Fat-free mass

Fat-free mass (FFM) is body mass minus fat mass.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 188.} It includes muscle, bone, water, connective tissue, organs and teeth.^{2International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 155.} Bone and muscle are denser than water while fat is less dense, so of two people with the same body weight, the one with more FFM has a higher body density and a lower body fat percentage.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 188.}

Muscle is largely water, and in a two-compartment model all body water counts as fat-free mass.^{3Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 64.} A change in FFM therefore includes changes in water as well as tissue.^{3Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 64.}

In practice

• Gaining lean tissue raises resting metabolic rate, and losing it lowers it.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 121.}
• In a calorie surplus with resistance training, untrained clients gain mostly FFM; well-trained lifters put more of a surplus into fat.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 221.}
• Substantial weight loss through calorie restriction leads to marked losses of lean mass.^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 131.}
• Read FFM changes during a diet with care: methods that assume a fixed water content can report water lost from fat tissue as lost FFM.^{7Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 67.}