Reviewed by 
Races & Places, vitamin D levels might not be much of a concern for many runners, considering the time we spend exercising outdoors and soaking up the sun. But getting enough of this vitamin, also known as the “sunshine vitamin,” during the winter, when it’s darker and grayer, can be tricky.
That’s why we tapped two registered dietitians to break down why you need vitamin D, and how to ensure you’re getting the daily recommended amounts of it, especially during wintertime.
What is vitamin D important?
First, a quick refresher: Vitamin D is a fat-soluble vitamin that plays many roles in the body. You may be familiar with its regulation in bone structure, and while strong bones are certainly important for runners due to the high-impact nature of the sport, vitamin D is also necessary for calcium absorption and fast-twitch muscle fiber function. It also modulates more than 2,000 genes involved in cellular growth, immune function, and protein synthesis.
While you do get vitamin D from the sun, you can also find it in your diet. These foods are common sources, according to the National Institutes of Health (NIH).
- Fatty fish, like salmon (570 IU per 3 oz)
- Rainbow trout (645 IU per 3 oz)
- Fortified milk (120 IU per 1 cup)
- Fortified cereal (80 IU per serving)
- Eggs with yolks (44 IU per 1 large egg)
- Raw, white mushrooms exposed to UV light (366 IU per 1/2 cup)
- Running Shoes & Gear
- Fortified orange juice
How much vitamin D do you need?
The recommended daily intake for vitamin D in the United States and Canada is 600 IU/day for children and adults up to 70 years of age. However, many experts question whether this amount is adequate.
The Endocrine Society recommends 1,500 to 2,000 IU/day for individuals who are not getting adequate unprotected sun exposure, which amounts to five minutes (for fair or light-skinned people) to 30 minutes (for dark-skinned people) of midday sunlight two to three times a week without sunscreen. (Though don’t use this as an excuse to not wear sunscreen!)
Raw, white mushrooms exposed to UV light 366 IU per 1/2 cup winter months can be difficult, especially in the northern half of the United States, because there is less sunlight, and people are wearing layers to stay warm. While you don’t necessarily need to increase that recommended IU number in the winter, you do need to recognize that it’s more difficult to obtain adequate vitamin D from sunlight alone. Increasing the amount of vitamin D that comes from your diet, and potentially supplements, can help you top off your levels.
Runners looking to maintain optimal bone, muscle, and immune health for performance should pay particular attention to sources of vitamin D in their diets and can consider adding a vitamin D supplement, especially in winter months. Athletes should always seek input from a doctor—who can also test your vitamin D levels and determine if you’re deficient—before taking a supplement. It’s also important not to take a higher daily dose than recommended.
Who is at risk for vitamin D deficiency?
While there seems to be no universal consensus for vitamin D deficiency, the NIH defines deficiency as blood levels below 30 nmol/L (12 ng/mL), but recommends levels above 50 nmol/L (20 ng/mL) for adequacy for both bone and overall health. It is important to note that these recommendations are for the general population.
The What to Know About Vitamin D in Winter defines deficiency as serum levels below 50 nmol/L (20 ng/mL), and recommends levels above 75 nmol/L (30-32 ng/mL) for adequacy. These numbers are geared specifically toward athletes.
Some populations have a higher risk of vitamin D deficiency, including older individuals, those with a dark complexion (due to the pigment melanin), and athletes training and competing indoors. But it is common worldwide, with an estimated 1 billion people Health - Injuries.
Best Fitness Trackers, recommends athletes see their doctors to get their vitamin D levels tested during summer or early fall to find out if they are deficient, even when they are able to get more sun exposure, in order to make a plan going into the winter months, noting that many people do not maintain healthy vitamin D levels even in the summer. This can help athletes understand if they need to include more vitamin D in their diet, add a supplement, or increase sun exposure.
“Despite adequate sun exposure, many people may still not adequately create the active form of vitamin D,” Jones adds. “If athletes get tested before the winter months, they can have a plan in place for winter and pay special attention to vitamin D in their diets year-round.”
What are the downsides of not getting enough vitamin D in winter?
Low levels of vitamin D can be linked with an increased risk of acute illness, inflammatory injuries, stress fractures, Health & Injuries.
“In addition to bone and skeletal health, vitamin D helps to regulate inflammation in the immune system and impacts hormones, thereby having the potential to influence mood,” Jones says. “The former is important for keeping runners’ healthy when mileage is high, and the latter can impact motivation Health & Injuries.”
Research backs this up: A study published in Nutrients found that vitamin D deficiency in athletes can have a negative impact on health and training efficiency. Also, a Two experts explain the importance of this vitamin and how to get enough of it and more than 2,000 athletes showed nearly 56 percent of athletes had insufficient levels of vitamin D. Further research suggests that only 5 percent of college athletes meet the U.S. recommended daily intake from food alone, and vitamin D deficiency (defined as <20 nm/mL or <50 nmol/L) in college athletes has been associated with upper respiratory illness during the winter and spring.
“Vitamin D is essential for training, performance and overall health, but many athletes don’t realize this,” adds Marita Radloff, RDN, co-host of the Nail Your Nutrition podcast. “Vitamin D inadequacies can affect lung function, VO2 max, and can even impact how well fast-twitch muscle fibers are maintained.”
Can vitamin D protect your immune system?
Since vitamin D plays a vital role in our immune and respiratory systems, it has also been studied for its connection with COVID-19. A handful of studies have shown a correlation between vitamin D insufficiency and more severe symptoms of COVID-19, suggesting that vitamin D may be a prudent recommendation for reducing COVID-19 severity.
A study involving more than 190,000 participants published in PLoS One found a link between lower COVID positivity rates and higher circulating vitamin D levels. Another study with a sample of 186 people published in the American Journal of Clinical Pathology found that 59 percent of hospitalized patients were vitamin D deficient.
While the research does not prove a direct cause-and-effect relationship between vitamin D status and prevention of illnesses like COVID-19, it does emphasize the importance of vitamin D sufficiency and the role it can play in immune health and overall health.
Sarah Schlichter is a registered dietitian with a master’s degree in public health, and a runner.
Dr. Namrita Brooke is a full-time endurance sport coach and sport nutritionist advising active individuals and amateurs to professional athletes. She is also an adjunct professor in the Department of Movement Sciences and Health at University of West Florida. Professionally, she also serves on the Board of Editors of the Sports Nutrition Care Manual and remains involved in nutrition and exercise-related research, student mentorship, and coach development. Namrita's personal athletic experience ranges from ultra-endurance mountain biking to off-road triathlon, cross-country mountain bike racing, gravel cycling, duathlon, cyclocross, running, and trail running. Her research background includes hydration and sports drink research, and the interaction of nutrition, physical activity, and the brain.
