VITAMIN D: MORE THAN JUST STRONG BONES

Dr. Jay Williams, Ph. D., is a professor of Exercise Science in the Department of Human Nutrition, Foods and Exercise at Virginia Tech. His research focuses on the responses and adaptations of muscle to activity, inactivity and disease. He also has a long history of working with athletes, ranging from kindergarten soccer players to Olympic track & field athletes.


In previous articles, we discussed how nutrition has a remarkable influence on a player’s performance.  Also, proper nutrition is a key to good health.  Much of the interest in diet and performance focuses on what are called macronutrients.  That is, the balance between carbohydrates, proteins and fats.  However, micronutrients are equally important.  Micronutrients include specific vitamins and minerals.  Even though players may eat the proper amounts of macronutrients, they may be missing key micronutrients such as calcium, magnesium and vitamins A, B and C.   One micronutrient that is sometimes lacking in the diet is vitamin D.  While we normally associate vitamin D with strong bones and teeth, a growing body of research indicates that vitamin D may play an important role in athletic performance.  In this article, we look at how vitamin D status affects performance and what can be done to insure adequate vitamin D intake.

Vitamin D status is determined by obtaining a blood sample and analyzing for the active form of vitamin D, known as 25(OH)D.  Optimal concentrations of vitamin D are in the range of 40-70 ng/ml.  Most currently define vitamin D insufficiency as a concentration below 30 mg/ml and vitamin D deficiency less than 20 ng/ml. 

What affects vitamin D status?  In a normal, healthy individual, vitamin D status is determined by two key factors.  The first is sunlight.  When the skin is exposed to UV light, production of vitamin D is stimulated.  Given this, it is not surprising that plasma levels of vitamin D can be affected by geography and time of year (seasonal variation).  In the winter, especially in the northern climates, exposure to sunlight is limited due to shorter days and moving indoors.  As a result, some players may experience seasonal vitamin D levels that fall below the optimal range. 

The second factor that affects vitamin D status is the diet.  Health and nutrition experts recommend a daily vitamin intake of at least 600 IU or 15 ug.  They also recommend that the intake of vitamin D come primarily from whole foods.  Fatty fish contains the largest amount of vitamin D.  This includes cod, salmon and tuna.  Fortified orange juice, milk and yogurt are also good sources.  For example, 3 oz of wild salmon contains nearly 1000 IU and a cup of fortified orange juice contains about 140 IU of vitamin D.

Is poor vitamin D status a problem for athletes?  In many athletes, suboptimal vitamin D levels exist.  In the mid 1950’s, researchers noted that peak athletic performance occurred in the late summer months then declined to a low point in the mid winter.  That is, performance varied based on the length of daily sun exposure.  A recent study of English Premier League players found that during the winter months, 65% of the players were considered vitamin D deficient (<20ng/ml).  Studies of youth and college players reveal both inadequate intake and suboptimal vitamin D levels.  In fact, only 5% of college athletes meet the US Recommended Daily Allowance (RDA) for vitamin D intake (600 IU).  Thus, it’s reasonable to assume that many of today’s youth, college and professional players are competing with vitamin D levels that are less than optimal for health and performance.

Much of the concern with vitamin D status centers on bone health.  However, vitamin D can affect a player’s performance on the field.  It should be noted that much of the research linking vitamin D and athletic performance is circumstantial.  But a compelling case can be made. A 2014 study of Greek professional players found that vitamin D status was directly correlated with sprint performance, vertical jump height, and maximal oxygen consumption (VO2max).  Those with the highest vitamin D levels performed better on the performance tests than did those with lower levels. Studies in young athletes also show that aerobic fitness, power, strength and fatigue are associated with vitamin D status. 

Vitamin D insufficiency and deficiency can also impact the immune system and may keep players off the field.  Several studies propose that suboptimal vitamin D status can raise the potential for developing illnesses such as respiratory infections.   In a recent study of college athletes, poor vitamin D status was linked to increased frequency of the common cold and flu.

Given these studies, it seems reasonable to suggest that optimal vitamin D status is important to insure peak performance and to reduce the risk of illness.  How can a player ensure that he or she has adequate vitamin D status?  The best way to determine this is through a blood test.  In fact, many recommend this as part of routine medical examinations.  This can rule out possible health problems that might compromise vitamin D status.  It is also wise to consult with a registered dietician before making large changes in the diet to rectify suboptimal vitamin D intake.

Taking sunlight out of the equation, there are two ways to insure adequate vitamin D status.  The first is through the diet.  Unfortunately, there are no firm vitamin D intake recommendations for athletes.  In the US, adequate intake is currently is 200 IU and the recommended intake (RDA) is 600 IU.  However, some researchers suggest that the RDA should be raised to 800-2000 IU especially in athletes.  Part of the confusion over recommended intakes arises from the influence of sunlight.  For individuals who are exposed to sunlight on a regular basis, optimal dietary intake may be less.  Those who spend much or all of their day indoors may require more vitamin D containing foods in their diet.  Nevertheless, increasing intake of fish (cod, tuna and salmon) as well as fortified milk, yogurt, orange juice and cereal should result in optimal vitamin D levels, regardless of sunlight exposure.  The good news is that all of these items are included in a solid diet designed to enhance performance (low in fat, high in carbohydrates).

A second strategy is supplementation.  While whole food sources of vitamins and mineral are preferable, some athletes who have health issues or various dietary restrictions may need supplementation.  Most multivitamins contain vitamin D and specific vitamin D supplements readily available.  Read the labels carefully to determine how much vitamin D is included as different supplements can contain from 400 to 2500 IU.  Again, it is important to have vitamin D status checked by a health care professional before considering taking a supplement. 

The bottom line, there is reason to suggest that many athletes compete with suboptimal vitamin D status.  Lack of sunlight in the winter months and poor diet can both contribute.  Poor vitamin D status can impact performance on the field by affecting fitness, strength and power.  There are also negative impacts on bone health and increased risk of illness when vitamin D levels are low.  The influence of vitamin on health and performance emphasizes how important it is for players to focus on a solid diet throughout the year.  Eating a diet that contains fish and fortified products such as milk, yogurt and cereals can go a long way in helping athletes maintain proper vitamin D levels.

References:

Angeline ME, Gee, AO, Shindle M, Warren RF, Rodeo SA (2013) The effects of vitamin D deficiency in athletes. American Journal of Sports Medicine, 41: 461-464.

Larson-Meyer DE, Burke LM, Castell LM (2013) A-Z of nutritional supplements: Dietary supplements, sports nutrition, foods and ergogenic aids for health and performance: Part 40. British Journal of Sports Medicine, 47: 118-120.

Larson-Meyer DE, Willis KS (2010) Vitamin D and athletes. Current Sports Medicine Reports, 9: 220-226. 

https://nscaa.com/news/2015/02/williams--vitamin-d