1. Body Composition

Body composition is the proportional distribution of the human body's physical constituents — primarily fat mass and fat-free mass — and the quantitative analysis of those constituents to assess health status, monitor physiological change, and guide clinical and athletic interventions. Unlike body weight alone, which provides no information about what the body is made of, body composition analysis distinguishes between the metabolically and functionally distinct components of tissue, providing a more precise and meaningful measure of physical health.

Compartment Models

Body composition is conceptualized and measured using compartment models, which divide total body mass into a defined number of chemically or physiologically distinct components. The choice of model determines the precision of the analysis and the method required to measure it.

The two-compartment model divides the body into fat mass and fat-free mass. It is the most widely used in clinical and field settings due to its simplicity. The three-compartment model further separates fat-free mass into bone mineral content and lean soft tissue, increasing accuracy by accounting for individual variation in bone density. The four-compartment model additionally separates body water from residual protein, providing the highest accuracy and serving as the reference standard against which other methods are validated.

Fat Mass and Fat-Free Mass

Fat mass (FM) encompasses all lipid-containing tissue in the body. It is subdivided into essential fat — the minimum quantity necessary for normal physiological function, including organ protection, nerve myelin sheaths, reproductive hormones, and fat-soluble vitamin storage (approximately 3% of body mass in men, 10–13% in women) — and storage fat, which accumulates in adipose tissue as an energy reserve and thermal insulator.

Fat-free mass (FFM), also called lean body mass, comprises everything that is not fat: skeletal muscle, bone mineral, organ tissue, connective tissue, and total body water. It is the metabolically active component of the body, determining basal metabolic rate, physical strength, immune competence, and the capacity for recovery from illness.

Fat Distribution: Subcutaneous and Visceral Fat

The anatomical location of fat storage is as clinically important as its total quantity. Fat is stored in two principal depots with profoundly different metabolic properties.

Subcutaneous fat is deposited beneath the skin and constitutes the majority of total body fat in most individuals. While excess subcutaneous fat contributes to obesity-related risk, it is metabolically less active and less directly associated with cardiometabolic disease than visceral fat.

Visceral fat is deposited within the abdominal cavity, surrounding and infiltrating the liver, pancreas, and intestines. It is highly metabolically active, releasing elevated levels of free fatty acids directly into the portal circulation and secreting pro-inflammatory adipokines — including tumor necrosis factor-alpha and interleukin-6 — that promote insulin resistance, hepatic steatosis, dyslipidemia, and systemic inflammation. Visceral adiposity is an independent risk factor for type 2 diabetes, cardiovascular disease, non-alcoholic fatty liver disease, and certain cancers, even at normal total body weight.

Skeletal Muscle Mass

Skeletal muscle is the largest single component of fat-free mass, comprising approximately 40% of total body weight in healthy adults. Beyond its role in movement and posture, skeletal muscle is the primary site of insulin-stimulated glucose uptake, making it central to glycemic regulation. It is an endocrine organ, secreting myokines that exert systemic effects on metabolism, inflammation, bone health, and cognitive function.

Sarcopenia is the progressive, generalized loss of skeletal muscle mass, strength, and physical performance associated with aging. It begins in the fourth decade of life and accelerates after 60, contributing to frailty, falls, metabolic dysfunction, increased surgical risk, and reduced independence. Sarcopenia is assessed by combining measures of muscle mass (typically DXA-derived appendicular lean mass indexed to height squared) with measures of muscle strength (handgrip dynamometry) and physical performance (gait speed or five-times sit-to-stand test).

Methods of Body Composition Assessment

Multiple methods exist for quantifying body composition, each with distinct principles, accuracy levels, cost, accessibility, and suitability for different clinical and research contexts.

Dual-energy X-ray absorptiometry (DXA) is the clinical gold standard for three-compartment body composition assessment. It directs two X-ray beams of different energies through the body and measures differential attenuation to distinguish and quantify bone mineral content, lean soft tissue, and fat mass in regional and whole-body scans. DXA also provides regional analysis — separating trunk, arm, and leg compartments — enabling quantification of android (central) versus gynoid (peripheral) fat distribution.

Hydrostatic weighing applies Archimedes' principle: body volume is calculated from the difference between weight in air and weight submerged in water, then body density is computed and fat mass estimated using population-specific density equations. It is accurate but requires specialized equipment and patient cooperation.

Bioelectrical impedance analysis (BIA) passes a low-level alternating electrical current through the body and measures the opposition to current flow. Fat tissue, which contains little water, has high impedance; lean tissue, which is highly hydrated and electrolyte-rich, has low impedance. BIA estimates total body water, which is then used to calculate fat-free mass and fat mass. Its accuracy is sensitive to hydration status, food intake, skin temperature, and the quality of the prediction equations used.

Skinfold anthropometry measures subcutaneous fat thickness at standardized sites — triceps, biceps, subscapular, suprailiac, thigh — using calibrated calipers. Multiple site measurements are entered into validated prediction equations to estimate body density and percentage body fat. Accuracy depends heavily on the skill of the measurer and the appropriate selection of population-specific equations.

Anthropometric Indices

Several simple anthropometric measurements serve as surrogates for body composition assessment in population health and clinical screening contexts.

Body Mass Index (BMI) is the ratio of weight in kilograms to height in meters squared (kg/m²). It is the most widely used population-level screening tool for obesity and underweight, classified as underweight (<18.5), normal (18.5–24.9), overweight (25–29.9), and obese (≥30). BMI does not distinguish fat from muscle, does not capture fat distribution, and has lower predictive validity in athletes, the elderly, and populations with different body proportions.

Waist circumference and waist-to-hip ratio (WHR) are superior to BMI in predicting visceral adiposity and associated cardiometabolic risk. Waist circumference thresholds associated with substantially increased risk are ≥94 cm in men and ≥80 cm in women (WHO criteria). Waist-to-height ratio (waist circumference divided by height) is increasingly recognized as a simple, population-independent predictor of cardiometabolic risk, with a threshold of 0.5 widely proposed as a universal health boundary.

Body Composition Across the Lifespan

Body composition undergoes systematic changes throughout life that have significant health implications.

In childhood and adolescence, lean mass — particularly skeletal muscle — and bone mineral density accumulate rapidly, driven by growth hormone, sex steroids, and physical activity. Peak bone mass is attained in the late twenties and represents a critical determinant of osteoporosis risk in later life.

In adulthood, lean mass is relatively stable with adequate nutrition and activity. After the age of 30, muscle mass declines at approximately 1–2% per year without resistance training, while fat mass — particularly visceral fat — tends to increase even at stable body weight as fat is redistributed from subcutaneous to central depots.

In older age, the accelerated loss of muscle and bone mass (sarcopenia and osteoporosis), combined with persistent or increasing fat mass — a phenotype termed sarcopenic obesity — markedly increases risk of disability, falls, fractures, metabolic disease, and mortality.

Clinical Applications

Body composition assessment serves distinct purposes across clinical and performance contexts. In oncology, the identification of cancer cachexia — characterized by loss of skeletal muscle with or without fat loss — guides nutritional support, predicts treatment toxicity, and is an independent prognosticator of survival. In bariatric medicine, tracking fat mass and lean mass separately during weight loss intervention distinguishes favorable (fat) from unfavorable (muscle) tissue loss. In sports science, body composition monitoring enables the optimization of power-to-weight ratio, hydration, and nutritional periodization. In critical care, depletion of lean mass is a predictor of prolonged mechanical ventilation, infection susceptibility, and delayed rehabilitation. Across all contexts, body composition provides information that body weight alone cannot supply.