What is unique about vitamin D in the Pacific Northwest?

November 14, 2025 •

4 min read

According to a UW Medicine expert, during the fall and winter, the sun's rays are too weak above the 37th parallel (which includes the entire Pacific Northwest) to produce any vitamin D, even on a sunny day. This unique geographical and seasonal challenge means residents must proactively manage their vitamin D levels to avoid deficiency and its associated health risks.

Quick Summary

The Pacific Northwest's unique combination of high latitude and frequent cloud cover creates significant challenges for natural vitamin D production through sunlight exposure. This article explains the scientific reasons for seasonal vitamin D deficiency in the region and offers practical solutions, including dietary sources and effective supplementation strategies.

Key Points

Seasonal Inadequacy: Due to its high latitude, the Pacific Northwest experiences limited effective UVB radiation from the sun between fall and spring, making natural vitamin D production nearly impossible during these months.
Unique Challenges: Frequent overcast weather and a generally indoor-focused lifestyle for much of the year exacerbate the seasonal deficiency problem.
Consider Supplementation: Medical experts often recommend vitamin D supplementation for Pacific Northwest residents, especially during the darker, colder months.
Dietary Support: While insufficient on its own, a diet featuring local fatty fish like salmon, fortified dairy, and other rich foods can help support vitamin D intake.
Associated Health Risks: Insufficient vitamin D levels are linked to bone health issues, seasonal mood disorders like SAD, and compromised immune function.
No Tanning Bed Solution: Tanning beds should not be used as a source of vitamin D due to the high risk of skin cancer.

Why the Pacific Northwest's Latitude Matters

At latitudes above approximately 37 degrees north, the sun's angle during the winter months is too low for the ultraviolet B (UVB) radiation needed for vitamin D synthesis to effectively penetrate the atmosphere. This places major population centers in the Pacific Northwest, like Seattle and Portland, firmly in the "winter vitamin D production deficit" zone. Unlike regions closer to the equator, Pacific Northwesterners cannot rely on sun exposure alone to maintain sufficient vitamin D levels year-round. This geographical reality is the single most unique factor influencing vitamin D status in the region and is exacerbated by the area's notoriously overcast weather.

The Role of Cloud Cover and Lifestyle

While the sun's low angle is the primary issue, frequent cloud cover amplifies the problem by further reducing UVB penetration. Moreover, lifestyle factors typical of modern urban living, such as spending most of the day indoors and using sunscreen, contribute to insufficient sun exposure even during the sunnier summer months.

Health Implications of Vitamin D Deficiency in the PNW

Persistent vitamin D insufficiency in the Pacific Northwest is linked to several health concerns, many of which can be exacerbated by the region's climate.

Bone Health: Vitamin D is essential for calcium absorption. Chronic low levels can lead to weakened bones (osteoporosis) in adults and, in severe cases, rickets in children.
Mental Health: The "winter blues," or Seasonal Affective Disorder (SAD), is often linked to low vitamin D levels, affecting mood and energy.
Immune Function: Adequate vitamin D supports a healthy immune system, and lower levels can compromise the body's ability to fight off infections.
Other Conditions: Research suggests possible connections between vitamin D deficiency and an increased risk of chronic diseases such as heart disease, diabetes, and certain cancers.

Strategies for Maintaining Healthy Vitamin D Levels

Given the challenges, Pacific Northwesterners must be proactive. A multi-pronged approach combining diet, supplementation, and informed sun exposure is recommended.

Dietary Sources of Vitamin D

While food alone is rarely enough, especially in winter, certain items can help boost your intake.

Fatty Fish: Salmon, sardines, and tuna are excellent natural sources of vitamin D. The region's access to fresh, wild-caught salmon offers a local advantage.
Fortified Foods: Many common foods like milk, cereals, and orange juice are fortified with vitamin D.
Other Sources: Egg yolks, beef liver, and mushrooms grown under ultraviolet light also contain some vitamin D.

Supplements: A Necessary Part of the Strategy

Supplementation is often essential for Pacific Northwest residents, particularly during the winter months. Consulting with a healthcare provider can help determine appropriate supplementation strategies based on individual needs.

Comparison of Vitamin D Strategies


Strategy	Pros	Cons	Best Application in the PNW	Comments
Sun Exposure	Natural, free, and efficient when possible.	Ineffective from fall through early spring due to latitude; requires unprotected skin exposure.	Limited to summer months (May-September), ideally between 10 a.m. and 3 p.m.	Not a reliable year-round source. UV lamps are not a safe alternative.
Dietary Intake	Offers additional nutrients and benefits from whole foods.	Limited natural sources; requires consuming large quantities of specific foods.	Use as a supportive measure year-round, focusing on fatty fish and fortified products.	Essential but insufficient on its own for most people.
Supplementation	Reliable, consistent method.	Requires daily adherence; can lead to toxicity at extremely high levels.	Recommended for many residents from fall to spring; consider year-round for some.	The most effective and predictable method for maintaining levels.

The Pacific Northwest Lifestyle and Vitamin D

Beyond the gray skies, a unique culture and lifestyle in the PNW impact vitamin D. An emphasis on outdoor activities, from hiking to skiing, can increase summer sun exposure, while a preference for indoor activities during the rainy season decreases it. The region's thriving food scene, which includes abundant seafood, provides access to excellent dietary sources. However, as medical professionals and nutrition specialists point out, the reliance on supplementation is a cornerstone of public health recommendations in this area. For many, the vitamin D conversation in the PNW isn't about getting a little extra, but about consistently addressing a seasonal, geographical deficit.

Conclusion

What is unique about vitamin D in the Pacific Northwest is the scientific certainty that natural production from sun exposure is severely limited for much of the year due to the region's latitude. This geographical reality, combined with lifestyle habits and frequent cloud cover, creates a significant risk of deficiency for residents. While a diet rich in fatty fish and fortified foods can help, regular supplementation is often a reliable strategy to ensure adequate vitamin D levels, support bone and immune health, and combat seasonal mood changes. Awareness and proactive measures, guided by healthcare professionals, are key to staying healthy in the PNW's unique climate.

Frequently Asked Questions

The Pacific Northwest is located at a high latitude (above the 37th parallel), which means during the fall and winter, the sun's angle is too low for the UVB rays needed for vitamin D synthesis to penetrate the atmosphere effectively. This makes natural production impossible, even on a clear, sunny day.

A reliable method often involves daily vitamin D supplementation, especially during the fall, winter, and spring. While dietary sources and summer sun exposure can help, supplements can provide support for consistent levels in this climate. Consulting a healthcare provider is recommended to determine the best approach.

During the months when the sun's angle is too low for vitamin D synthesis, cloud cover or not, your body cannot produce vitamin D effectively. While some UV light gets through clouds, it's generally not enough during these seasons.

Anyone living in the Pacific Northwest is at risk for seasonal vitamin D deficiency, regardless of how long they have lived there. People with darker skin tones, older adults, and those who spend more time indoors are at an even higher risk due to reduced natural synthesis.

Excellent food sources include fatty fish like salmon, mackerel, and sardines. Many milk, cereal, and orange juice products are also fortified with vitamin D. Other sources include egg yolks and UV-exposed mushrooms.

No. Medical professionals advise against using tanning beds as a source of vitamin D due to the serious risk of skin cancer associated with high-intensity UV exposure.

Lower vitamin D levels have been linked to an increased incidence of Seasonal Affective Disorder (SAD) and general depression symptoms. Supplementation may help support mood in some individuals.