The body can burn muscle for energy, a process known as muscle catabolism. Muscle tissue is a major component of lean body mass and is highly metabolically active. When the body’s primary fuel sources are depleted or unavailable, it begins to break down this tissue to meet its energy demands.
The Body’s Fuel Hierarchy and Muscle Breakdown
The human body operates on a strict fuel hierarchy, prioritizing sources that are easiest to convert into Adenosine Triphosphate (ATP), the cellular energy currency. Carbohydrates, stored as glycogen in the liver and muscles, are the first and most readily available fuel source. Once glycogen stores are significantly reduced, the body shifts its focus to fat reserves, which offer a highly concentrated energy source. Fat becomes the predominant fuel during rest and lower-intensity, longer-duration activities.
Muscle breakdown occurs when the body requires glucose but cannot generate enough from carbohydrate or fat stores. This process is called gluconeogenesis, meaning “the creation of new glucose.” The liver, and to a lesser extent the kidneys, performs this metabolic task using non-carbohydrate sources. Muscle protein is broken down into amino acids, which are then transported to the liver.
These amino acids, particularly alanine and glutamine, are converted into glucose in the liver. This glucose supplies energy to tissues that rely exclusively on it, such as the brain and red blood cells. Hormones, especially cortisol, signal this catabolic state. Cortisol, a stress hormone, promotes the breakdown of muscle protein to mobilize amino acids for emergency glucose production. When sustained, this hormonal signal tips the balance from muscle building (anabolism) to muscle wasting (catabolism).
Lifestyle Conditions That Accelerate Muscle Loss
One of the most common triggers for muscle catabolism is extreme caloric restriction, often seen in crash dieting or prolonged fasting. When a calorie deficit is too large, the body cannot spare the time to slowly mobilize fat reserves, and it breaks down muscle to quickly release amino acids for gluconeogenesis. Studies suggest that when weight loss is too rapid, between 20% and 40% of the total weight lost can come from lean muscle mass. The lack of sufficient protein intake during these deficits further compounds the problem, as the body lacks the raw materials to rebuild and repair muscle tissue.
Chronic, high-volume endurance exercise, particularly without sufficient rest or fuel, can also push the body into a catabolic state known as overtraining syndrome. When an athlete consistently trains intensely without adequate recovery, the stress hormone cortisol remains elevated, which drives continuous protein breakdown. In extreme cases of overexertion, a condition called exertional rhabdomyolysis can occur, where muscle tissue breaks down rapidly, releasing proteins that can damage the kidneys.
Prolonged inactivity, or disuse atrophy, leads to muscle loss due to a lack of mechanical stimulus. This is frequently seen in cases of limb immobilization, extended bed rest, or a highly sedentary lifestyle. When muscle fibers are not subjected to mechanical load, the balance between muscle protein synthesis and breakdown shifts toward breakdown. This catabolic process is amplified by the persistent inactivation of signaling pathways responsible for muscle growth and maintenance.
Practical Steps to Preserve Muscle Mass
Preserving muscle mass during periods of weight loss or increased activity requires a strategic approach focused on nutrition and exercise stimulus. The most effective way to protect muscle is by avoiding aggressive, rapid weight loss goals. Health experts recommend aiming for a weight loss rate no greater than 0.5% to 1% of total body weight per week. This slower pace allows the body to prioritize fat loss over muscle tissue.
Strategic protein intake is also necessary to provide the body with the amino acids it needs without resorting to muscle catabolism. For active individuals in a calorie deficit, protein intake should be significantly higher than the standard recommendation, typically ranging from 1.6 to 2.2 grams per kilogram of body weight daily. Spreading protein evenly throughout the day, aiming for about 20 to 30 grams per meal, helps maximize muscle protein synthesis.
Resistance training is a powerful stimulus for muscle preservation because it signals to the body that the muscle tissue is still needed. Incorporating weightlifting or bodyweight exercises two to three times a week is effective for maintaining muscle mass, even in a calorie deficit. This training stimulus helps maintain the anabolic state and counteracts the catabolic signals triggered by reduced calories or stress.