Acute effects of 2 hours of moderate-intensity cycling on serum parathyroid hormone and calcium.

Previous studies have found that serum parathyroid hormone (PTH) increases in response to relatively short (<60 minutes), intense bouts of exercise, possibly as a result of decreases in serum calcium. Whether longer, less intense exercise also stimulates an increase in PTH is not known. The effects of 2 hours of moderate-intensity cycling on serum PTH and calcium were investigated in 20 competitive male cyclists, aged 22-45 years. Serum concentrations of PTH and calcium were measured before and after exercise. Dermal calcium loss was estimated using patch collections and loss of sweat. There were increases in PTH from 40.6 +/- 15.6 to 69.5 +/- 25.5 pg/mL (P < 0.001) and in serum calcium from 9.3 +/- 0.3 to 9.6 +/- 0.5 mg/dL (mean +/- standard deviation, P = 0.001) in response to exercise. Contraction of plasma volume explained the rise in calcium but not PTH. Dermal calcium loss was estimated at 138.0 +/- 71.9 mg for the 2-hour exercise bout. Neither the change in serum calcium nor the dermal calcium loss was significantly related to the increase in PTH. The study demonstrated that prolonged exercise stimulates PTH secretion. The effects of such transient increases in PTH on bone metabolism are not known.