As horses age, they might start losing muscle mass, a condition known as sarcopenia. This can lead to reduced strength, decreased mobility, and a general decline in health.

Muscle Mass in Aged Horses

A recent survey by the University of Kentucky reported low muscle mass in 17.2% of horses over 15 years of age.

Protein is the largest non-water component of muscle (about 70% of the dry weight of muscle). Protein gives muscle both its structural and functional properties. Therefore, the amount of protein present in the muscle is the major determinant of muscle mass.  

The amount of protein in a muscle cell is determined by the balance between the rates of muscle protein synthesis and degradation. The factors that regulate muscle protein synthesis are well-researched and understood.

For horse owners, understanding how protein synthesis works is key to managing muscle loss and keeping your senior horse strong and healthy.

What is Protein Synthesis?

Protein synthesis is the process of joining amino acids, the ‘building blocks’ of protein, together. There are 21 amino acids in protein. The order and relative proportions of each of these amino acids is encoded by the DNA.

DNA is used to make mRNA, which is the blueprint for protein synthesis. It also dictates the order of amino acids in each protein.

There is cellular machinery necessary for protein synthesis, known as the ribosomes. For protein synthesis to occur, ribosomes need to assemble around the mRNA. There needs to be enough of each of the individual amino acids available in the cell for the process to work.

Where Do Amino Acids Come From?

The horse’s body is capable of making some amino acids from scratch. However, some amino acids, known as the essential amino acids, must come from the dietary protein that a horse consumes.

Some of the essential amino acids include lysine, threonine, methionine, and leucine.

Once consumed, dietary protein is broken down into amino acids in the gastrointestinal tract. Then the amino acids are absorbed into the bloodstream. Once there, they can then be taken up by the individual cells to support the process of protein synthesis.

Key sources of protein in the diet include forage (grass or hay). Legume forages are higher in protein than grass forages. Commercial feeds that include ingredients such as soybean meal also have high protein levels.

How is Protein Synthesis Regulated?

In addition to having enough of each of the individual amino acids, the ribosomes also need to assemble around the mRNA for protein synthesis to occur. This is the major regulatory step of protein synthesis.

Ribosomal assembly is controlled by a series of regulatory proteins known as the mTOR signaling pathway. This pathway can be activated by a variety of anabolic stimuli. These can include amino acids, certain hormones (especially insulin), and exercise.

The mTOR signaling pathway has been studied extensively in humans, rodents, and piglets. It has been studied to a more limited extent in horses.

Why Do Old Horses Lose Muscle?

In other species, there is evidence to support both a decrease in rates of muscle protein synthesis and an increase in muscle protein breakdown. This has not been directly studied in horses.

In the single study in horses in which aged horses were compared to younger horses, there was evidence that some of the components of the mTOR signaling pathway are activated to a lesser extent in the old horses. That supports what has been seen in other species with old age—protein synthesis is less responsive to the anabolic stimuli.

Other factors associated with old age in horses could contribute to these reductions in the activation of mTOR signaling. Those could include impairments in insulin sensitivity, a reduction in exercise due to soundness problems, and nutritional factors. Nutritional factors could include poor dentition affecting the horse’s ability to chew and digest its feed or changes in appetite.

How to Support Protein Synthesis and Muscles in Aged Horses?

There are several management strategies you can take to help support protein synthesis and muscle mass in old horses.

  1. Provide adequate amounts of a high-quality, easily digested protein. In general, the protein originating from complementary feeds tends to be more digestible than forage-based proteins. Therefore, you might consider the addition of a commercial feed formulated specifically for older horses. These “senior feeds” are formulated to include digestible sources of protein. They often have added amounts of the amino acids most likely to be limiting in the diet.
  2. Provide routine dental care.  Ensuring that your horse can effectively and painlessly chew will promote the optimal digestion of all nutrients, including protein.
  3. Allow regular, moderate exercise. Whenever possible, light, consistent exercise should be incorporated into the horse’s management routine. While not all aged horses will be able to tolerate a structured riding program, other ways to increase activity include regular turnout and hand walking. 
  4. Monitor body condition. Researchers at the University of Kentucky recently developed a Muscle Atrophy Scoring System (MASS) that can be used to monitor changes in muscle mass in your aged horses. Good monitoring will allow you to notice changes in muscle mass early so you can consult with members of your horse’s health care team (i.e., veterinarian and nutritionist) to make changes to support the maintenance of muscle mass.

Final Words

Protein synthesis is an important determinant of the amount of protein in the muscle, which subsequently determines muscle mass. Therefore, to manage or mitigate muscle loss in older horse, it is important to manage the factors that regulate muscle protein synthesis. This specifically includes diet and exercise.

Owners should provide high-quality, easily digestible dietary protein in the senior horse diet. Combine that with appropriate amounts of exercise that can be tolerated by your aged horse. In this way, you can help your senior horse maintain muscle mass, strength, and overall health.

Further Reading