by Marilyn Munzert
The latest buzz on obese horses and ponies is insulin resistance, a condition where cells are resistant to the effects of insulin, and a new condition termed Equine metabolic syndrome has been used to describe these “easy keepers.”
In the normal state, insulin is secreted when blood glucose (sugar) levels are increased, usually after a meal. Insulin suppresses liver production of glucose and increases uptake and utilization of glucose by muscle and fat cells, therefore lowering overall circulating blood glucose. Insulin resistance causes increased blood glucose levels, which actually stimulates more insulin production in an effort to lower the glucose, causing a state of hyperglycemia and hyperinsulinemia (excess sugar and insulin) and glucose intolerance. In humans this condition often leads to type II diabetes, but in horses this doesn’t seem to occur.
The main causes of this condition are inactivity and feeding high carbohydrate feed (high in sugar feeds). It’s believed that equines with this condition are genetically predisposed to glucose intolerance and insulin resistance, and once obesity occurs, the state of insulin resistance remains, making it very difficult to solve this problem once it’s started. It’s also believed the tendency toward obesity begins in young animals fed high glycemic diets (feeds that increase blood glucose) even before obesity is seen. The most common high glycemic feeds are grains, namely oats.
Once the fat gets put out, it seems to be a self-perpetuating cycle. In contrast to the belief that fat cells are just energy storage cells, they are very active, secreting hormones that cause many different reactions. Circulating fatty acids in the blood have been shown to decrease sensitivity to insulin, adding to insulin resistance. Also, fat cells in different parts of the body act differently. One substance in particular, 11Beta-HSD-1, which converts inactive circulating cortisone into cortisone, is secreted by fat cells. In humans it has been shown that this substance is released by abdominal fat cells, causing increased cortisone in the abdominal fat only. Cortisone in humans has been shown to stimulate many reactions, including increased fat production by fat cells, causing fat to accumulate in the abdomen only, versus other parts of the body. This substance has also been found in different sites in obese horses and ponies, which explains the fat bellies, cresty necks, etc.
From here the reactions keep coming, with one of the more significant being increased local production of cortisone by 11Beta-HSD-1 in the sensitive lamina (tissue that holds the coffin bone to the hoof) — possibly causing Cushing’s syndrome, where total circulating levels of cortisone are not elevated due to a problem with the pituitary gland, but rather elevated cortisone levels are seen in specific tissues only from local increased 11Beta-HSD-1. Most horses with peripheral Cushing’s syndrome develop laminitis of varying degrees that appears resistant to treatment.
Overall, the hormone interactions, genetics and body chemistries are complex, and all of the links are not yet known. However, it still comes down to the basics that we have been aware of all along: diet, and exercise. Exercise increases insulin sensitivity. High sugar diets decrease insulin sensitivity. To counteract this syndrome, we must not only reduce an animal’s feed, but we have to exercise him. In addition, since this problem seems to occur early in life long before obesity is seen, we need to feed young horses and ponies low-sugar diets and give them plenty of exercise as well. Exercise should consist of not only walking but trotting and some canter/gallop work as well so that heart rate and metabolism are increased.
Replacing grain-based feeds with protein and fiber-based feeds is a great way to reduce the sugar in the diet. Often, hay only is just fine. Inactive animals don’t need grain of any type — this includes the senior diets. (Vitamin and joint supplements can be added to beet pulp.) Work with your vet to determine the best diet for your horse or pony.