PURPOSE: Objective 1 - To determine whether trained cyclists exhibit lower bone mineral density (BMD) than recreational cyclists. Objectives 2&3 - To determine the effects of 12 weeks supplementation with calcium collagen chelate (CCC) on body composition, bone and biomarkers of bone metabolism during habitual training in competitive cyclists. A group of 29 male cyclists [9 recreational (<8 h/wk) and 20 trained (≥8 h/wk] participated in the study. METHODS: Maximal exercise testing and 40-k time trials (TT) were performed on an electronically braked cycle ergometer. BMD of the whole body, lumbar spine (L1-L4) and both hips were measured using a Hologic Discovery-W (Hologic, Waltham, MA, USA). For research Objective 1, a one-way analysis of variance (ANOVA) was used to evaluate the differences on dependent measures of between the recreational and trained cyclists. Pearson product moment correlations and multiple regressions were used to examine relationships between the dependent variables. For research Objectives 2 and 3, trained cyclists were assigned to one of two groups: 1) 6 g/d of CCC or 2) placebo control (CON) composed of an inert compound with calcium and vitamin D equivalent to that found in the CCC. One-way ANOVA were used to compare baseline variables between the CON and CCC. Possible effects of the independent variable, CCC or CON supplementation, on the dependent variables, BMD (whole body, total hip, lumbar spine), bone alkaline phosphatase (BAP) and tartrate resistant acid phosphatase 5b (TRAP5b) were evaluated statistically by two-way repeated measures ANOVA (group x time). The null hypothesis was that CCC supplementation would have no effect on the dependent variables. When interactions were significant, a Tukey post hoc test was used to compare between group values. Significance was accepted at p<0.05. RESULTS: Objective 1 - There were no differences in BMD at any site between recreational and trained cyclists. T-scores identified both recreational and trained cyclists as osteopenic (-1.16 and -1.49, respectively) at the lumbar spine. Evaluated individually, 12 trained cyclists and two recreational cyclists were identified as osteopenic and three trained cyclists were identified as osteoporotic. Objectives 2 & 3 - No differences in BMD, body composition or biomarkers of bone metabolism were found between the CCC and CON groups. There were no group*time effects found for BMD, body composition or biomarkers of bone metabolism. Strong Pearson moment correlations were found between weekly training hours and TRAP5b (r = 0.531), BAP and VO2max (r = -0.561), and BAP/TRAP5b ratio and right/left hip BMD (r = -0.649 and r = -0.646, respectively). CONCLUSION: Our findings demonstrate that male cyclists riding more than six hours per week have reduced BMD, particularly at the lumbar spine. While increased training volume leads to improved aerobic capacity, it may increase bone turnover and promote an environment that leads to significant bone loss over time. Additionally, 12 weeks supplementation of CCC does not affect body composition, BMD or biomarkers of bone metabolism. Further research is needed to determine whether low BMD compromises bone strength in male cyclists. Supported in part by the Institute of Sports Sciences and Medicine.