The purpose of this study is to evaluate the change in vitamin D status with the change in muscular strength and muscular power in highly trained athletes over a 9-month study period. The investigators hypothesize that the greatest muscular strength and power gains will be demonstrated in the athletes who achieve and maintain a 25(OH)-D > 50 ng/mL (but < 100 ng/mL) over the 6 month study duration.

This hypothesis will be tested by carrying out the following specific aims: Aim 1. to determine the extent to which increasing serum 25(OH)D to > 50 ng/mL is associated with improved muscular power by assessing specific performance measures (e.g. Vertical Jump Test and Wingate test) relative to lean body mass; and Aim 2. to examine the correlation between vitamin D status and muscular power over time.

Study will be advertised through flyers at the training facilities and by word of mouth. Researchers will schedule meeting with club members at the training facilities. At the meetings, the researchers will give an explanation of the study and address any concerns or questions that they may have. Cyclist will have an opportunity to sign up for participation on a sign-up sheet or e-mail the PI directly to minimize any chance of coercion. Meetings will take place in the training facilities in Tulsa, Oklahoma or the Oklahoma City Metropolitan Area. At this time, subjects will complete an informed consent along with demographic, anthropometric and health history, sunlight exposure, calcium and food frequency questionnaires and PAR Q. All subjects will be asked to fill out the questionnaires in the training facilities at various locations to ensure privacy. After completion of the appropriate questionnaires, cyclist will be have venous blood from the dominant arm (i.e., ~ 5 cc of blood) collected by a licensed medical professional and serum samples stored for future analysis of vitamin D metabolites (i.e., 25-OH-vitamin D and 1,25-OH-vitamin D) and parathyroid hormone (PTH) concentrations. The cyclist's arm will be cleaned with an alcohol swab, the blood will be drawn into a sterile vacutainer using a sterile needle. At the completion of blood collection, the arm will again be cleaned using sterile gauze and covered by a band aid to minimize the risk of infection. Samples will be placed on ice in an ice chest until they are processed in the Nutritional Sciences laboratories at Oklahoma State University. Subjects will then undergo skin tone analyses followed by evaluation of specific performance measures of muscular strength and power. These performance measures include: vertical jump test and a Wingate Power Test. All testing and blood collection will take place at the training facilities.

Within 4 weeks of completing the baseline assessment, the investigators will meet with the cyclist at their training facilities. Participants will be randomly assigned to placebo or the vitamin D supplemented group. The vitamin D supplementation will consist of 100 mcg of vitamin D/d, to be taken throughout the day. Participants will achieve 100 mcg by taking 2 x 25 mcg capsules per day. One will be taken at each time point (i.e., morning and evening) throughout the day. Currently, there is no evidence suggesting that athletes require more vitamin D then the general population for their age. Although we do not anticipate that the dose of vitamin D used in this study, will produce intoxication (150 ng/mL), subjects who exceed 100 ng/ml (i.e., upper end of the normal range) at any time will be asked to discontinue the study. Studies that have demonstrated the benefits of vitamin D supplementation on muscular strength and muscular power have combined vitamin D with calcium. Therefore, all subjects (placebo and vitamin D supplemented group) will be given a commercially available supplement of calcium and instructed to take with breakfast. The calcium supplement is in the form of calcium citrate (500 mg per capsule).

Inclusion Criteria:

• Elite Cyclist (level 1,2 or 3)

Exclusion Criteria:

• Currently prescribed vitamin D without physician approval to participate in study
• Participants who respond "yes' to PAR-Q and do not get physician approval to participate in study