Effects of different levels of calcium and phosphorus intake on calcium homeostasis in exercising horses

Abstract

Increasing levels of dietary calcium (Ca) and phosphorous (P) might have a negative impact on parathyroid hormone (PTH) response and result in a more pronounced hypocalcaemia during high-speed exercise in horses. In successive order, five trained horses were fed 33 g Ca and 19 g P (approximately 100% NRC, adequate intake), 64 g Ca and 38 g P (moderately high intake) or 96 g Ca and 56 g P (high intake). Each horse was adapted to each diet over a 21 day period before undergoing a standardised exercise test (SET) on a treadmill. The SET comprised 5 steps (each step 4 min duration, 3% incline, first step 5 m/sec, followed by increments of 1 m/sec). Blood samples were taken at defined times. Blood lactate, plasma intact PTH (PTHi) and plasma inorganic P (Pi) increased during exercise (P<0.05), blood pH and blood ionised Ca2+ (Ca2+, adjusted to a pH of 7.4) declined during SETs (P<0.05) whereas total plasma Ca (CaT) and Mg (MgT) remained constant. The most exaggerated drop in Ca2+ and lactate peaks were found for the adequate Ca and P intake at 7, 8, and 9 m/sec during SET (treatment P<0.05). At 120 min after exercise, Ca2+ levels exceeded resting concentrations with higher Ca2+ values for the higher Ca intake (Ca2+: adequate 1.58 +/- 0.07 mmol/, moderately high 1.63 +/- 0.04 mmol/l, high 1.65 +/- 0.02 mmol/l, P<0.05). Concomitantly, 120 min after exercise, PTHi returned to basal concentrations with higher PTH concentrations for the adequate Ca intake than the other two treatments (PTHi: adequate 45.6 +/- 72.8 pg/ml, moderately high 11.6 +/- 13.6 pg/ml, high 2.6 +/- 2.3 pg/ml, P<0.05). During recovery (30 min, 120 min and 24 h after exercise) MgT concentrations were depressed (P<0.05) for the high Ca intake (P<0.05). These results suggest that high dietary Ca and P intake did not impair PTH responses during exercise and recovery, but might depress Mg metabolism. High Ca and P intake might have some potential to increase the availability of blood Ca2+ and Pi in the recovery phase after intensive exercise, but detrimental effects on Mg homeostasis might offset this potential benefit.