Vitamin D is certainly essential for normal calcium metabolism and plays a vital function in maintaining bone and muscle health.  Many athletes are vitamin D deficient simply because they shun vitamin D-fortified dairy products and don’t get much outdoor activity.  Vitamin D is produced naturallywithin the skin as part of a reaction involving sunlight,however it is also consumed in the diet.  A review of literature by Enette Larson-Meyer and Kentz Willis concluded that adequate vitamin D levelsare essential for promoting muscle and strength, and that it plays a crucial role in muscle protein synthesis.  Vitamin D deficient athletes should strive to achieve blood levels of vitamin D greater than 40 nanograms per milliliter of blood.  (Current Sports Medicine Reports, 9:  220-226, 2010)