Globe-trotting horses may be prone to jet lag just like the rest of us, New Zealand research suggests.
A study carried out at Massey University analysed levels of vitamin D metabolites, also known as calciotropic hormones, taken from blood drawn from horses around the shortest and longest days of the year.
The researchers wanted to gain insights into the differences in circadian rhythms at these seasonal extremes.
The term circadian is derived from Latin and means “around the day”. A circadian rhythm is a biological process that is driven by an “internal clock”.
Vitamin D is a hormone required for the body to absorb calcium and phosphorus. It can be obtained through diet or produced in skin in the presence of enough sunlight.
Sara Azarpeykan and her colleagues used five healthy adult horses which were on constant pasture for the research, the findings of which have been published in Journal of Animal Physiology and Animal Nutrition.
Blood samples were collected from each horse every three hours over a 48-hour period around the shortest and longest days of the year.
Vitamin D metabolites measured including 25-hydroxyvitamin D3, 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D and parathyroid hormone, in addition to serum concentrations of calcium, phosphorus and magnesium.
The study team then looked for any differences between the two periods. They found what they described as significant differences between the serum concentration of calciotropic hormones between the shortest and longest days of the year in horses.
The concentrations of some of them indicated they were regulated on a circadian (24-hour) rhythm on the longest days of the year, while others revealed a summer pattern related to day and night – a diurnal rhythm.
None of the analytes showed any circadian rhythm on the shortest days of the year.
“The result of this study could have significant relevance to equine athletes travelling to international equestrian competitions and facing a huge time and seasonal differences that might affect their ability to adjust their circadian rhythms to new time zones,” the researchers reported.
The study team noted that previous work on rhythm patterns related to serum concentrations of calcium, phosphorus and vitamin D metabolites in horses suggested that calcium, phosphorus and 25-hydroxyvitamin D3 had a circadian rhythm.
This, they said, was not consistent with the findings in the current study.
“Light has a direct effect on circadian rhythms, although some non-photic parameters [unrelated to light] such as exercise patterns and food availability may also play an important role on these rhythms.
“It is suggested that the reason experimental horses and trained racehorses in other studies showed stable rhythms might be because of the daily routine of the horses’ environment with constant management regimens (e.g. feeding and exercise time).”
They pointed to previous research showing a stronger rhythm of the stress hormone cortisol in horses used to a management routine that included stabling, feeding and sometimes exercise.
“The fact that most horses in New Zealand are mainly kept in paddocks with limited stabling might reduce the management effects on horses, and therefore the rhythms of different analytes in their body.”
Horses in the current study also had unlimited access to grass, which was similar to horses’ natural feeding habits. “Food intake and the time between ingestion of food and blood sample collection have a direct effect on the plasma concentration of many analytes, suggesting that any rhythm in serum analytes measurements found in other studies might be the direct result of feeding and management practices, rather than changes in photoperiod.”
The researchers noted that, despite the fact that concentrations of calcium, phosphorus, magnesium and vitamin D in pasture and hay eaten by the horses exceeded their minimum daily recommended intake requirements, there was a significant variation in the concentration of most serum analytes between the shortest and longest days of the year.
The predominance of 25-hydroxyvitamin D2 suggested that vitamin D2 obtained from food was the principal source of vitamin D for horses.
The amount of sunshine and ultraviolet radiation has a direct effect on the synthesis of vitamin D2 in the pasture and hay that is consumed by the horses. “Perhaps the differences in vitamin D concentration due to the greater availability of vitamin D in the longest days of the year compared with the shortest days of the year contributed to the variability in measurements of other analytes.”
Another possible reason for the differences between the longest and shortest days was the fact that all the horses were mares – seasonal breeders for whom light is the main environmental factor. Differences might relate to a need to establish hormonal and mineral balances to meet the requirements of pregnancy, they suggested.
“This study could have significant relevance to horses in New Zealand and abroad, as there was a significant difference between serum concentrations of measured analytes between summer and winter,” they said, while stressing that the implications of the seasonal differences were not yet known.
“Most international travelers encounter jetlag, which is directly linked to circadian clocks. Circadian clocks and circadian rhythms have important roles in the daily regulation of human physiology.
“Equine athletes that travel by air to international equestrian competitions face huge time and seasonal differences that might affect their health, ability to adjust their circadian rhythms to new time zones and their performances during the competition events.”
Perhaps maintaining a regular feeding and handling schedule might reduce some of these effects, they suggested.
Sara Azarpeykan was joined in the research by Keren Dittmer, Erica Gee, Jonathan Marshall, Els Acke and Keith Thompson, all from Massey University; and Peter Elder, from Canterbury Health Laboratories.
Circadian rhythm of calciotropic hormones, serum calcium, phosphorus and magnesium during the shortest and longest days of the year in horses in New Zealand.
Azarpeykan S, Dittmer KE, Gee EK, Marshall JC, Elder P, Acke E, Thompson KG.
J Anim Physiol Anim Nutr (Berl). 2016 Feb 3. doi: 10.1111/jpn.12477