In Scandinavia, your body can produce vitamin D from sun exposure primarily during the summer months, roughly from May to August. This period is often referred to as the “vitamin D summer.”
The ability to synthesize vitamin D is dependent on the sun’s ultraviolet B (UVB) rays reaching the skin. In northern latitudes like Scandinavia (which are above 54°N), the sun’s angle during the rest of the year is too low for sufficient UVB to penetrate the atmosphere and trigger this process.
The period of the year when the sun’s radiation is insufficient for vitamin D production is called the “vitamin D winter.” The duration of this “winter” increases as you go further north. For example:
- In Copenhagen, Denmark (55°N), the vitamin D winter lasts about six months.
- In Tromsø, Norway (69°N), it can last for up to eight months.
- In Finland, the period is from the beginning of October to the end of March.
Because of this prolonged “vitamin D winter,” it is often recommended for people in Scandinavia to get their vitamin D from dietary sources (like fatty fish and fortified foods) and supplements during the non-summer months.
The ability to produce vitamin D from sunlight in North America depends heavily on your latitude, as well as the season. The key factor is the sun’s angle and the intensity of its ultraviolet B (UVB) rays.
Generally, in most of the United States and Canada, you can produce vitamin D from sun exposure during the spring, summer, and early fall. The “vitamin D winter,” when UVB rays are too weak for synthesis, typically begins and ends on a schedule that is tied to a specific latitude line: the 37th parallel north.
- South of the 37th Parallel: (e.g., Los Angeles, CA; Phoenix, AZ; San Antonio, TX; Atlanta, GA)
- People in this region can produce vitamin D from sun exposure year-round, though the amount and duration needed will vary by season. You may need more time in the sun during the winter months when the sun is lower in the sky.
- North of the 37th Parallel: (e.g., Seattle, WA; Chicago, IL; New York, NY; Toronto, ON)
- For people in these areas, the vitamin D “winter” is a reality. The sun is not high enough in the sky for sufficient UVB to reach the ground for roughly five to seven months of the year.
- During the fall and winter (typically from late October to early March), it is nearly impossible to produce enough vitamin D from the sun alone. During this period, it’s recommended to get vitamin D from food sources (like fortified milk, cereals, and fatty fish) and/or supplements.
How to Check if You Can Produce Vitamin D: The “Shadow Rule”
A simple rule of thumb for all locations is the “shadow rule”:
- If your shadow is shorter than you are, the sun is high enough in the sky for you to produce vitamin D. This usually occurs around midday.
- If your shadow is longer than you are, the sun is too low for effective vitamin D synthesis.
This simple test can help you determine the optimal time of day to get sun exposure for vitamin D production, regardless of your location.