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The horse's metabolic processes provide the body with the fuels it needs to ­sustain itself

Stroll past any magazine display or newsstand and chances are some health or lifestyle cover is boasting of new ways to boost your metabolism. The term "metabolism," however, encompasses far more than simple weight gain, weight loss, or calorie burning. Whether in humans or horses, it refers to chemical reactions occuring within the body that ultimately provide it with the components required to sustain life.

The metabolic process typically is divided into categories: Anabolic reactions use energy to build structural components of the body such as muscle. Catabolic reactions break down large particles to smaller particles and produce energy. Many anabolic reactions occur just after feeding, with the body storing and building substrates for later use, while catabolic reactions tend to occur several hours (or days) after a meal, or with exercise, when fuels need to be made available. Adenosine triphosphate (ATP) is the unit of energy the body uses. "Metabolic rate" refers to the total amount of energy a body uses at rest in a day (for tasks such as breathing, maintaining body temperature, etc.) and differs based on factors such as body size and horse breed.

Metabolic Reactions

Following a meal, the body of a horse at rest breaks down feed substrates within its cells and stores them for later use. Feeds high in starch and sugar (cereal grains) are ultimately digested into glucose, which is absorbed into the bloodstream. This increase in blood glucose concentration triggers the pancreas to release the hormone insulin, which allows glucose to leave the bloodstream and enter muscle or adipose (fat) tissue (among other body tissues), thereby returning blood glucose concentrations to baseline.

"Certain feeds create more of an increase in glucose and, therefore, insulin than others. These feeds are called high glycemic index feeds," explains Carey Williams, PhD, associate director of the Rutgers University Equine Science Center, in New Brunswick, N.J. "In horses the glycemic index has been placed against whole oats in so things like average grass hay are going to have a lower 'index,' where feeds like corn will be higher."

Once inside a tissue glucose can be catabolized (broken down) to ATP or can be stored as glycogen (the storage form of glucose). At rest, because the cell's fuel needs are lower, most glucose is stored as glycogen rather than broken down to energy. Insulin is also responsible for stimulating glycogen synthesis. Feeds higher in fiber (e.g., forages such as hay and pasture) are fermented in the large intestine to produce the volatile fatty acids (VFAs), namely acetate, propionate, and butyrate. Once absorbed, propionate is converted to glucose in the liver, while butyrate is converted to acetate. Acetate can then be metabolized to ATP by a variety of tissues or can be incorporated into fat synthesis. Fat metabolism is poorly understood in the horse, but ultimately dietary triglycerides (fats) can be stored for future use in the muscle or adipose tissue.

Kristine Urschel, PhD, assistant professor of equine science at the University of Kentucky, who specializes in protein nutrition in horses, points out, "following a meal, dietary protein is broken down to amino acids, which are mostly absorbed in the small intestine. The main use for these amino acids is to make protein, a key component of all tissues, but of particular interest in an athletic animal such as the horse is the muscle. Muscle protein synthesis is activated by amino acids, insulin, certain hormones, and by exercise stimuli. The greatest rates of muscle protein synthesis happen during growth, and rates decline as the animal ages. Dietary amino acids that are provided in excess of what can be used to make protein are degraded by the body, and the resulting ammonia is converted to urea (a nitrogen-containing molecule) and excreted in the urine."

It should be noted that while glucose (from starch and sugar digestion) levels will increase rapidly after a meal, fiber fermentation is a longer process and, thus, peak VFA concentrations occur much later (two to eight hours) after feeding a high-fiber meal. In fact, fermentation might occur upwards of 24-48 hours after a meal, with VFAs providing the animal with a prolonged and sustained fuel source.

If a horse goes too long without eating, blood glucose concentrations will start to decrease. This decrease triggers the release of another pancreatic hormone, glucagon. While insulin functions to promote substrate storage, glucagon stimulates the mobilization of energetic substrates. For example, glucagon triggers glycogen breakdown in the liver to release glucose to blood, and it causes fat mobilization from adipose tissue. With extended feed restriction, amino acids can be broken down to provide energy or be converted to glucose. "However, using amino acids as an energy or glucose source is somewhat of a last resort because this process is energetically costly and results in ammonia that needs to be excreted in the urine," Urschel adds.

With exercise, fuels also need to be mobilized so the skeletal muscle can have ATP available for sustained muscle contractions. At the onset of exercise, the hormone epinephrine (adrenaline) increases. Epinephrine acts to mobilize free fatty acids from the adipose tissue, so that skeletal muscle can use them as fuel. Epinephrine also acts to break down skeletal muscle glycogen to free glucose for catabolism and energy (ATP) production. At all levels of work intensity, both carbohydrate (muscle glycogen, blood glucose) and fat are made available as fuels to the muscle. However, at lower levels of work intensity, proportionally more energy is derived from fat oxidation. At higher levels of intensity, more energy is derived from carbohydrate metabolism. It should be noted that protein and amino acids can be used for energy sources during exercise, though their overall contribution to fuel production is minimal.

"Depending on your discipline and level of competition, your horse's diet can be adjusted to maximize the fuel efficiency," says Williams. "For example, if you are competing in endurance races, you will want your horse on a higher fat diet to help increase the fatty acid stores that will be the main source of energy used during training and competition. However, the racehorse, while still using fatty acids to train, also needs higher levels of glycogen during a race over a few furlongs. Therefore, it is not a good idea to feed a racehorse a diet low in sugars and starches."

Metabolic Rate

An individual animal's metabolic rate is based on many factors and ultimately determines how much sustaining dietary energy he requires. Body size is a major determinant of metabolic rate. For instance, a smaller animal has more surface area per unit weight; thus, he has more area for heat loss (which is important as a large portion of an animal's energy is used simply to maintain body temperature). Therefore, a mouse generally will have a higher metabolic rate per unit weight than an elephant. But within the same species, a heavier animal typically will have greater energy requirements than a smaller ¬animal.

There also appear to be breed-dependent differences, as some breeds are notoriously "easier keepers" than others. For example, many gaited breeds and pony breeds are considered easy keepers and have a slower metabolic rate, while other breeds such as Arabians and Thoroughbreds tend to be harder keepers. The National Research Council's Nutrient Requirements of Horses (2007) therefore determines "maintenance" (not growing, lactating, exercising, etc.) dietary energy requirements based on body weight and also if the horse has an "average," "low" (easy keeper), or "high" (hard keeper) metabolism.

In addition to individual differences, ambient temperature plays a role in a horse's metabolic rate. The "thermoneutral zone" is the range of temperatures at which metabolic rate doesn't need to change simply to maintain body temperature. This zone changes throughout the year (which is why 50°F might seem cold in summer, but balmy in winter), differs among individual horses, and varies based on factors such as hair coat thickness. As the ambient temperatures drop below the horse's "lower critical temperature" (the lowest temperature he can tolerate prior to a drop in body temperature), his metabolic rate will increase to maintain body temperature, thus increasing his dietary energy needs.

"The best way to increase the energy needs during colder months is to add extra hay to the diet (see illustrated explanation on page 58)," Williams suggests. "A good guideline is one extra flake of hay for every 10°F below the horse's critical temperature the ambient temperature drops."

Hormonal Control of Metabolism

As noted above, insulin and glucagon are very important in regulating fuel metabolism based on the animal's needs and feeding status. However, other hormones such as cortisol, growth hormone, and epinephrine help regulate energy status and substrate use. The key hormones involved in controlling overall metabolic rate are the thyroid hormones thyroxine and triiodothyronine. These are less influenced by the animal's energy status on a day-to-day (or hour-to-hour) basis, but instead affect general metabolic processes such as heat production, growth, cardiac function, and red blood cell synthesis. These hormones are in turn regulated by thyrotropin-releasing hormone from the hypothalamus (in the brain) and thyroid-stimulating hormone from the anterior pituitary gland (endocrine gland located at the base of the brain).

Take-Home Message

Ultimately, the horse's metabolic processes provide the body with fuels it needs to sustain itself. Feeding, fasting, exercise, and the environment can pose challenges to the body that are met through hormonal actions to help it regain the "status quo." Working with your veterinarian, equine nutritionist, and exercise physiologist can help you and your horse best meet any metabolic challenges he might face in the field or in the show ring.