behaviors like sun avoidance and clothing choices can also contribute to widespread deficiency, even in warm regions. Ultimately, maintaining a healthy vitamin D level depends on a combination of adequate sun exposure (when safe and possible), dietary intake, and, for many, strategic supplementation, tailored to one's specific climate and lifestyle. For more information on vitamin D metabolism and function, refer to the detailed resources from the National Institutes of Health.

Frequently Asked Questions

During the summer months, yes, but in winter, there is not enough UVB radiation at high latitudes for the body to produce significant vitamin D. You will likely need to rely on diet and supplementation.

Yes, heavy or completely overcast cloud cover can significantly reduce or block the UVB rays needed for vitamin D synthesis. It is harder to produce vitamin D on cloudy days, even in summer.

'Vitamin D winter' refers to the period during colder months at mid to high latitudes when the sun's angle is too low for effective UVB radiation to reach the Earth's surface, making vitamin D synthesis impossible.

Darker skin tones have more melanin, which acts as a natural sunscreen and reduces the amount of UVB radiation absorbed. This means individuals with darker skin require longer sun exposure than those with lighter skin to produce the same amount of vitamin D, potentially increasing deficiency risk in climates with limited sun.

No, it is not possible to get toxic levels of vitamin D from prolonged sun exposure. The body has a self-regulating mechanism where previtamin D3 and vitamin D3 are converted into inactive forms when exposed to too much sunlight.

Factors other than pure sunlight availability, such as cultural practices involving full-body clothing, urban lifestyles that limit outdoor activity, and heavy air pollution, can all block UVB exposure and lead to deficiency even in sun-rich areas.

No, standard window glass blocks the UVB radiation necessary for vitamin D synthesis. Only UVA rays can penetrate glass, and these do not trigger vitamin D production.