Are Lean Body Mass and Fat-free Mass the Same or Different Body Components? A Critical Perspective

Steven B Heymsfield; Jasmine Brown; Sophia Ramirez; Carla M Prado; Grant M Tinsley; Maria Cristina Gonzalez

doi:10.1016/j.advnut.2024.100335

Are Lean Body Mass and Fat-free Mass the Same or Different Body Components? A Critical Perspective

Adv Nutr. 2024 Nov 5:100335. doi: 10.1016/j.advnut.2024.100335. Online ahead of print.

Authors

Steven B Heymsfield¹, Jasmine Brown², Sophia Ramirez², Carla M Prado³, Grant M Tinsley⁴, Maria Cristina Gonzalez⁵

Affiliations

¹ Pennington Biomedical Research Center, Louisiana State University System, Baton Rouge, LA, USA. Electronic address: Steven.Heymsfield@pbrc.edu.
² Pennington Biomedical Research Center, Louisiana State University System, Baton Rouge, LA, USA.
³ Department of Agricultural, Food & Nutritional Science, Human Nutrition Research Unit, University of Alberta, Edmonton, Alberta, Canada.
⁴ Department of Kinesiology and Sport Management, Texas Tech University, Lubbock, TX, USA.
⁵ Federal University of Pelotas, Post-graduate Program in Nutrition and Food, Pelotas, Brazil.

PMID: 39510253
DOI: 10.1016/j.advnut.2024.100335

Abstract

The two-component molecular level model dividing body mass into fat and fat-free mass (FFM) is a cornerstone of contemporary body composition research across multiple disciplines. Confusion prevails, however, as the term lean body mass (LBM) is frequently used interchangeably with FFM in scientific discourse. Are LBM and FFM the same or different body components? Captain Albert R. Behnke originated the LBM concept in 1942 and he argued that his "physiological" LBM component included "essential" fat or structural lipids while FFM is a chemical entity "free" of fat. Classical experimental animal and human studies conducted during Behnke's era laid the foundation for the widely used body density and total body water two-component molecular level body composition models. Refined body composition models, organization of lipids into structural and functional groupings, and lipid extraction methods all have advanced since Behnke's era. Our review provides an in-depth analysis of these developments with the aim of clarifying distinctions between the chemical composition of LBM and FFM. Our retrospective analysis reveals that FFM, derived experimentally as the difference between body weight and extracted neutral or non-polar lipids (mainly triglycerides), includes polar or structural lipids (i.e., Behnke's "essential" fat). Accordingly, LBM as originally proposed by Behnke has the same chemical composition as FFM, thus answering a longstanding ambiguity in the body composition literature. Bringing body composition science into the modern era mandates use of the chemically correct term FFM with elimination of the duplicative term LBM that today has value primarily in a historical context. Avoiding use of the term LBM additionally limits confusion surrounding similar widely used body composition terms such as lean mass, lean soft tissue mass, and lean muscle mass.

Keywords: Adiposity; Body Composition; Fat-free Mass; Lean Body Mass; Malnutrition; Muscle Mass; Obesity.

Publication types

Review