During winter months, particularly in regions where sunlight hours are low, vitamin D synthesis remains low throughout the season. While your summer tan may last into October and November, the vitamin D synthesized throughout the summer is often not sufficient to compensate. It is no coincidence that cold and flu rates surge dramatically during the winter months.
The "flu season" occurs during the cold half of the year of each hemisphere, and it has been regularly hypothesized that this is the result of vitamin D seasonal deficiency (4). Of course, the best source of vitamin D is still sunlight, in the correct dosages. However, not all of us are fortunate enough to have daily sun exposure all year round — sometimes not even during the summer months! And yet vitamin D is essential for normal function of numerous physiological functions. At least 1,000 different genes, governing virtually every tissue in the body, are thought to be regulated by 1,25-dihydroxyvitamin D3, the active form of the vitamin. Unfortunately, a large percentage of the population is deficient. Here are some steps you can take to keep your vitamin D levels high this winter.
How to Boost Your Vitamin D Levels
If you live in an area with year-round sunshine, then natural sunlight provides the best source of vitamin D. "20 to 25 minutes of exposure is helpful," according to Stephen Honig, MD, director of the Osteoporosis Center at the Hospital for Joint Diseases. Photochemical vitamin D synthesis varies greatly with age, skin color and the amount of skin coverage during sun exposure. Make sure to take these into account when calculating how much natural sunlight you may need. However, natural sunlight does come with risks. The disadvantage of sun exposure is the risk of skin cancer from prolonged exposure, due to DNA damage caused by UV radiation. However, the majority of people tend to be deficient in vitamin D because of limited sun exposure — so getting some sun exposure, but not too much, is key.
With limited sunlight during the winter months, one of the safest and cost-effective methods of boosting your vitamin D levels is through supplementation. A large systematic review and meta-analysis looking at the optimal supplementation dosage around the globe (Indsæt link) suggest a intake of 2000-3000IU a day is appropriate to maintain adequate healthy levels of vitamin D. It is important to note that sometimes with a good amount of sun exposure less supplementation is necessary, but sun exposure should not be excessive. The best and most accurate way to tell if you are supplementing correctly is with a blood test.
Certain foods are relatively high in vitamin D. However, this can be a very costly source. As you can see from the levels below, you would need to eat a large volume of these food sources in order to achieve the recommended daily allowance. But, it is certainly a great way to help boost your levels:
- Fresh fish: 100g of a fatty fish such as salmon may contain around 240IU per serving.
- Mushrooms: Depending on the variety, mushrooms can contain around 450IU per 100g serving.
- Eggs: One egg contains approximately 36IU of vitamin D.
- Liver: 100g of lamb's liver may contain approximately 36IU of vitamin D.
On a doctor's recommendation, some severely vitamin D deficient patients can utilize UV emitting indoor lighting to synthesize vitamin D. However, these pose the same risks of skin cancer as long-term sun exposure and should be used carefully.
- Zhu, Gui-Dong, and William H. Okamura. "Synthesis of vitamin D (calciferol)." Chemical reviews 95.6 (1995): 1877-1952.
- Silveira, Sineli R., and Walter A. Hadler. "A histochemical study on the vitamin D synthesis into the epidermis." Acta histochemica 76.2 (1985): 225-IN3.
- Mead, M. Nathaniel. "Benefits of sunlight: A bright spot for human health." Environmental health perspectives 116.4 (2008): A160.
- Urashima, Mitsuyoshi, et al. "Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren." The American journal of clinical nutrition 91.5 (2010): 1255-1260.