Journal Title (Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Year: 2010 Epub ahead of print
Summary of Background and Research Design
Background:Although cross-sectional studies show that athletes have a higher bone mineral density (BMD) than nonathletes, some studies show that competitive cyclists have lower BMDs than other athletes such as runners. In a recent study it was shown that total hip (TH) BMD decreased 1.5% in male cyclists over a 1 year training period, a rate comparable with that observed in early menopause. The mechanism for such BMD loss in cyclists is unknown.
Hypothesis: Prolonged and excessive sweating during exercise will decrease serum ionized calcium concentration, triggering an increase in parathyroid hormone (PTH) and subsequent bone resorption that will disrupt normal bone homeostasis in favor of a BMD decline. Supplemental calcium before or during exercise will prevent decreased serum calcium levels.
Subjects:Adult male cyclists and triathletes with mean age of 37 years, a mean of 6 years of cycling competition experience, a mean maximal oxygen consumption (VO2max) of 53 mL/kg/min, a mean lumbar T-score of -0.56, and a mean serum vitamin D level of 32.6 ng/mL participated in the study.
Experimental design: Double-blind cross-over trial
Treatments and protocol: Participants performed 4 different exercise routines at the same time of day. Aerobic VO2max was measured at the first exercise visit. Three subsequent routines of 35-km time trials were performed, 2 to 7 days apart, with 3 levels of calcium supplementation. In the first level, participants consumed 1 L of calcium-fortified beverage 20 min before exercise, and 250 mL of a placebo beverage at 15, 30, and 45 min of the exercise round. In the second test condition, participants consumed 1 L of placebo before exercise and 4 x 250 mL of calcium-supplemented beverage at 15, 30, and 45 min of the exercise round. In the third test condition, participants consumed placebo throughout. The first 2 test conditions both provided 1,000 mg of calcium. Parathyroid hormone (PTH), carboxy-terminal collagen crosslinks (CTX), bone-specific alkaline phosphatase (BAP; a marker of bone formation), ionized calcium (iCa), and serum calcium were measured before and immediately after exercise.
Summary of Research Findings
- There were no significant differences in pre-exercise values of PTH, CTX, or iCa across test conditions.
- Calcium supplementation did not affect cycling performance.
- Calcium supplementation before exercise attenuated the exercise-induced increase in PTH relative to placebo (55.8 ± 15.0 vs 74.0 ± 14.2 pg/mL; P = .04) and this difference remained significant when adjusted for hemoconcentration (P = .05).
- A similar trend for suppression of the PTH increase (58.0 ± 17.4 pg/mL vs 74.0 ± 14.2 pg/mL; P = .07) for calcium supplementation during exercise vs placebo was observed.
- CTX increased and iCa decreased similarly in response to exercise under all test conditions.
- Sweat calcium data were incomplete and not reported.
- There were no significant effects of calcium supplementation on changes in CTX, BAP, and iCa.
Interpretation of findings/Key practice applications:
Supplemental calcium provides a non-skeletal calcium source to stabilize serum levels and may minimize skeletal calcium loss. In this study, calcium supplementation before exercise attenuated the increase of PTH although no overall effects were observed on the levels of markers of bone turnover or serum calcium (ie, CTX). It may be that changes in CTX may appear at a later time point than measured in this study. Additional study limitations were not standardizing test time between participants (PTH has diurnal variation) and not standardizing the diet for calcium and vitamin D intake. It also would have been interesting to see whether this calcium supplementation effect on PTH held true in individuals having higher serum 25-hydroxyvitamin D levels (eg, >= 40 ng/mL), as the levels in this study were close to vitamin D insufficiency (eg, < 30 ng/mL). Finally, further research is needed to determine the effects of repeated increases in PTH and CTX on bone (ie, exercise training), and whether calcium supplementation can diminish any exercise-induced demineralization over a longer-term observation period.