The Importance of Sunlight for Vitamin D Production
For those who live in northern climates, winter can be hard. Not only do the weather and darkness affect how we physically feel, but our mental health can suffer as well. One of the key factors of this is the lack of vitamin D many of us experience during the winter months.
This vitamin plays an important role in a variety of functions. It’s produced when our skin is exposed to UV-B rays from sunlight, which is why those living in dark and cloudy regions often struggle to maintain adequate levels, especially during the colder months. Knowing how to boost your vitamin D level is an important part of staying healthy throughout the winter.
What is Vitamin D?
Vitamin D is a fat-soluble vitamin naturally produced in our bodies. Historically, it has been shown to be a critical player in maintaining healthy bones, but over the years, research has discovered that it also may play a role in many other areas of health, too.
Brain health: Vitamin D may play an important role in brain development and function. Emerging research suggests it has neuroprotective properties, helping to maintain a healthy brain as we age, and potentially reducing cognitive decline.1
Depression: Studies have shown that vitamin D helps regulate dopamine levels, a brain chemical that plays a key role in mood regulation and reducing symptoms of depression.2
Heart health: Although still debated, some research has found a connection between low levels of vitamin D and a higher risk of developing cardiovascular disease. 3
Metabolic health: Vitamin D may play a role in your metabolic health. A recent study found that getting enough vitamin D can reduce the risk of developing type 2 diabetes in people with prediabetes and even help some return to normal blood sugar levels.4
Immune function: Low vitamin D levels have been linked to an increased risk of autoimmune conditions, including Multiple Sclerosis, and a higher risk of infections. Research suggests that vitamin D supports both the adaptive immune system, which provides targeted defense, and the innate immune system, which acts as our first line of defense. 5,6
Multiple Sclerosis: Low levels of vitamin D have been associated with a higher risk of multiple sclerosis, an autoimmune condition that affects the central nervous system. However, more research is needed to determine whether vitamin D supplementation provides benefit to patients with this condition.5
Bone health: Due to its importance in maintaining our bone structure and health, vitamin D supplementation can help reduce the risk of fractures related to osteoporosis.7
Many of the conditions and health challenges mentioned above have been linked to low vitamin D levels. Research indicates that around 30% of adults have a vitamin D deficiency, due to factors such as limited sunshine exposure, dietary restrictions, environmental pollution, genetics, and health conditions like obesity.8
The Two Forms of Vitamin D
Vitamin D comes in two forms: D2 and D3. The differences lie in their chemical make-up and where they are produced. D2, also known as ergocalciferol, naturally comes from plants like mushrooms, or can be synthetically made in a lab.
D3, or cholecalciferol, is produced in animals and humans. In humans, it is made in the skin when exposed to UV-B rays from sunlight, triggering a series of steps that lead to the production of vitamin D.
Both forms need to be converted in the liver and kidneys to become biologically active in the body. However, research has shown that D3 is more effective at raising overall vitamin D levels in the blood.9
The Importance of Light
90% of vitamin D in the body is obtained from the sun, which is why those who live in dark and cloudy climates are more likely to be deficient. When sunlight hits the skin, it triggers the production of vitamin D by acting on a chemical found in the skin. This process uses UV-B rays from the sun to produce cholecalciferol (D3).
Once this is complete, D3 is transported to the liver, where it is converted to the storage form of vitamin D, 25-hydroxyvitamin D (25(OH)D). It’s then further processed into its active form 1,25-dihydroxyvitamin D (1,25(OH)2D) in the kidneys, which helps regulate calcium and phosphorus to support bone health and other physiological functions.
In addition to the kidneys, other tissues like skin, immune cells, and parathyroid gland can produce the active form to use locally. Without enough sunlight these processes can’t happen, depleting our store of vitamin D. 10
How We Can Maintain Vitamin D During the Winter
To help support the body in maintaining healthy vitamin D levels year-round, there are 3 areas we can focus on: eating foods rich in vitamin D, supplementing as needed, and monitoring our sunlight exposure.
Foods with Vitamin D
Only a few foods contain a significant amount of vitamin D. Fatty fish like rainbow trout, salmon, mackerel, tuna and sardines are some of the best sources. And wild sources have higher levels than farmed, especially when it comes to salmon. Eggs, particularly the yolks, are another natural source of Vitamin D. Mushrooms are one of the only natural, plant-based sources of vitamin D2, with wild varieties such as Morels containing the highest levels.
Some foods, such as cereals, orange juice, cow’s milk, and plant-based milks, are fortified with vitamin D. Fortification adds nutrients to foods during processing to replace lost nutrients or to address widespread deficiency. Many people don’t realize that vitamin D is essential for absorbing calcium from food. This is why many dairy products are fortified with both calcium and vitamin D—to ensure adequate intake of both nutrients.
While many of these foods provide valuable nutrients, the amount of vitamin D they contain is relatively small, so they’re not the most effective way to maintain or boost your vitamin D levels.11,12
Supplementation
Supplementing vitamin D is the most effective way to increase your levels, especially during the winter or when sun exposure is limited. Supplements provide a concentrated daily dose that is absorbed into your bloodstream, offering much higher amounts of vitamin D than food alone.
It’s important to work with a healthcare practitioner to check your vitamin D levels before starting supplementation. They can help you determine the right dosage based on your specific needs and ensure your levels stay within a safe range. While most labs consider 30 ng/mL to be a sufficient level, research has found the optimal range of vitamin D in the blood to be between 50-80 ng/mL.13
When choosing a supplement, opt for vitamin D3, as it is more effective than D2 at raising and maintaining vitamin D levels.14 Consider pairing it with vitamin K2, which helps direct calcium to the bones and away from soft tissue.15 Additionally, ensure you’re selecting high-quality products. Look for brands that are third-party tested and verify their ingredients and potency. For tips on finding quality supplements, check out this post.
Monitor Vitamin D Exposure
Keeping track of your vitamin D and sun exposure is a great way to ensure you’re getting enough throughout the year. One tool I use is the Dminder app, which tracks sunlight and lets you know when you can get vitamin D from the sun. By entering your location, skin type, and other details, the app calculates how much vitamin D you can produce from the sun in real time based on the UV index.
The app not only tells you when you can get vitamin D from the sun but also estimates how long you should stay outside to maximize vitamin D production while avoiding overexposure. It alerts you when it’s time to seek shade or apply sunscreen to prevent burning. For me, this “sun time” typically ranges between 15–40 minutes depending on the UV index, my clothing, and the time of year.
Living in Seattle, this app has been an invaluable tool for tracking my narrow windows of opportunity for vitamin D production. For example, during the winter months (November to March), UV-B levels are often too low to make vitamin D. The app will let me know when my next chance to get vitamin D is—sometimes it’s 100+ days away!
What This All Means
Vitamin D is essential for maintaining both physical and mental health, especially in climates where it is unavailable from sunlight during parts of the year. Incorporating vitamin D-rich foods, taking a high-quality supplement (after checking your levels), and monitoring your sun exposure can help ensure your body gets the vitamin D it needs to support strong bones, a balanced mood, optimal brain function, and a resilient immune system.
By prioritizing your vitamin D intake and sun exposure, you’ll be on your way to a happier and healthier winter.
-This blog was written in collaboration with Hadley Pearce, MSc, MSiMR, CGE
References
Eyles DW. Vitamin D: Brain and Behavior. JBMR Plus. 2021;5(1):e10419. doi:10.1002/jbm4.10419.
Pertile RAN, Brigden R, Raman V, Cui X, Du Z, Eyles D. Vitamin D: A potent regulator of dopaminergic neuron differentiation and function. J Neurochem. 2023;166(5):779-789. doi:10.1111/jnc.15829.
Cosentino N, Campodonico J, Milazzo V, De Metrio M, Brambilla M, Camera M, Marenzi G. Vitamin D and cardiovascular disease: Current evidence and future perspectives. Nutrients. 2021;13(10):3603. doi:10.3390/nu13103603.
Pittas AG, Kawahara T, Jorde R, Dawson-Hughes B, Vickery EM, Angellotti E, Nelson J, Trikalinos TA, Balk EM. Vitamin D and risk for type 2 diabetes in people with prediabetes: A systematic review and meta-analysis of individual participant data from 3 randomized clinical trials. Ann Intern Med. 2023;176(3):355-363. doi:10.7326/M22-3018.
Feige J, Moser T, Bieler L, Schwenker K, Hauer L, Sellner J. Vitamin D supplementation in multiple sclerosis: A critical analysis of potentials and threats. Nutrients. 2020;12(3):783. doi:10.3390/nu12030783.
Martens PJ, Gysemans C, Verstuyf A, Mathieu C. Vitamin D’s effect on immune function. Nutrients. 2020;12(5):1248. doi:10.3390/nu12051248.
Hill TR, Aspray TJ. The role of vitamin D in maintaining bone health in older people. Ther Adv Musculoskelet Dis. 2017;9(4):89-95. doi:10.1177/1759720X17692502.
Balachandar R, Pullakhandam R, Kulkarni B, Sachdev HS. Relative efficacy of vitamin D2 and vitamin D3 in improving vitamin D status: Systematic review and meta-analysis. Nutrients. 2021;13(10):3328. doi:10.3390/nu13103328.
Alayed Albarri EM, Sameer Alnuaimi A, Abdelghani D. Effectiveness of vitamin D2 compared with vitamin D3 replacement therapy in a primary healthcare setting: A retrospective cohort study. Qatar Med J. 2022;(3):29. doi:10.5339/qmj.2022.35.
Bikle DD. Vitamin D: Production, metabolism and mechanisms of action. In: Feingold KR, Anawalt B, Blackman MR, et al., eds. Endotext. MDText.com, Inc.; 2000. https://www.ncbi.nlm.nih.gov/books/NBK278935/.
Benedik E. Sources of vitamin D for humans. Int J Vitam Nutr Res. 2022;92(2):118-125. doi:10.1024/0300-9831/a000733.
Fleet JC. Vitamin D-mediated regulation of intestinal calcium absorption. Nutrients. 2022;14(16):3351. doi:10.3390/nu14163351.
Wimalawansa SJ. Physiology of vitamin D: Focusing on disease prevention. Nutrients. 2024;16(11):1666. doi:10.3390/nu16111666.
Logan VF, Gray AR, Peddie MC, Harper MJ, Houghton LA. Long-term vitamin D3 supplementation is more effective than vitamin D2 in maintaining serum 25-hydroxyvitamin D status over the winter months. Br J Nutr. 2013;109(6):1082-1088. doi:10.1017/S0007114512002851.
Kuang X, Liu C, Guo X, Li K, Deng Q, Li D. The combination effect of vitamin K and vitamin D on human bone quality: A meta-analysis of randomized controlled trials. Food Funct. 2020;11(4):3280-3297. doi:10.1039/c9fo03063h.
