Can Vitamin D enhance exercise performance?  Vitamin D has been a topic of discussion lately as experts debate whether or not the current recommended daily intake is sufficient to meet the needs of the body.  Vitamin D is a fat-soluble vitamin (can be stored in body fat) and a hormone (conversion occurs in the skin to be sent to the liver and kidneys for further conversion to a useable form in other areas of the body).  Our primary source of Vitamin D comes from the sun.  Fortified foods, such as milk, are the next prominent source.

     Vitamin D is "inactive" and in order for it to become "active", and useable by the body, it has to go through hydroxylation.  This process starts in the liver where Vitamin D becomes 25-hydroxyvitamin D (25(OH)D).  Further conversion occurs in the kidneys where it becomes 1,25-dihydroxyvitamin D (1,25(OH)2D).  Adequate blood levels of Vitamin D, 40-100 nmol/L, are determined by measuring blood levels of 25(OH)D.  Modern day man's habits, such as sunscreen use and increased participation in indoor activities, have reduced our exposure to the sun's Vitamin D producing rays.  This, in turn, increases the risk for becoming deficient.  In order to achieve sufficient levels for body systems to function, dietary intake of Vitamin D needs to be increased to compensate for the decreased sun exposure.

     Adequate levels of Vitamin D are needed by the body to promote calcium absorption, bone health and muscle function, to maintain normal blood levels of calcium and phosphorus, for homeostasis of the nervous system, to regulate cell production, and for protein synthesis.  Vitamin D's role in enhancing exercise performance has been recently speculated.  An extensive review of the literature by Cannell et al., published in the May 2009 issue of Medicine & Science in Sports & Exercise, suggests that it can for individuals who are Vitamin D-deficient, especially for those who participate in indoor athletics (gymnastics) or who live at the northern latitudes.  The extra stress that exercise places on the metabolic pathways in which micronutrients, such as Vitamin D, play a role may indicate that there is a greater need for, and hence a greater intake required, of Vitamin D.  In addition, athletes are more likely to engage in dietary practices that limit Vitamin D intake, such as restricting total energy consumption and eliminating one or more of the food groups.

     Cannell et al.'s review of the literataure discovered that many studies have shown exercise performance to be seasonal.  That is, individuals perform at their best when 25(OH)D is at its peak in the late summer (when sun exposure has been greatest).  Exercise performance then starts to decline in early autumn as sun exposure decreases, and subsequently 25(OH)D levels decline.  In late winter, when sun exposure is very limited, and 25(OH)D levels are at their lowest, so is athletic performance.  This phenomenon suggests adequate levels of vitamin D are needed for optimum performance.  Cannell et al. also cited literature that indicated Vitamin D has been shown to increase the size and number of Type II (fast twitch) muscle fibers.

     Do these findings mean that everyone who wants to enhance their physical performance should start supplementing with Vitamin D?  No.  If you consume a nutritionally adequate, well-balanced diet, your Vitamin D levels should be sufficient to meet the demands of exercise.  There is no evidence that Vitamin D levels above normal will increase exercise performance.  In fact, taking mega-doses of Vitamin D can lead to toxicity.  Scientific findings only indicate that individuals who are Vitamin D-deficient can benefit from increased sun exposure and dietary intake or through Vitamin D supplements.

     If you think you may be Vitamin D deficient you should consult your physician to have your blood levels tested.  You may also want to consider having your diet reviewed by a registered dietician who has experience counseling active and athletic individuals.

Note:  Before beginning an exercise program or increasing the intensity level of a current routine, a physician's approval should be obtained, especially for older adults and those at risk for or who currently have chronic health conditions.

Resources:

Medicine & Science in Sports & Exercise, Vol. 41 No.5 May 2009, "Athletic Performance and Vitamin D,"pp. 1102-1109.  Cannell, JJ et al.

ACSM's Health & Fitness Journal Vol. 12, No. 5 September/October 2008, "Vitamin D and Health: Do We Need More Than the Current 

DRI?," pp. 34-36. Volpe, S.L.

The Encyclopedia of Healing Foods, Murray M., Pizzorno, J., and Pizzorno L.

American College of Sports Medicine, American Dietetic Association, and Dietitians of Canada Joint Position Statement "Nutrition and Athletic Performance," Medicine & Science in Sports & Exercise, pp. 709-731.