Acute calcium ingestion attenuates exercise-induced disruption of calcium homeostasis


Journal Title (Medline/Pubmed accepted abbreviation): Med. Sci. Sports Exerc..
Year: 2011
Volume: 43
Number: 4
Page numbers: 617-623
doi (if applicable): 10.1249/MSS.0b013e3181f79fa8

Summary of Background and Research Design

Background: Although athletes generally have higher bone mineral density (BMD) than non-athletes, cyclists are an exception. It is thought that the act of sweating indirectly activates bone resorption (minerals leaving the bone). This, in combination with the weight-supported exercise (as opposed to weight-bearing exercise), is possibly responsible for the reduction in BMD among cyclers.

Parathyroid hormone (PTH) is a hormone that is released when calcium blood levels decrease. It activates bone resorption. Another protein, C-terminal telopeptide of Type I collagen (CTX) was measured in this study because it is a marker of bone resorption.


Hypothesis: Calcium supplementation either before or during exercise will reduce the amount of circulating PTH and CTX compared to a placebo. Decreases in PTH and CTX imply a lower rate of bone resorption after exercise, which could prevent the loss of BMD.

Subjects: Amateur male road cyclists and triathletes competing at the state to international level (n = 20). Participants were 37.0 ± 7.6 yrs old.

Experimental design: double-blind, placebo-controlled, randomized, counterbalanced, cross-over design.

Treatments: A calcium-fortified sports beverage containing 1000 mg/L of calcium or a placebo, which was the same beverage without calcium fortification. Subjects were assigned to one of the following groups: 1) a 1000 mL beverage before a cycling time trial (placebo or fortified) followed by placebo during exercise; 2) Placebo before exercise followed by 4 × 250 mL beverages during the time trial (either placebo or fortified); 3) placebo both before or during exercise (control).

Protocol: Before the time trials, the participants’ VO2max was determined using a cycle ergometer. Subsequently, the participants performed 3 × 35 km time trials with 2-7 days between visits. Venous blood was sampled before the time trial, every 15 min during the time trial, and then immediately after exercise and analyzed for ionized calcium (iCa), PTH, CTX, and bone-specific alkaline phosphatase (BAP, a marker of bone turnover). Sweat was collected during the time trials with epidermal patches. They were analyzed for sweat quantity and calcium concentration. There were 2 sweat collection periods: minutes 0-20 and 30-50 of the time trial.

Summary of research findings
  • Calcium supplementation at any time did not affect performance during the time trial.
  • Calcium supplementation before the time trial was effective at attenuating PTH concentrations (p = 0.04). A similar trend was present for calcium supplementation during exercise, although the decrease in PTH was not significant (p = 0.07). When the PTH concentrations were normalized to hemoconcentration (takes into account dehydration), calcium supplementation statistically significantly attenuated PTH concentrations regardless of timing (p < 0.05).
  • CTX was not influenced by calcium supplementation before or during exercise.
  • Neither iCA nor BAP were affected by calcium supplementation.
  • There were no differences among treatments in sweat calcium concentrations.

Interpretation of findings/Key practice applications

Without affecting performance, calcium supplementation either before or during exercise attenuated PTH, the hormone that activates mineral loss from bone. This is an exciting finding, but more research is necessary to see if calcium supplementation could prevent the loss of BMD in cyclers. When combining these data with other research on calcium supplementation in athletes, it appears that the timing of calcium ingestion is important; more research is needed to determine the optimum timing of calcium ingestion. Cyclers need to be aware that developing a low BMD is likely and ensure adequate amounts of calcium. Incorporating weight-bearing exercise into their workouts may also help.

Limitations

The dietary intake of calcium and vitamin D (affects calcium absorption) was not controlled during the duration of the study. However, the calcium intake of the participants immediately before the trials appeared to be consistent, so it is not expected that this limitation would cause inadequacies of the data. The subjects in this study had relatively normal BMD (t-scores < 1 at lumbar and hip sites), so it would have been interesting to repeat this study in subjects having lower BMD.
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