Circadian rhythm of calciotropic hormones, serum calcium, phosphorus and magnesium during the shortest and longest days of the year in horses in New Zealand.
Authors: Azarpeykan S, Dittmer K E, Gee E K, Marshall J C, Elder P, Acke E, Thompson K G
Journal: Journal of animal physiology and animal nutrition
Summary
# Editorial Summary New Zealand researchers tracked circadian fluctuations in calcium-regulating hormones and minerals across 48-hour periods on the winter and summer solstices in five pastured horses, collecting blood samples every three hours to measure vitamin D metabolites (including 25-hydroxyvitamin D3 and its active form 1,25-dihydroxyvitamin D), parathyroid hormone, and serum calcium, phosphorus and magnesium. During the longest days of summer, ionised calcium, active vitamin D and parathyroid hormone demonstrated clear circadian rhythmicity, whilst total calcium, phosphorus and magnesium showed distinct diurnal patterns; conversely, no analyte exhibited circadian variation during the shortest winter days. Notably, 25-hydroxyvitamin D3 concentrations were persistently low and often undetectable, suggesting limited endogenous synthesis even during summer photoperiods in New Zealand's temperate climate. These findings carry particular significance for equine athletes competing internationally, as the combined disruption of circadian rhythm adjustment during travel, coupled with seasonal variation in mineral metabolism, may impact performance recovery, bone metabolism and overall physiological adaptation—factors worth considering when conditioning horses for competitions across different latitudes and seasons.
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Practical Takeaways
- •Horses show seasonal variation in mineral metabolism and hormone regulation; those competing internationally during winter may experience disrupted calcium-phosphorus homeostasis affecting performance and recovery
- •Equine athletes travelling across significant time zones and seasonal changes may require nutritional or management adjustments to support circadian rhythm re-entrainment and maintain mineral balance
- •Vitamin D supplementation strategies may need seasonal consideration given the very low baseline 25-hydroxyvitamin D3 concentrations observed year-round in pasture-kept horses in New Zealand
Key Findings
- •Serum ionized calcium, 1,25-dihydroxyvitamin D, and parathyroid hormone showed clear circadian rhythms on the longest (summer) days but not on shortest (winter) days
- •Total calcium, phosphorus, and magnesium concentrations demonstrated diurnal patterns only during summer, with significant seasonal differences between winter and summer
- •Serum 25-hydroxyvitamin D3 was very low and mostly undetectable across both measurement periods
- •Circadian rhythm disruption during winter months may have implications for horses travelling to international competitions across time zones and seasonal changes