The winter poses a unique challenge to maintaining optimal vitamin D, often called the "sunshine vitamin." Studies consistently show that vitamin D levels drop during the winter when the days get shorter and colder. This article will address the seasonal nuances that affect vitamin D status, supplying readers with the knowledge to proactively manage their vitamin D levels and prevent deficiency during the winter season.

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Understanding Vitamin D and Its Health Implications

Vitamin D is a fat-soluble vitamin that plays a crucial role in various physiological functions within the body. One of its primary functions is to promote calcium absorption in the gut and maintain serum levels of calcium and phosphate to maintain normal bone mineralization. It is also involved in bone growth by modulating the cells involved in bone remodeling processes. Vitamin D deficiency in children is linked to rickets. Vitamin D insufficiency in adults can lead to osteomalacia and osteoporosis. 

Vitamin D is involved in the epigenetic regulation of at least one thousand genes. This means that the role of vitamin D in health extends far beyond the maintenance of bones. Other roles of vitamin D in the body include reducing inflammation, supporting the body's defense against infections, and modulating processes like cell growth, neuromuscular function, and glucose metabolism. (17, 20) 

Prolonged vitamin D deficiency can manifest as low serum calcium levels (hypocalcemia) and hyperparathyroidism. Emerging research also indicates that vitamin D inadequacy is linked to a wide range of health concerns. These include loss of muscle strength, preeclampsia during pregnancy a higher risk of poor birth outcomes, cancer, type 1 and 2 diabetes mellitus, cardiovascular disease, multiple sclerosis, neurodegeneration, and mood disorders like seasonal affective disorder (SAD). (17)

Factors Affecting Vitamin D Levels in Winter

The concern for low vitamin D levels heightens in winter. The body synthesizes vitamin D when the skin is exposed to sunlight, specifically ultraviolet B (UVB) rays. When UVB rays interact with the skin, 7-dehydrocholesterol is converted into vitamin D3 (cholecalciferol). Vitamin D3 is transported to the liver and kidneys, where it undergoes further transformations to become the biologically active form of vitamin D called calcitriol. (16)

Several factors affect vitamin D synthesis during the winter, with reduced sunlight exposure being a primary one. In the colder months, people tend to spend more time indoors, limiting their exposure to sunlight. Days during the winter are also shorter; this reduced sunlight availability makes it challenging for individuals to produce an adequate amount of vitamin D solely through sun exposure. Individuals living at higher latitudes, where sunlight is less direct, may experience more pronounced decreases in vitamin D levels during winter. In these regions, the angle of the sun's rays is generally lower throughout the year, especially during winter months, which results in fewer UVB rays reaching the Earth's surface. (17)

Skin type also influences vitamin D synthesis. People with darker skin pigmentation have higher levels of melanin, which can act as a natural sunscreen, reducing the skin's ability to produce vitamin D in response to sunlight exposure. As a result, individuals with darker skin may need more extended exposure to the sun to generate the same amount of vitamin D as those with lighter skin. This can be particularly significant during winter when sunlight exposure is already limited. (1) 

Age-related considerations also play a role in vitamin D levels. Older adults have a decreased capacity to produce vitamin D in response to sunlight. Additionally, older individuals may spend more time indoors or have limited mobility, further diminishing sunlight exposure. Age-related changes in kidney function can also affect the conversion of vitamin D into its active form, potentially leading to lower vitamin D levels. (15) 

Considering these factors, it becomes evident that maintaining optimal vitamin D levels in winter requires a multifaceted approach, including dietary sources, supplements, and mindful sun exposure. It's essential for individuals, especially those at higher risk for deficiency, to be aware of these considerations and take proactive measures to ensure sufficient vitamin D intake for overall health and well-being.

Testing and Monitoring Vitamin D Levels

Testing for vitamin D is done with a blood test. Both calcidiol (produced from cholecalciferol in the liver) and calcitriol (produced from calcidiol in the kidneys) can be measured. 

Calcitriol (1,25-dihydroxyvitamin D) is the bioactive form of vitamin D. It is used during the diagnostic evaluation of abnormal calcium levels and to monitor patients with kidney disease. However, for most patients, it is not recommended to diagnose vitamin D deficiency or monitor vitamin D supplementation. (11, 12, 20) 

Calcidiol (25-hydroxyvitamin D) provides a good index of circulating vitamin D in patients without renal disease because it has a much longer half-life and is less prone to short-term fluctuations than calcitriol. This simple test is performed via blood spot or blood draw, as US BioTek offers. (12, 13)

Vitamin D has a wide normal reference range, often reported as 25-80 ng/mL. The vitamin D levels in the table below are reference ranges reported by Mayo Medical Laboratories. Many functional medicine doctors believe optimal vitamin D levels for most people are 50-80 ng/mL.

Monitoring vitamin D levels should be done at least twice a year. Measuring calcidiol in the spring will reflect vitamin D status after the winter, and in the fall will reflect the status after the summer. (13)

Dietary Sources of Vitamin D

Few foods naturally contain vitamin D. Fatty fish and fish liver oil are among the best dietary sources, but beef liver, egg yolks, and cheese also have small amounts of vitamin D. Mushrooms provide variable amounts of natural vitamin D2. Fortified foods provide the most vitamin D in American diets. Vitamin D-fortified foods include milk, plant milk alternatives, margarine, breakfast cereals, and orange juice. (21)  

Vitamin D Content of Foods 

Tips for Incorporating Vitamin D Into Your Daily Diet

1. Aim to eat at least two servings of fatty fish, such as salmon, mackerel, trout, and sardines, weekly. Grill or bake fish for a healthier cooking method than frying. 
2. Include eggs in your breakfast, or add them to a salad. 
3. Check food labels to identify products fortified with vitamin D
4. Some mushrooms on the market have been treated with UV light to increase their vitamin D concentration (21). If possible, place mushrooms in direct sunlight before eating to improve their vitamin D content. 
5. Cod liver is an excellent source of vitamin D but can have an unpleasant taste. Try blending it into a smoothie or mix it with an acidic juice to mask the flavor.

Vitamin D Supplementation Strategies

Vitamin D supplements are available in two bioequivalent forms: D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D2 is primarily derived from plant sources, whereas vitamin D3 is predominantly derived from animal sources and is naturally produced by humans. Both forms are used in supplements and fortified foods and are equally well absorbed in the small intestine. However, when choosing vitamin D supplements, D3 appears to be the superior form as it has been shown to increase serum levels better and longer. (2, 12) 

Because foods typically lack sufficient amounts of vitamin D to meet human needs and the obstacles that arise for endogenous vitamin D synthesis during the winter, many people must supplement vitamin D to prevent insufficiency and deficiency. Dosing vitamin D depends on various factors, including baseline serum levels and the body's ability to absorb fat. A standard regimen for correcting vitamin D deficiency includes a "loading dose" of 50,000 IU vitamin D for eight weeks until serum levels are replete, followed by a maintenance dose of 800-1,000 IU daily. Monitoring vitamin D levels more frequently can help tailor dosage to an individual's nutritional needs. For example, a daily vitamin D dose of 800 IU may be sufficient to maintain optimal serum levels in the summer; however, increasing this during the winter may be necessary for someone living in northern latitudes. (12)

Lifestyle Modifications to Enhance Vitamin D Levels

Humans obtain 90% of vitamin D from sunlight, so sun exposure is one of the best ways to improve endogenous vitamin D synthesis. As mentioned above, it can be more challenging to do this during the winter – but not impossible! Take advantage of sunny winter days by participating in outdoor activities. During the winter, the time required outdoors to generate adequate vitamin D increases. For instance, research indicates that in spring and summer, exposing the skin to sunlight for 8-10 minutes around noon can yield the recommended vitamin D levels. In contrast, during the winter months, when more of the body is typically covered by clothing, a substantial increase in sun exposure time is necessary. Specifically, around two hours of sun exposure at noon becomes essential to produce sufficient vitamin D. (3) 

Ultraviolet light therapy may be an appropriate option for enhancing vitamin D synthesis during the winter months. UV lamps emit rays similar to sunshine and have been shown to maintain or increase serum calcidiol levels. This can be an effective option for patients with fat malabsorption or who cannot meet vitamin D requirements through natural sunlight exposure and diet during the winter. (6) 

Achieving a balance between meeting vitamin D requirements and preventing sun damage through sun exposure requires a thoughtful approach, considering factors like skin type, location, and the time of day. Understanding your skin type and susceptibility to sunburn is crucial; individuals with fair skin may need less sun exposure for vitamin D synthesis but are more prone to sunburn. Use the UV index score, available on weather reports, to help determine how long it is safe to stay outside. The higher the UV index, the greater the potential for harm from UV radiation. If the UV index is high, or during prolonged outdoor activities, apply sunscreen and take breaks in the shade to help protect the skin from the harmful effects of UV radiation. (24) 

Addressing Vitamin D in Special Populations

Special attention is required for vitamin D in special populations at high risk for deficiency. 

For exclusively breastfed infants, the American Academy of Pediatrics recommends 10 mcg (400 IU) of supplemental vitamin D daily starting shortly after birth until they are weaned and consuming at least 400 IU of vitamin D through fortified foods. This is because breast milk may not provide sufficient vitamin D, and adequate levels are crucial for bone development. (22)

Elderly individuals may be at an increased risk of vitamin D deficiency due to the natural decline in appetite and the skin's ability to synthesize vitamin D with age. Supplementation or increased dietary intake is often recommended, as maintaining adequate vitamin D levels is essential for bone health and reducing the risk of fractures.

Melanin is the pigment in skin that results in darker skin tone. It also reduces the skin's ability to produce vitamin D from sunlight. Compared to people with lighter skin, those with darker skin tones require longer exposure to the sun to synthesize equivalent amounts of vitamin D. Therefore, vitamin D supplementation may be necessary to prevent vitamin D deficiency during the winter. (23) 

Individuals with conditions affecting fat absorption, such as celiac disease and inflammatory bowel disease, or who have undergone gastric bypass surgery may have difficulty absorbing fat-soluble vitamins (18). These individuals may require higher doses of vitamin D or different forms, and healthcare professionals should monitor their levels regularly.

Patients with chronic kidney disease may experience impaired activation of vitamin D in the kidneys. Monitoring and managing their vitamin D levels are essential to prevent complications related to calcium and phosphorus metabolism. Treatment guidelines recommend using prescription calcitriol replacement beginning in stage 3 chronic kidney disease. 

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Optimizing Vitamin D During Winter: Key Takeaways

Optimizing vitamin D levels is pivotal in promoting winter health. Adopting proactive measures becomes paramount as sunlight exposure diminishes during these colder and darker months. Encouraging a well-rounded approach that includes dietary adjustments, lifestyle changes, and, when necessary, judicious supplementation is key to maintaining optimal vitamin D levels. Recognizing the diverse needs of different populations, it is essential to tailor strategies to individual requirements, ensuring that everyone, from infants to the elderly, receives the benefits of adequate vitamin D.