Sun Exposure and Vitamin D: Essentials for Athletic Performance

July 27, 2017 / Lindsay Christensen, MS, CNS, LDN


  1. Sun exposure and human physiology: A brief review

  2. How does sunlight enhance athletic performance?

  3. Vitamin D builds muscle

  4. Low vitamin D may increase risk of injury

  5. How much vitamin D do I need to optimize my athletic performance?

  6. How much sun exposure do I need to raise vitamin D? 

  7. What about vitamin D supplementation?

  8. How sun exposure has improved my athletic performance

The ancient Greeks held the belief that plentiful sun exposure was vital for physical performance, and they required Olympian athletes to bathe in the sun as part of their training. A growing body of research indicates that the sunbathing practices of these ancient athletes were well-informed. Sun exposure influences a wide variety of physiological processes that affect athletic performance, including testosterone and nitric oxide production, circadian rhythms, alertness, skeletal muscle function, and vitamin D status. Read on to learn how safe sun exposure can help you reach your highest athletic potential.

Sun exposure and human physiology: A brief review

Humans are essentially solar-powered beings. This planet that we call home revolves around the exchange and transformation of solar energy; without the sun, Earth would not exist as we know it. Humans evolved with abundant sun exposure, and the full spectrum of light available from the sun influences our physiology in countless ways. The human body requires sunlight to stimulate the production of vitamin D, which is a crucial regulator of many biochemical processes, to maintain a balanced immune system, to regulate circadian rhythms, and to potentiate mitochondrial energy production. (1) Within the past century, we have been indoctrinated into believing that sun exposure is harmful, and will ultimately give us cancer. Based on my research and experience, I believe this couldn’t be further from the truth. Safe sun exposure, which refers to sunscreen-free sun exposure that makes your skin lightly pink and does not result in a burn, is essential for promoting optimal health and can significantly impact your athletic performance. When people slather on sunscreen every time they head outside, or simply avoid the sun, they are physically blocking the beneficial wavelengths of light from the sun that promote vitamin D production in the skin, and may also be missing out on the many other health benefits sunlight has to offer. 

How does sunlight enhance athletic performance?

The history of sunlight and athletic performance is fascinating! In the early part of the 20th century, before research has been conducted on the topic, coaches and athletes intuitively used the sun and ultraviolet B (UVB) light (a wavelength of light present in sunlight) as an ergogenic aid. (2) In the mid-20th century, scientific research on the relationship between ultraviolet (UV) light and athletic performance began to emerge. In 1957, researchers found that exposing athletes to UV light improved their athletic performance and muscle response to training. (3) However, when just supplemental vitamin D3 was provided and not UV light, the previously-noted performance-enhancing effects were not realized. This indicates that UV light, and not simply vitamin D, has special properties that enhance athletic performance. Additional research further supports this hypothesis; in 1938, Russian researchers found that UV light treatments improved athletes’ speed in the 100-meter dash, and studies conducted by German researchers revealed that UV light exposure improved endurance and strength gains. (4) Based on the available research, it appears that UV/sunlight exposure benefits athletic performance by increasing testosterone and nitric oxide, entraining circadian rhythms and enhancing alertness, and by increasing vitamin D, which has a direct effect on muscle growth and musculoskeletal health.  

Sunlight optimizes testosterone levels

Testosterone is a hormone with many physiological functions, one of which is enhanced muscle protein synthesis. Research has found that UV light increases testosterone; (5) this may help explain why UV light has been found to enhances the response of muscle to exercise and produce strength gains in scientific research.  

Sunlight increases nitric oxide

Nitric oxide is a signaling molecule in the body that causes vasodilation, which is a widening of blood vessels. Dilated blood vessels allow for increased blood flow and oxygen transport throughout the body, a phenomenon that directly benefits athletic performance and recovery. (6)

Sunlight entrains circadian rhythms to enhance alertness

Circadian rhythms are the internal organization of biochemical processes within our bodies that follow an approximately 24-hour cycle and regulate many aspects of our behavior and physiology. One of the primary factors that influences circadian rhythms is light exposure. While this is a gross oversimplification of how light affects circadian function, the premise is that bright light in the morning signals to the body that it is time to wake up and be productive, whereas gradually increasing darkness in the evening indicates that it is time to wind down and prepare for sleep. In a study published in PLoS One, researchers found that exposing subjects to bright light significantly increased their work output (i.e. athletic performance) on a bicycle ergometer test. (7) The subjects indicated that the bright light increased their alertness, an effect that may have contributed to the enhanced physical performance. This is similar to the effect that bright light has on the human body in the morning, stimulating wakefulness. I suspect that the circadian rhythm-entraining effects of bright light also improve athletic performance by optimizing hormone balance and increasing testosterone levels. Disrupted circadian rhythms, such as in shift-workers, have been found to lower testosterone; this may ultimately have a detrimental effect on muscle growth and recovery in response to exercise. (8)

Vitamin D builds muscle

The human body is designed to synthesize vitamin D in response to cutaneous UV light exposure. The process of vitamin D production begins when ultraviolet B (UVB) photons from the sun penetrate the skin and interact with a substance synthesized in sebaceous glands called 7-dehydrocholesterol. 7-dehydrocholesterol absorbs the photons from sunlight, forming previtamin D3. Over the course of several hours to a few days, the structure of previtamin D3 is rearranged, resulting in the synthesis of vitamin D3. Vitamin D3 in the skin gradually diffuses into the blood, where it is transported to the liver and kidneys, and ultimately transformed into its active metabolite, 1,25 (OH) vitamin D. In its active form, vitamin D3 influences gene expression and cell signaling. Vitamin D receptors (VDR) are a primary target of vitamin D3. Vitamin D receptors are prevalent in muscle tissue, and provide a direct pathway by which vitamin D can affect the structure and function of skeletal muscle, and thus influence athletic performance. Vitamin D may enhance skeletal muscle function, and thus athletic performance, by influencing maximal oxygen uptake (VO2 max), potentiating the transport and utilization of oxygen by body tissues. (9) Vitamin D promotes the differentiation and proliferation of muscle cells, and decreases myostatin, a protein that inhibits muscle cell growth. (10)  Vitamin D also increases force and power production, potentially by sensitizing calcium-binding sites in the sarcoplasmic reticulum, a structure in muscle cells involved in muscular contraction. (11) Finally, vitamin D, like UV light, enhances testosterone production, which has implications for muscular adaptation to training. (12)

Low vitamin D may increase risk of injury

An inadequate vitamin D level may increase your risk of experiencing a sports-related injury by compromising the integrity of your musculoskeletal system. Adequate vitamin D is crucial for maintaining strong bones, and emerging research indicates that it is also important for regulating the health of soft tissues such as muscle, ligaments, and tendons. Low vitamin D predisposes to stress fractures, a common injury in athletes. (13) In addition, interindividual genetic differences in a gene called VDR (vitamin D receptor) may make some athletes more susceptible to stress fractures than others. Polymorphisms (or genetic variations) in VDR FokI and BsmI affect the body’s responsiveness to vitamin D; several of these polymorphisms are associated with lower bone mineral density and an increased risk of stress fractures. Polymorphisms in VDR can be detected using a genetic testing kit, such as 23andme. Athletes with certain VDR polymorphisms may need to take extra care when it comes to their bone health, significantly increasing their sun exposure and intake of vitamin D and other bone-health promoting foods (see my previous post Nutrition for Building Healthy Bones for more information on this topic), and perhaps avoid high-impact exercise, which can cause serious damage to bones in the long-term.

How much vitamin D do I need to optimize my athletic performance?

So, what level of vitamin D in the body is best for optimizing muscle function and sports performance? While the research is not conclusive, the available evidence suggests that a serum vitamin D level above 50 ng/ml may be required for athletes to achieve maximal physical performance. Vitamin D levels lower than this, (between 30 and 40 ng/ml) are sufficient to promote normal bone mineralization, regeneration of muscular force, and prevention of stress fractures, but may not be enough to optimize musculoskeletal function and recovery. (14)

How much sun exposure do I need to raise vitamin D?

The efficiency of vitamin D production in the skin depends on the number of UVB photons that penetrate the skin, a process that it affected by clothing, excess body fat, sunscreen, and the skin pigment melanin. For most Caucasian people, a half-hour in the summer sun in a bathing suit can initiate the production of 50,000 IU of vitamin D within 24 hours of sun exposure; this same amount of sun exposure time will yield approximately 20,000–30,000 IU in individuals with tan skin and 8,000–10,000 IU in dark-skinned people. (15) In the summer, I recommend that people expose a large amount of their skin to the sun at solar noon, for as long as it takes for skin to turn a light shade of pink (but not burn), on a daily basis. Solar noon, or midday, lasts from around 10 am to 2 pm, and is when UVB rays from the sun are the strongest; these are the wavelengths of light your body needs to initiate cutaneous vitamin D production. In light-skinned people, it may take only 10-20 minutes for sun-exposed skin to turn lightly pink, though people can build up their sun tolerance over time.

Remember that if you live above 37 degrees latitude, summer is the best time to optimize your sun exposure and vitamin D stores; the sun is too low in these areas from November to March, which means the body can’t produce much vitamin D during this time of year and will need to rely partly on vitamin D stores that were built up during the summer (vitamin D is fat-soluble, and can be stored in the body for a couple of months). However, since vitamin D stores gradually become depleted during winter months, I also recommend that people increase their intake of vitamin D-rich foods in the fall and winter, such as fatty cold water fish and egg yolks, or as a last resort, take a vitamin D supplement.

What about vitamin D supplementation?

I recommend sunlight first-and-foremost as the best source of vitamin D because it not only stimulates vitamin D production, but also has a host of other effects on the body and athletic performance. However, if regular sun exposure or use of a UVB lamp are not possible, a vitamin D supplement is the next best option. 

Conventional medicine tells us that a dose of 100 IU of vitamin D per day can raise serum vitamin D by 1 ng/ml after two to three months of supplementation. (16) However, recent research indicates that the average adult needs to take far more than 100 IU of vitamin D per day to maintain an adequate serum vitamin D level; a dose of at least 2,000 IU per day has been associated with maintenance of adequate serum vitamin D levels of around 40 ng/ml. However, peoples’ responses to vitamin D supplements can vary quite widely. Some people can mega-dose on vitamin D, taking above 5,000 IU per day, and see their serum vitamin D barely budge, whereas others can consume moderate amounts, such as 2,000 IU per day, and keep their serum vitamin D at a healthy level. This spectrum of responses may be due to genetic differences that affect the body’s responsiveness to vitamin D, such as the polymorphisms in VDR that I mentioned above. If you choose to supplement with vitamin D, I would suggest starting with 2,000 IU per day, though you will need to do some self-experimentation to see whether this dosage is sufficient to increase your serum vitamin D level. I would also highly recommend consuming a vitamin K2 supplement (around 100 mcg of K2) along with vitamin D, as vitamin K2 helps direct vitamin D to where it is needed in the body (i.e. bone) and prevents it from being deposited in inappropriate places, such as blood vessels, where it can cause vascular calcification. Keep in mind that it is possible to overdose on synthetic vitamin D supplements, leading to a toxic buildup of vitamin D in the body. Hypervitaminosis D (a toxic overload of vitamin D) can cause calcification of soft tissues, including organs such as the kidneys, heart, blood vessels, and lungs. It can also cause kidney stones, headache, weakness, and nausea. For this reason, I would be careful about following the vitamin D mega-dosing trend that has recently become de rigueur. I do not recommend dosages above 5,000 IU per day long-term, as we really have no idea how such a high dose of synthetic vitamin D will affect the body in the long run. 

Finally, regular blood testing for 25(OH)D status should be a key part of any athlete’s training program. Be sure to get tested for 25(OH)D, which is the best marker of vitamin D status, rather than 1,25(OH)D. If your serum vitamin D is less than 30 ng/ml, I suggest you work on increasing your sun exposure using the recommendations I have made here – increasing your safe sun exposure in the summer, upping your intake of vitamin D-rich foods, using a UVB lamp. and if necessary, taking a vitamin D supplement. 

How sun exposure has improved my athletic performance

I personally have found that since increasing my sun exposure (I now receive about 1 hour of unprotected sun exposure daily), my athletic performance in my chosen sport, rock climbing, has improved dramatically. I have witnessed almost effortless gains in muscle growth and strength, as well as increased power and energy during my workouts. To prevent sunburn, I have designed my diet in such a way that it provides plenty of nutrients that support the overall health of my skin. For some of my dietary tips on preventing sunburn, and optimizing your skin’s response to sunlight, check out my previous post Nutrition for Sun Protection

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