Muscle Tissue and Energy Use
Understanding the metabolic role of lean tissue in the body
The Role of Skeletal Muscle
Skeletal muscle is the tissue that enables voluntary movement and makes up approximately 30-40% of body mass in adults. Beyond its primary function of enabling motion, muscle tissue plays crucial roles in energy metabolism, glucose regulation, and overall physiological function.
Muscle and Basal Metabolic Rate
One of the most significant metabolic roles of muscle is its contribution to basal metabolic rate (BMR)—the amount of energy expended by the body at rest. Muscle tissue is metabolically active; it requires energy (ATP) simply to maintain its structure, even when not contracting.
Individuals with higher muscle mass typically have higher basal metabolic rates. This is one reason why resistance training is recommended for overall metabolic health—increased muscle mass supports higher energy expenditure at rest.
Muscle Glucose Uptake and Insulin Sensitivity
Muscle tissue serves as a primary site for glucose uptake, particularly during and after physical activity. When muscles contract, they take up glucose from the bloodstream through both insulin-independent and insulin-dependent mechanisms.
Regular resistance exercise and aerobic activity enhance this glucose uptake capacity, improving glucose metabolism and often improving insulin sensitivity. This metabolic benefit contributes to long-term health advantages associated with regular physical activity.
Building and Maintaining Muscle
Muscle tissue is dynamic—it is constantly broken down and rebuilt. Several factors influence this process:
Resistance Exercise
Resistance training creates mechanical tension and microscopic damage to muscle fibers, triggering adaptive responses that lead to increased muscle protein synthesis. Consistent resistance training over time leads to increased muscle size (hypertrophy) and strength.
Protein Intake
Adequate protein intake provides amino acids necessary for muscle protein synthesis. General recommendations for muscle maintenance are approximately 0.8 grams of protein per kilogram of body weight daily, though requirements may be higher with resistance training.
Age-Related Changes
Muscle mass naturally declines with age—a process called sarcopenia. Starting around age 30, individuals may lose approximately 3-8% of muscle mass per decade, with acceleration after age 60. However, resistance exercise remains effective at maintaining and building muscle throughout the lifespan.
Total Energy Expenditure
While basal metabolic rate is important, it represents only part of total daily energy expenditure. Total expenditure includes:
- Basal metabolic rate - Energy expended at rest (approximately 60-75% of total)
- Thermic effect of food - Energy expended in digestion and processing nutrients (approximately 10%)
- Physical activity - Energy expended during intentional exercise and daily movement (approximately 15-30%)
Individual Variation
Energy expenditure varies considerably between individuals due to differences in muscle mass, genetics, hormonal factors, and environmental temperature. This variation means that "standard" recommendations for energy intake will not be appropriate for all individuals.