Reference
Lean Body Mass
**Lean body mass** (LBM) is total body weight minus fat mass — the combined weight of skeletal muscle, bone, organs, connective tissue, and body water. It is the metabolically active portion of body composition, and skeletal muscle is its largest and most modifiable component.
What lean body mass includes
Lean body mass is everything in the body that is not stored fat. It is sometimes used interchangeably with fat-free mass, though strict definitions place the small amount of essential lipid inside cell membranes and the nervous system within LBM and outside fat-free mass. For practical purposes — body-composition tracking, dosing protein, judging the results of a diet — the two terms describe the same thing: weight that is not adipose tissue.
Skeletal muscle accounts for the share of lean mass most responsive to diet and training. Bone and organ mass change slowly; muscle can be gained or lost over weeks. This is why “preserving lean body mass” almost always means preserving muscle.
How protein and training preserve it
Lean body mass is maintained by the daily balance between muscle protein synthesis and muscle protein breakdown. Dietary protein and resistance exercise both shift that balance toward retention. In a blinded randomized trial, whey protein combined with resistance training increased fat-free mass with no significant change in body fat (PMID:31565912), and plant proteins can produce a comparable result: an 84-day trial reported gains in muscle mass of 2.3% with pea protein versus 2.4% with whey, with no significant difference between groups (Nutrients, 2024).
The stakes rise in an energy deficit. When calories are restricted for fat loss, the body can draw on muscle as well as fat unless protein intake and a training stimulus signal that muscle should be kept. Adequate protein during a deficit is the difference between losing weight and losing the lean tissue that determines strength and resting metabolic rate. Athletes managing body composition can read the broader case for higher intakes in this guide to protein for athletes.
Lean body mass and aging
Lean mass declines with age, a process termed sarcopenia, and protein intake strongly modifies the rate. In a cohort of adults aged 70–79, those eating roughly 91 grams of protein daily lost 40% less lean mass over three years than those eating less (American Journal of Clinical Nutrition, 2008, PMID:18175749). The ESPEN Expert Group recommends 1.0–1.2 g of protein per kilogram of body weight per day for older adults to maintain muscle mass and function (Clinical Nutrition, 2014, PMID:24814383) — above the standard RDA of 0.8 g/kg.
Related terms