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Vitamin D: The Performance Vitamin

Vitamin D: The Performance Vitamin

Vitamin D is commonly known as the Sunshine Vitamin, as it is the only vitamin that we are actually able to synthesize through exposure to sunlight. Aside from the sun thing, I would guess that the next thing you would say about Vitamin D is that it is important for bone health. Correct!
That may be its claim to fame, but this vitamin does a lot more than just keep our bones strong, and research has been showing just how important Vitamin D can be for athletes’ health and performance. In addition to supporting bone health, it also plays crucial roles in regulating the immune system, muscle function, and more. The trickle down effects of the amount of Vitamin D in your system can make serious impacts on your physical performance. This article takes a dive into the fascinating sea of Vitamin D research and brings to the surface the info you need to benefit from this Performance Vitamin.
If you only have a minute, you can skip to the final section for essential information and recommendations for Vitamin D. Otherwise, let’s go!

How Vitamin D Came To Be
Long before humans even knew that Vitamin D existed, they knew about the power of sunlight exposure. Hippocrates, the “father of medicine”, was said to have travelled to Egypt and there learned about the practice of treating some diseases with time in the sun. He returned to his native Greece and began to practice this as “heliotherapy” to heal various conditions. Hippocrates and others before him realized there was a real connection between sun exposure and health. (1)
Fast forward to the 1900’s. Light therapy was a well established treatment, but it wasn’t until 1922 when a scientist by the name of Elmer McCollum actually “discovered” Vitamin D. (2) Researchers soon started to look into the impacts of sunlight and UV rays on athletic performance. By the 30’s in Germany, scientists were already well aware that athletic performance could be improved with exposure to sunlight and UV rays. Multiple studies had shown significant improvements in performance measures after UV exposure. German scientific literature stated that it might be unfair for athletes to use a sunlamp, as ultraviolet radiation could be a form of “doping” it was so effective at improving performance. One researcher’s convincing findings prompted him to notify the National German and International Olympic Committee of the effects of UV light on performance. (3)
The connection between UV rays, Vitamin D, and physical performance wasn’t established until 1952. A German researcher performed a study with school children where fitness was assessed in two groups of children; one exposed to UV light in the classroom for 9 months prior, and one group without exposure. The UV kids performed 56% better than the non-UV kids. He then gave some of the children a mega dose of 250,000 IU Vitamin D and observed significant fitness improvements in subsequent performance tests. (This study would absolutely not fly today, obviously.) The researcher was able to conclude that it was Vitamin D, or a related substance, that could explain why UV light could impact fitness and performance. (3)
Since this time, research has continued to explore the hows and whys of the influence of Vitamin D on performance. We now know a pretty amazing amount about Vit. D and its relationship with health, disease, and helping athletes improve their game. My question is: If people were so well aware of the performance enhancing potential of UV light/Vitamin D several decades ago, why is it not a bigger deal in the sports medicine world today? I think it is getting there though, as more and more studies are able to show what a difference adequate Vit. D levels can make for an athlete. I hope this article provides you enough evidence to convince you too that Vitamin D should be an important part of every athlete’s performance nutrition plan. 

Soaking Up The Rays
I don’t know about you, but the fact that we are able to make a vitamin simply with a little time in the sun is pretty amazing to me. How does this happen? When cholesterol-like molecule 7-DHC (7-dehydrocholesterol) gets hit with sunshine, specifically UVB rays, it turns into Vitamin D3, or cholecalciferol. D3 attaches to a Vitamin D binding protein in the blood and is brought to the liver to be transformed to 25-hydroxy Vitamin D (25OHD3). This is the main circulating form of Vitamin D, and this is what we look at when testing someone’s Vit. D status. 25-hydroxy Vitamin D then undergoes a final transformation in the kidney to the active form of 1,25-dihydroxy Vitamin D. It is this active form that functions like a hormone in the body, impacting and regulating bioactive pathways. Technically, Vitamin D truly is a hormone and not a vitamin. (4)
Sun exposure is by far the leading way in which we obtain Vitamin D. There are actually surprisingly few foods that are decent sources of Vit. D. Dairy products, cereals, some alternative milks, and occasionally juices are fortified with D. This means that these foods don’t naturally contain the vitamin, but it has been added in during processing. Significant natural sources of Vitamin D are fatty fish, egg yolks, beef liver, and shiitake mushrooms. Seems a little random, I know. The fortified foods definitely help, but the fact that so few foods contain Vit. D makes it no surprise that Americans generally only get up to 25-50% of the recommended daily allowance for D. (5)
There are other factors that make it difficult to get enough Vitamin D too. Season, time of day, latitude, sunscreen use, and clothing can all affect just how much UVB is reaching your skin for Vit. D synthesis. The skin pigment melanin acts like a sunscreen by competing with 7-DHC in the skin for the absorption of UVB radiation. This means that people with darker skin require longer time in the sun to make the same amount of Vitamin D as those with lighter skin. (6,7)

How much Vitamin D do you need though? This seemingly simple question is met with a complicated response. Based on what is needed to maintain good bone health and calcium metabolism, the Institute of Medicine established the following Vitamin D (blood serum tested for 25OHD3) status guidelines: <12 nmol/L is severely deficient, 12-30 nmol/L is deficient, 30-50 nmol/L is inadequate, >50 nmol/L is adequate, and levels >125-150 nmol/L may have the potential to cause adverse effects. (6)
A couple factors challenge these guidelines though. The nutritional needs of athletes/active individuals tend to be different from the general “healthy” population. When it comes to nutrition for athletic performance, the idea is to optimize performance and health through nutrition. These guidelines are based on levels of Vitamin D for bone maintenance though, and not optimization. Many researchers feel that the IoM established Vitamin D status levels are too low or conservative. The American College of Sports Medicine’s 2016 Joint Position Statement on Nutrition and Athletic Performance suggests that a blood serum of 25OHD3 of <50 nmol/L is deficient and 50-75 nmol/L is insufficient. (8)
Optimal Vitamin D levels for athletic performance have yet to be established. Research of course continues to explore Vit. D and performance in hopes of one day illuminating if there is truly an optimal level of D for athletes. In the mean time, let’s take a look at what we know so far about the several areas of health affected by Vitamin D status and how they influence athletic performance. 


Vitamin D impacts bone health in a couple of ways. Calcium, the main mineral needed for strong bones, is poorly absorbed in the absence of Vit. D. Without D, only 10-15% of dietary calcium is absorbed in the gut, but with Vitamin D present, 30-40% of ingested calcium can be absorbed. It is a similar story with phosphorus, another mineral important for bone health. Alone, about 60% can be absorbed versus 80% in the presence of Vit. D. (9) Along with helping you absorb these minerals, Vitamin D also helps newly formed bone to mineralize. Adequate Vit. D is associated with greater bone mineral density, a measure of bone strength. (6)
Bones are clearly important for the health and activity of any human, let alone athletes. Having a strong skeleton is literally the foundation of a strong, able-bodied athlete. Therefore, inadequate Vitamin D can compromise athletic performance through increased risk and occurrence of fractures. (6)
How much is enough vitamin D for strong bones? Studies suggest that serum Vit. D of ~40-50 nmol/L may be the optimal level for bone health and strength. So far, research doesn’t show any greater benefit for bone health when Vitamin D levels are greater than 75 nmol/L. (9)

Immune System
There are two parts to the immune system, innate and adaptive immunity, both of which are influenced by Vitamin D. Vit. D regulates of proliferation of B and T cells of the adaptive immune system. Within innate immunity, D is important for the proper function of macrophages and monocytes. Vitamin D also upregulates antimicrobial peptides that act as a first line of defense against harmful bacteria. Through these impacts Vitamin D supports the ability of your body to identify, attack, and destroy foreign invaders to keep you healthy and able to compete. (6) A study done in endurance athletes found that those with inadequate Vitamin D levels experienced more upper respiratory infections as well as lower infection-fighting cytokine production. (10)
It is interesting to note that when macrophages and monocytes are exposed to the tuberculosis bacteria, they upregulate the genes for their Vitamin D receptors and enzymes, allowing them to produce a compound capable of destroying these and other bacteria. There is a theory that African Americans may have greater risk of contracting tuberculosis, and having more severe forms, due to the fact that darker skin makes it harder to synthesize and reach adequate Vitamin D levels. The ability of macrophages and monocytes to initiate their response to harmful pathogens may be prevented by serum Vit. D levels less than 50 nmol/L. (9) Many studies have reported average Vitamin D levels in African Americans to be less than 50 nmol/L. (11, 12)
While moderate habitual exercise is associated with a lower risk of infections, a single bout of prolonged strenuous exercise can result in impaired immune function for up to 24 hours. Athletes who push too hard in their activity over time can develop symptoms of overreaching, which includes suppressed immunity. (6) A couple of studies have reported incidences where athletes experienced low levels of Vitamin D despite training over several weeks outdoors during the summer months. It is believed that stress from strenuous training causes decreases in Vitamin D, further compounding the strain on the immune system during periods of regular intense activity. (13)
The stress of exercise and training is what leads athletes’ bodies to adapt, grow, and improve. However, this same stress can cause inflammation. Inflammation results from the reaction of the immune system to a harmful or abnormal presence. Although for the most part inflammation is considered a negative occurrence, to some degree the inflammatory response is necessary in the recovery and rebuilding process in muscle after strenuous exercise. It is also useful in the healing process clearing damaged cells and in defending from foreign invaders or pathogens. Too much inflammation is of course problematic as it can result in pain and decreased performance abilities. There is evidence that Vitamin D may be able to mediate and decrease inflammation through mediating certain aspects of the immune system. (14)
For bone health, research points to Vitamin D levels of about 50 nmol/L as enough to support a strong skeleton. But, studies have found that this level is likely not adequate to optimize immune function and avoid sickness. Current literature indicates that >75 nmol/L serum Vitamin D is perhaps the ideal when it comes to maximizing immunity. (6, 15) 


Another important area of impact of Vitamin D for athletes is the muscle. It goes without saying that muscle and proper muscle function is essential for athletic performance, or for just being a human in general really. Vitamin D is believed to regulate the calcium pumps in muscle responsible for muscle contractions, and therefore movement. In line with this, low serum Vit. D is associated with decreased measures of muscle function and performance such as grip and quad strength, and general fitness. (4, 13)
Many studies support adequate Vitamin D as an important factor influencing muscle function and therefore performance. A trial with Division I and II collegiate athletes in Oklahoma reported low Vit. D to be associated with decreased muscle strength and power. (16) Another study performed with elite UK soccer players found muscle function and physical performance (10 meter sprints and vertical jumping) were improved with Vitamin D supplementation versus no improvements with a placebo. (17)
Strength and power exercises (like sprinting and jumping) are common measurements of muscle function and performance used in many Vit. D research studies. The major type of muscle driving these kinds of movements is the fast twitch, type II, muscle fibers. While the vast majority of athletes use a combination of this and the slow twitch, type I, muscle fibers, certain types of athletes rely more heavily on type II for their sport. For example, weight lifters, sprinters, and other athletes who need to effectively perform quick, powerful movements, heavily recruit type II fibers. Several studies have shown that Vitamin D supplementation can significantly increase the amount and growth of type II muscle fibers. (3, 13) It should be noted that these trials focused on older individuals deficient in Vitamin D. Nonetheless, they illuminate the potential ergogenic effects of Vitamin D on strength and power performance through these fast twitch muscle fibers.
An interesting theme in Vitamin D research is that athletic performance seems to peak with peaks in serum Vit. D. However, this is not to say that Vitamin D is the “magic pill” for performance. There are many other factors that play into the end result seen in competition, and above all, training still unsurprisingly seems to be the biggest influence. With these ideas in mind, one study looked at Vit. D levels and muscle strength in Greek professional soccer players before and after the 6-week offseason. There were a couple of interesting findings: 1. Higher Vitamin D was associated with better neuromuscular performance; 2. The 6-week offseason resulted in decreased performance, but increased Vitamin D levels; 3. Even though performance decreased after the offseason, performance measures carried the same positive association with Vitamin D status. (13) The take aways here are that training ultimately dictates performance, but Vitamin D plays an important supporting role that can boost performance, and that decreasing the exercise stress can allow Vitamin D levels to rise.
Strenuous exercise and training can result in muscle damage, and it is important for athletes to recover as quickly and effectively as they can to continue to perform at a high level. Vitamin D plays a critical role in the recovery process as it regulates the growth and regeneration of muscle cells (myocytes). Studies performed with rodents have shown that Vit. D status before a tough training session forecasts the degree of muscle weakness experienced afterwards. (6)
According to the ACSM Joint Position Statement, Vit. D levels of about 80 nmol/L to 100-125 nmol/L may be an ideal range for overall performance. (8) However, most researchers conclude that we so far do not have enough answers to definitively determine what Vitamin D levels truly optimize muscle function. There are many variables that affect the end result, athletic performance, which make it challenging to uncover a clear answer as to what ideal Vit. D levels may be for muscular function. As Vitamin D research continues, hopefully in the not too distant future we will be able to establish more specific optimal Vit. D levels for athletes to achieve greatest benefit for muscle.

VO2max is an assessment of physical fitness that measures the maximal amount of oxygen a person can uptake during exercise. Generally a higher VO2max indicates better fitness. It can be a useful way to evaluate the aerobic fitness and performance potential of athletes in research. For example, the positive correlation between VO2max and distance run during a soccer match has been well established. (13)
Vitamin D is believed to augment lung function, which of course can influence the VO2max of an athlete. A study with rodents reported the mechanism of Vit. D on lung function may be through modulating lung volume. Deficiency of Vitamin D resulted in decreased lung function mostly explained by a smaller lung volume. (18) The previously mentioned study involving Greek professional soccer players also examined aerobic capacity through VO2max. Athletes with lower serum Vitamin D were found to exhibit lower VO2max measurements both before and after the offseason period. (13)
As with muscle, the nature of athletic performance can make it difficult to answer a core question of if there is a certain level of Vitamin D for optimizing VO2max. An interesting trend in Vitamin D research is the otherwise unexplained phenomenon of performance (measured by various parameters like aerobic capacity and strength) peaking around August, then sharply dropping in September, followed by a bottoming out in January. These findings take into account confounding factors such as training. There is an intriguing theory that these performance trends are related to, if not a result of, the strength of and exposure to sunlight that peaks in the summer and is minimal during the winter months. (3)

Further Impacts
It is (at least for me, and maybe for you too if you are still reading, hi!) super interesting how many ways in which Vitamin D can impact the health and performance of athletes. But, there is more! Most of Vit. D research doesn’t even center on athletes, but other at risk populations and diseases. Because the focus here is how Vitamin D impacts athletes specifically, I am not going to go into as much detail as the areas of influence previously discussed. Although these other aspects of health may not impact athletes in the acute sense, they are nonetheless important to note for general and long-term health.
Bone Abnormalities
Vitamin D is likely best known for its relationship to bone health. Likewise, Vit. D deficiencies are most commonly connected with weak, brittle bones. In childhood, a deficiency of D can result in rickets. In adulthood, too little Vit. D can cause osteomalacia. Both of these are softening of the bones as a result of inadequate calcium secondary to a Vitamin D deficiency. Soft bones greatly increase risk of fractures. (9)
Research indicates that low Vitamin D levels can significantly raise the risk of some types of cancers. Studies found people with serum Vit. D less than 50 nmol/L had a 30-50% increased risk of developing colon, prostate, and breast cancer, as well as higher mortality rates from these cancers. Even just living at higher latitudes where there is decreased exposure to sunlight increases the likelihood of ovarian, prostate, breast, pancreatic, colon, Hodgkin’s lymphoma, and other types of cancer. (9, 19) The obvious next question is, what about skin cancer? Because UVB rays that induce synthesis of Vit. D in the skin can also cause skin cancer, it is wise to be smart about your exposure to sunlight. There is clearly such thing as too much, and depending on skin tone and genetics, the safe limit for sun exposure can vary greatly per individual. Supplementation with Vitamin D is the safest route to reach adequate serum Vit. D levels for people with increased risk for skin cancer. (20) The relationship between Vitamin D status and some cancers is not necessarily causational, but the consistent associations are something to be thoughtful of.
Respiratory Diseases
As discussed above, Vitamin D is linked to aerobic capacity and lung function. Some researchers believe deficiency of this vitamin may cause the development of asthma and chronic obstructive pulmonary disease (COPD). Indeed, low Vitamin D is associated with increased rates of these two conditions, as well as more severe forms of them. Low vitamin D status during pregnancy is associated with wheezing and asthma in children. (18)
Cardiovascular Disease and Diabetes
Many studies have shown that lower levels of Vitamin D are associated with greater risk for cardiovascular disease (CVD) and hypertension (HTN). Increasing Vit. D status or increasing UVB exposure may be able to decrease hypertension, perhaps mediated by blood pressure regulation. (19) Higher serum Vitamin D is associated with lower risk of developing type 2 diabetes versus low serum D, and with better glycemic control. In addition, this vitamin may be important in improving insulin sensitivity and decreasing inflammation. These are two factors that play a role in the diabetic state, and can influence of development of type 2 diabetes. (19, 21)
Autoimmune Diseases
There is evidence that people living at higher latitudes experience higher rates of certain autoimmune diseases such as Crohn’s, type 1 diabetes, and multiple sclerosis. Studies have shown an inverse relationship between Vit. D status and risk of developing multiple sclerosis, type 1 diabetes, and rheumatoid arthritis. It is theorized that Vitamin D’s ability to modulate inflammation and differentiation of immune cells may be behind these relationships. (19)
And more!


Getting Enough Vitamin D
Current research supports serum Vitamin D levels of about 75-125 nmol/L as a goal range for athletes to adequately support health and performance. (8, 22) Even though you would think most athletes have good Vitamin D levels due to training outdoors, that is not the case. Here are a couple of examples of Vit. D status in a few athlete groups:

  • 225 endurance athletes in the UK, 38% with 30-50 nmol/L before winter, 55% with <30 nmol/L after winter (10)
  • 61 professional athletes in the UK, 62% with <50 nmol/L (17)
  • 103 college athletes in Oklahoma, 32% with <75 nmol/L (16)
  • 67 professional soccer players in Greece, 55% with <75 nmol/L (13)
  • 89 NFL players, 51% with <75 nmol/L (22)

To recap, the IoM states serum Vit. D (all in nmol/L) < 12 is severely deficient, 12-30 is deficient, 30-50 is inadequate, and >50 is adequate. (6) However, many think these ranges are too conservative (especially for athletes) and the 2016 ACSM Sports Nutrition paper states that Vit. D <50 is deficient and 50-75 is insufficient. Current research seems to support the ACSM guideline of >75 nmol/L as adequate for athletes and performance. (8) With that said, the above examples clearly show that many athletes are holding back their performance potential by failing to reach adequate Vitamin D levels.
So how do you make sure you aren’t falling behind on Vitamin D? The simplest way is sun exposure. It depends on skin tone, season, time of day, etc., but about 5-30 minutes of sunlight during peak hours (10am-3pm) twice per week is generally enough to maintain adequate Vit. D levels. If wearing a bathing suit during of these “exposure sessions”, the body can produce about 20,000 IU Vitamin D. (9)
Factors that can make achieving adequate sun exposure difficult include regular practice indoors (think basketball, ice hockey, etc.), a dark skin tone, living at high latitudes, heavy sunscreen use, little time spent outdoors, and clothing covering the body. Plus, even if you do get outside for some sunshine, if it is not summer or in the middle of the day, the UV rays may not be strong enough to produce adequate Vit. D. What you do have working for you is that your body is able to store “extra” synthesized Vitamin D in fat cells, which can be released in the fall or winter when UV exposure declines. (9)
Adequate sun exposure may not be feasible for many, especially in those with higher skin cancer risk. While there are a few foods, natural and fortified, that provide vitamin D, athletes don’t tend to eat enough Vit. D foods. Additionally, it is usually not realistic or effective to treat a Vitamin D deficiency with foods alone. (8) Therefore, supplements are the likely best choice for reaching sufficient Vitamin D status.
While a blood test to assess a Vitamin D status would be ideal for all athletes, it is not really necessary. Research supports the safety of supplementing with 600 IU Vitamin D everyday to reach serum Vit. D levels of about 75- 125 nmol/L for most people. (22) If sun exposure is minimal, supplementation can be increased to 800-1000 IU per day. (9) An athlete with very low D status who may want to rapidly increase levels to >75 nmol/L should supplement with 2000-5000 IU per day for 8 weeks. (6) Although Vitamin D toxicity is very rare, it can result from very high doses of 50,000+ IU per day. Studies have not shown harm from taking 10,000 IU Vitamin D per day for 5 months, however these large doses may not be worth the money to achieve similar effects as the lower 600-1000 IU per day regimen. (9)
It is important to note that Vitamin D supplements come in two forms: D2 and D3. Vitamin D2 is only 30% as effective as D3 in raising serum Vit. D levels. You may choose to take the (generally less expensive) D2 supplements, but you would have to consume significantly more than a D3 dose to reach the same serum D levels. (9) It is probably easier just to look for Vitamin D3 (cholecalciferol) on supplement label. All the above dose recommendations are for Vitamin D3. 
THE END! Thank you so much for reading all this way! I know this was a long article, but there is just too much interesting and useful information to tell you about. Believe it or not, there is much, much more Vitamin D research out there that wasn’t covered here. If you like, keep exploring! The referenced articles below are great resources. I hope you learned a little something about Vitamin D and how you can use this Performance Vitamin to help yourself and/or others around you to improve health and athletic performance!

1. Photodynamic Therapy: From Theory to Application
2. The Discovery of Vitamin D: The Contribution of Adolf Windaus
3. Athletic Performance and Vitamin D
4. Emerging Biomolecular Role of Vitamin D in Skeletal Muscle
5. Vitamin D Fact Sheet for Health Professionals 
6. Vitamin D and the athlete: Emerging insights 
7. Vitamin D and Skin Health 
8. ACSM Joint Position Statement: Nutrition and Athletic Performance
9. Vitamin D deficiency 
10.Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes  
11.Assessing the Vitamin D status of the US population 
12.Vitamin D-Binding Protein and Vitamin D Status of Black Americans and White Americans
13. Vitamin D and exercise performance in professional soccer players 
14. Role of Redox and Signaling and Inflammation in Skeletal Muscle Adaptations to training
15. Is there an optimum Vitamin D status in athletes and military personnel?
16. Compromised Vitamin D Status Negatively Affects Muscular Strength and Power of Collegiate Athletes
17.Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK: implications for skeletal muscle function
18.Vitamin D deficiency causes deficits in lung function and alters lung structure
19.Vitamin D—Effects on Skeletal and Extraskeletal Health and the Need for Supplementation
20. Vitamin D at the Expense of Skin Cancer Protection: Is It Worth the Risk?
21. Role of Vitamin D on glycemic control and oxidative stress in type 2 diabetes mellitus
22. The effects of vitamin D deficiency in athletes 

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