Body composition refers to the proportion of different tissues that make up the body’s total mass. Unlike simple body weight, body composition is defined by the ratio of fat mass to fat-free mass (also known as lean mass). Fat-free mass comprises muscle, bone, organs, and water. A healthy body composition features a favorable ratio—more lean mass and less fat mass—which is associated with improved metabolic health and a lower risk of chronic disease. Physical activity reshapes this ratio by increasing lean mass components while decreasing fat storage.
Building and Maintaining Muscle Tissue (Lean Mass)
Exercise, particularly resistance training, improves body composition by stimulating muscle protein synthesis, a process known as hypertrophy. This growth is primarily driven by mechanical tension, which is the force placed on muscle fibers during the lifting of heavy loads. Mechanical tension signals specific cellular pathways, such as the mTOR pathway, which promotes the synthesis of new muscle proteins to adapt to the imposed stress.
Another mechanism contributing to muscle growth is metabolic stress, often experienced as the “pump” during high-repetition sets. This stress involves the accumulation of metabolic byproducts, which trigger anabolic signaling pathways within the muscle cell. The resulting increase in lean muscle mass directly improves the fat-free component of body composition. A greater amount of muscle tissue also elevates the body’s basal metabolic rate (BMR), meaning more calories are utilized at rest, supporting the maintenance of lower fat mass.
Targeting and Reducing Fat Stores (Adipose Tissue)
Physical activity reduces fat mass by creating a sustained energy deficit. When energy expenditure from exercise exceeds caloric intake, the body mobilizes stored fat from adipose tissue to be used as fuel. This process, called lipolysis, breaks down triglycerides within fat cells into free fatty acids and glycerol for release into the bloodstream.
Exercise intensity plays a part in determining the source of the fuel being oxidized. Moderate-intensity exercise is often most effective for maximizing fat oxidation, using lipids as the major energy substrate. Exercise training preferentially reduces visceral fat, which is the metabolically active fat stored around internal organs. This reduction is beneficial because visceral fat is linked to an increased risk of cardiovascular disease and type 2 diabetes.
Subcutaneous fat, stored just beneath the skin, is also reduced through exercise and energy deficit, though this reduction may be slower than for visceral fat. The mobilization of fat stores is a systemic, hormonal response that stimulates lipolysis in the fat cells. The resulting loss of adipose tissue, especially the visceral component, significantly improves overall body composition and long-term health.
Enhancing Skeletal Strength (Bone Density)
The quality and density of the skeletal system are a component of fat-free mass positively influenced by exercise. Bone tissue is dynamic and responds to the mechanical forces placed upon it, a biological principle described by Wolff’s Law. This means bone adapts and remodels to become stronger in response to applied stress.
Weight-bearing and high-impact activities, such as running, jumping, and resistance training, provide the necessary mechanical stress to the skeleton. These forces stimulate osteoblasts, the specialized cells responsible for forming new bone tissue. The activation of osteoblasts leads to increased bone mineral density (BMD), which makes bones more resilient and less prone to fracture.