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

The research investigates how differing levels of calcium and phosphorous intake impact calcium balance in horses during high-speed exercise. The study shows that a high intake of these minerals does not negatively affect parathyroid hormone responses but may interfere with magnesium metabolism.

Study Design and Methods

• The experiment was conducted on five trained horses that were sequentially fed diets containing three different levels of calcium (Ca) and phosphorous (P): 33 g Ca and 19 g P, 64 g Ca and 38 g P, and 96 g Ca and 56 g P. These represent an adequate, moderately high, and high intake, respectively.
• Each horse was acclimatised to the diets over 21 days before undergoing a standardised exercise test (SET) on a treadmill. This test included 5 stages, each lasting 4 minutes, with 3% incline. The speed started at 5 m/sec, with increments of 1 m/sec after each stage.
• Blood samples were drawn from the horses at specific intervals for analysis.

Results and Interpretations

• During the exercise, horses’ blood lactate, plasma parathyroid hormone (PTH), and plasma inorganic phosphorous (Pi) levels increased. At the same time, their blood pH and ionised calcium levels (Ca2+, adjusted to a pH of 7.4) decreased. Despite these changes, the total plasma calcium (CaT) and magnesium (Mg) levels remained constant.
• The most exaggerated decrease in Ca2+ and lactate peaks occurred under adequate Ca and P intake at speeds between 7, 8, and 9 m/sec which indicate how the different mineral intakes affected the horses under high-speed exercise.
• Two hours post-exercise, the study noted an increase in Ca2+ levels that exceeded resting concentrations. The high calcium diet correlated with higher levels of Ca2+ in the horses.
• Parathyroid hormone levels returned to baseline two hours post-exercise, with those on the adequate Ca intake showing higher PTH concentrations compared to those on the diets with higher calcium. During the recovery phase, horses on the high Ca diet had decreased Mg levels.

Conclusions

• High dietary intake of calcium and phosphorous did not impact the responses of the parathyroid hormone during exercise and recovery. However, the study hinted at depression in magnesium metabolism in the high Ca and P diets.
• Despite potential for an increased availability of Ca2+ and Pi post intensive exercise with high Ca and P intake, the negative effect on Mg balance may compromise this potential benefit. Further research is required to fully understand this dynamic and its potential implications on horse health and performance.