Lean mass

The working group says fat-free mass includes every component at the molecular level except fat, which is mainly triglycerides [1]. It counts lean mass as equivalent to fat-free mass but not to lean soft tissue, because fat-free mass includes bone mineral content [1]. The group discourages the phrase "lean body mass", because it is used for several different components [1]. Skeletal muscle is classified at a different level, the tissue and organ level, and should not be confused with fat-free mass [1].

One method trials use to measure these components is dual-energy X-ray absorptiometry (DXA) [2]. A review of the method reports that fat mass and lean mass values from DXA show very good accuracy compared with computed tomography and magnetic resonance imaging [2]. Creatinine offers another route: because most creatinine is made in muscle, a 2025 review calls it a valuable marker for assessing what it terms lean body mass, and describes its use in monitoring nutritional status in patients with chronic kidney disease [3].

Creatine research reports this outcome under more than one name. A 2026 three-level meta-analysis of 63 studies of creatine with resistance training listed lean mass among its outcomes [4], while a 2014 meta-analysis of the same pairing in older adults reported fat-free mass [5]. Under the working group's definitions those two labels name the same component [1].

A change in lean mass is not the same as a change in muscle, because lean mass covers everything that is not fat [1]. Check which measure and which method a trial used before comparing it with another.

• Creatinine: a muscle waste product that a review describes as a marker of "lean body mass" [3].
• Triglycerides: the main constituent of the fat that lean mass excludes [1].
• Creatine: the compound page grades the lean-mass evidence.