I speak from personal experience when I say that most people think of muscle only in regards to physical activity, regardless of age or health. I remember telling my 95-year-old mother several years ago about a paper I was writing titled “The Underappreciated Role of Muscle in Health and Disease” in which I reviewed the central role that muscle plays in the regulation of metabolism in the body, the support of bone health, greater survival in the case of heart failure and cancer, and even psychological benefit. I explained my work with essential amino acids and how they can help in her daily life by strengthening muscle and improving its functions. She listened very politely, but ultimately said, “What do I need muscle for? I’m just playing bridge all day. I don’t need to be strong for that.”
My experience with many, many more people since then is that most only think of physical performance when thinking about the importance of muscle. How about we take some time to recognize muscle for more than just brute strength?
The Many Roles of Muscle
Muscle plays a central role in amino acid and protein metabolism in the body. Crucial tissues and organs (e.g., heart, brain, skin) need a constant supply of amino acids to produce enough new protein to maintain organ and tissue functionality. However, there are normally many hours throughout the day when we are not eating protein/amino acids. During those times between meals muscle serves as the reservoir of amino acids for the rest of the body to maintain normal concentrations of blood amino acids.
Here’s how it works: the breakdown of muscle protein between meals releases amino acids into the blood that can be taken up by other tissues and organs. Muscle protein is then repleted when dietary protein and/or amino acids are consumed and absorbed into the body. The role of muscle as a reservoir to maintain normal blood amino acid levels in the absence of dietary intake is necessary for survival.
Muscle plays a variety of other metabolic roles in addition to maintaining normal amino acid levels, including regulating blood glucose concentration.
Muscle Helps Regulate Blood Sugar Levels
Type 2 diabetes occurs when muscle function is not optimal.
Under normal conditions, muscle plays a key role in regulating the blood concentration of glucose, as well as that of amino acids. Glucose is commonly called blood sugar. When all is functioning properly, the brain relies entirely on glucose from the blood for energy. A drop in blood glucose concentration can cause loss of consciousness and even death. In contrast, an increase in glucose concentration in the blood is responsible for many of the adverse effects of diabetes.
To understand how healthy muscle can help keep us diabetes free, we must first examine how glucose is processed in the body. All dietary carbohydrates are ultimately converted to glucose in order to be metabolized in the body. After you eat carbohydrates, your blood glucose level increases. The magnitude of this increase is moderated by the release of the hormone insulin. Insulin sets in motion the uptake of glucose by various tissues, most prominently muscle. Once in the muscle cell, glucose may either be converted to a chemical form of energy or stored as glycogen for later use during exercise when energy requirements are elevated in order to fuel the contraction of muscle.
Now, that’s all assuming your body is responding to insulin properly. If your body’s cells become resistant to the effects of insulin, then your pancreas pumps out more insulin to try to compensate. Eventually the pancreas can no longer produce enough insulin to satisfy the body’s needs, and blood sugar increases. This condition is termed insulin resistance, and it can lead to type 2 diabetes and heart disease if not managed. When you have diabetes, insulin no longer stimulates the clearance of glucose from the blood.
Muscle not only helps blunt the magnitude of increases in blood glucose after meals, but also helps prevent decreases in blood glucose levels between meals that could impair brain function. You see, some of the amino acids released from muscle in the post-absorptive state (between meals) become precursors for the production of glucose in the absence of dietary carbohydrate. Thus, healthy muscle helps regulate the concentration of glucose in the blood. This is imperative for the prevention of diabetes, as well as health problems caused by hypoglycemia (low blood glucose levels).
Muscle Supports Bone Health
Then there is muscle’s influence on bone to consider. When you contract your muscles, such as squeezing your muscle on a bicep curl, you exert mechanical force on bone, which is essential for bone strength and mass. It is difficult to distinguish the role of muscle on bone from other factors, since the amount of dietary protein, insulin growth factor, and testosterone that affect bone also directly affect muscle. We do know that weight-bearing exercises serve to increase not only muscle strength but also bone strength, and even obesity or a high body weight strengthens bone by providing a direct mechanical force. Prevention of bone loss due to aging (osteoporosis) is highly dependent on the maintenance of adequate muscle mass and function.
Muscle Increases Your Likelihood of Survival
Muscle mass is also associated with improved health outcomes in a number of serious conditions. Cancer is the most well-documented clinical state in which survival is directly linked to the maintenance of muscle mass. Cancer is associated with a rapid loss of muscle mass and strength at a rate faster than would normally occur because of decreased protein intake. This is a classic example of the catabolic state. Survival from a variety of cancers is directly related to how well muscle mass is maintained. How muscle exerts this effect is not certain, but one aspect seems to be muscle’s ability to withstand the rigors of chemotherapy and radiation therapy.
The all-cause morbidity and mortality due to adverse cardiovascular events (heart attacks, stroke) are also worse in individuals with depleted muscle mass. Interestingly, the loss of muscle strength is even more strongly related to mortality than the amount of muscle mass. Survival from other serious diseases, such as chronic obstructive lung disease and heart failure, is also better in individuals with greater muscle mass.