I am going to write about a hormone called leptin. This is produced by the fat in the horse and is involved with controlling the appetite. It also has other effects on the immune system and on regulation of temperature. It is controlled by the ventro-medial hypothalamus in the brain where there are receptors. A lot of the work that has been carried out into leptin has been done at mice. When it was first discovered, there was great interest into it's effects in relation to obesity in humans but it has been found to work differently in humans than other animals due to the type of diet we eat.
What is leptin?
Animals that have higher levels of fat will have higher levels of leptin circulating in the plasma. This will in turn reduce the appetite telling them to stop eating as much. It will also increase the appetite in horses that are thin, this is thought to be an adaptation to starvation in the wild and they will try and find food harder when it is scarce.
Affects with day-light
Melatonin levels are high in the night and low in the day and cause leptin levels to. There has been controversy in results regarding the effect melatonin has on leptin as some has found it increases it and others have found it decreases it (Kus et al, 2004). If it increases leptin levels then appetite would be down regulated during the night which would make sense. However, it is thought that the involvement with melatonin is a response to the scarce food available during winter. In the winter the days are shorter so there is more melatonin produced and if it decreased the levels of leptin it would increase the appetite. It is also thought to work the other way around during the summer (Alonso-Vale et al, 2005).
Leptin and feed restriction
When horses have had their feed restricted, the concentration of leptin decreased 6 hours after the restriction and the levels did not recover straight away when the were fed again. This shows that leptin is affected by what the horse is eating but it takes time to have an affect. The slow response to leptin increasing was thought to be due to the energy being eaten in the feed going to the tissues that needed it the most after feed restriction before being stored as fat. It was also thought that this could be an adaptation to when food is scarce, the leptin levels stay low so the horse will eat as much as possible for a period of time in case their food is restricted again (Buff et al, 2006).
When mares have a poor body condition they do not reproduce well. This is seen in lean mares that do not cycle through the winter (go into anoestrous). Leptin may act to signal to the reproductive system if sufficient body fat is present to support a pregnancy. Leptin treatments have been found to increase sexual development in both male and female mice. Leptin levels vary during foetal development and are correlated with foetal size.
A number of other factors have also been found to effect leptin concentrations. Males had higher serum concentrations than females, this was a surprising result as females are thought to have more fat.There has also been found to be an increase in leptin levels with age. This is as expected as young animals often have a lower level of body fat and a greater need of nutritional resources. The greatest levels were seen in horses older than 12 years (Buff et al, 2002).
So why do some horses get fat?
For some horses it may be down to the owner feeding them a high grain, high energy diet, they have not evolved to eat this kind of feed and the high energy intake from eating just a small amount will not be controlled by leptin. It has also been suggested that some ponies (small and fat ones) may be leptin resistant. In the same way people or horses that are diabetic are insulin resistant. This will therefore mean they have high levels of circulating leptin but it will not be having an effect on their appetite as the horse's tissues fail to respond to leptin.
Hyperleptinemia can happen. This is often seen at the same time as hyperinsulinaemia which is the equivalent of diabetes in horses. It has been suspected that hyperleptinaemia may be a result of reduced insulin sensitivity which causes an increase in insulin concentrations which effects the leptin levels (Caltabilota et al, 2010). This may also predispose horses to laminitis.
(Some of this information is taken from Applied Animal Endocrinology by Squires).
Love Laura
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