Lean Body Mass
Definition
Total body weight minus fat mass—includes muscle, bone, organs, water, and other non-fat tissue.
Calculated as: Lean Body Mass = Total Weight × (1 - Body Fat %)
Why It Matters
Lean mass is crucial for health and function: - Metabolic rate - More muscle = higher resting metabolism - Functional capacity - Strength, mobility, independence - Sarcopenia risk - Low lean mass in elderly linked to frailty - Athletic performance - Muscle mass affects power and endurance - Mortality association - Low lean mass associated with worse outcomes
How It's Measured
Derived from body composition measurements: - DEXA scan - Gold standard for body composition - Smart scales (BIA) - Calculate from impedance - Calculation - From weight and body fat percentage
Reference Considerations
Lean mass varies greatly by: - Body size (taller people have more) - Sex (men typically have more) - Age (decreases with aging) - Activity level (training increases muscle)
More useful metrics: - Lean mass index (lean mass/height²) - Change over time - Muscle mass relative to function
Confounders & Limitations
- Hydration - Water is counted in lean mass; affects readings
- Measurement accuracy - Consumer devices less precise
- Individual variation - "Normal" ranges are broad
- Edema - Fluid retention inflates lean mass
Use Cases
- Fitness tracking - Building or maintaining muscle
- Weight management - Ensuring fat loss, not muscle loss
- Aging - Monitoring for sarcopenia
- Rehabilitation - Recovery from illness/injury
Sarcopenia (Age-Related Muscle Loss)
- Begins around age 30
- Accelerates after 60
- Preventable/reversible with resistance training
- Associated with falls, fractures, disability
Practical Advice
- Prioritize strength training to maintain lean mass
- Adequate protein intake supports muscle
- Track trends over months
- Resistance exercise at any age is beneficial