The influence of different feeding times and frequencies on the circadian rhythm of serum cortisol concentration in horses (Equus caballus)

Abstract

With the exception of dogs, all mammals studied so far (including the horse) have shown a circadian rhythm in blood glucocorticoid concentration. The rhythm is tightly regulated and may be influenced by light, feeding, and sleep. Light is the strongest candidate for being the entrainer of the rhythm, although, meals and paradoxical sleep (PS) have been associated with increases in glucocorticoid concentration. Of most interest in this study is the meal associated rises in glucocorticoid concentration. This is because glucocorticoids are associated with glucose metabolism, and it is this association that may offer a functional explanation for the circadian rhythm in glucocorticoids.

The study was carried out on horses because of the large differences in feeding patterns between feral horses (continuously grazing throughout the day) and stabled domestic horses (one or two large high energy meals per day). The purpose of the study was to investigate whether feeding large meals of high DE (Digestible Energy) concentration once a day could be detrimental for horses.

Nine horses were fed over three periods either at 0900 hours, 2100 hours or hourly, for eight days in a cross over design so all horses were on each treatment once. Behaviour sampling occurred on day six and hourly blood samples were collected on day eight. Behaviour sampling consisted of monitoring sleep-wake patterns. Blood samples were analysed for serum cortisol concentration, and serum glucose concentration.

Horses showed a circadian rhythm in cortisol (p<0.05), but the circadian rhythm was not affected by feeding time or frequency (p>0.05). Sleep cycles were also not affected by feeding times or frequencies (p>0.05). Glucose concentration profiles differed markedly between treatments (p<0.05), a sharp rise in glucose occurred after feeding when fed at 2100 or 0900 hours. In hourly fed horses the glucose concentrations remain relatively constant. It did appear from the glucose concentration profiles, that mares fed at 2100 hours had a larger glucose rise and took longer to return to pre feeding levels than when fed at 0900 hours.

From this result I tentatively conclude that feeding at 2100 hours resulted in a lower glucose tolerance compared to when fed at 0900 hours, this may have been because feeding occurred at the different stages of the circadian rhythm in glucocorticoids. As glucocorticoids increase glycogen deposition through the action of insulin in the liver while at the same time decreasing insulin action in peripheral tissues, feeding during basal glucocorticoid concentrations may have resulted in decreased efficiency of glucose regulation mechanisms as well as decreased deposition in the liver. If horses are to be fed large high energy meals then these meals should be fed at the time of peak glucocorticoid concentration, that is, in the morning.