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Cited by 96Body Composition Measurement TechniquesMuscle metabolism and nutritionNutrition and Health in Aging

Are lean body mass and fat-free mass the same or different body components? A critical perspective

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Heymsfield, Steven B, Brown, Jasmine, Ramirez, Sophia, Prado, Carla M, Tinsley, Grant M, Gonzalez, Maria Cristina·2024·Advances in Nutrition
DOI 10.1016/j.advnut.2024.100335PMC PMC11625996
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Abstract

The 2-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 whereas 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 2-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 nonpolar lipids (mainly triglycerides), includes polar or structural lipids (that is, 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 the use of the chemically correct term FFM with the elimination of the duplicative term LBM that today has value primarily in a historical context. Avoiding the 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.

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  • Some individuals classified as overweight (BMI > 25 and < 30 kg/m2) may not have excess body fat, while others with BMIs within the normal range may exhibit elevated percentages of fat.

    “Some individuals who are classified as overweight (BMI > 25 and < 30 kg/m2) may not have excess body fat, while others with BMIs within the normal range may exhibit elevated percentages of fat [16, 31].”

    From Sarcopenic Obesity Phenotype Index (SOP <i>i</i> ): A Population‐Based Study (2025) · cites this paper as [31]

    The Summary and Perspective discusses methods for estimating body fat, but does not compare body-fat percentages with BMI classifications. Searches for BMI, overweight, and normal-weight obesity found no passage addressing either mismatch described in the claim. (LLM, conf=0.74)

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  • The distinction between fat-free mass and skeletal muscle mass is supported by methodological standards for body composition research and by proposed definitions of body composition components.

    “Distinguishing fat-free mass from skeletal muscle mass is well-supported by methodological standards for body composition research [2] and proposed definitions for various body composition components [3].”

    From Reply to GM Tinsley et al.; DB Ibsen et al.; and EJ Dhurandhar et al. (2025) · cites this paper as [3]

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Research works printed but not linked: 5

  1. [3]Behnke A.R. Feen B.G. Welham W.C. The specific gravity of healthy men: body weight and volume as an index to obesity J. Am. Med. Assoc. 118 1942 495 498 7627779
  2. [9]Agricultural Board; Division of Biology and Agriculture; National Research Council Body Composition in Animals and Man Proceedings of a Symposium Held May 4, 5, and 6, 1967, at the University of Missouri, Columbia 1968 The National Academies Press Washington, DC
  3. [13]Behnke A.R. Role of fat in gross body composition and confíguration Rodahl K. Issekutz B. Fat as a Tissue 1964 McGraw-Hill New York 285 313
  4. [20]Christie W.W. Lipid Analysis 2nd ed. 1982 Pergamon Press Oxford
  5. [26]Akoh C.C. Min D.B. Food Lipids: Chemistry, Nutrition, Biotechnology 2nd ed. 2002 CRC Press Boca Raton 1028

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Authors
Heymsfield, S. B., Brown, J., Ramirez, S., Prado, C. M., Tinsley, G. M., & Gonzalez, M. C.
Affiliations
  • Pennington Biomedical Research Center (US)
  • University of Alberta (CA)
  • Texas Tech University (US)
Corresponding author
Heymsfield, Steven B
ORCID
Venue
Advances in Nutrition
Publisher
Elsevier BV
Published
2024-12
Volume
15
Issue
12
Pages
100335
ISSN
  • 2156-5376 (ISSN-L)
  • 2161-8313
DOI
10.1016/j.advnut.2024.100335
PMC
PMC11625996
OpenAlex
W4404077146
Cited by
96
References
43
Citations by year
2026: 45 · 2025: 50 · 2024: 1
FWCI
5.48
Type
journal article
Language
en
Open access
Licence
Locations
  • Advances in Nutrition (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
Funders
  • Canada Research Chairs
  • National Institutes of Health
  • Nutrition Obesity Research Center, University of North Carolina (#P30DK072476, P30DK040561)
  • National Institute of Diabetes and Digestive and Kidney Diseases (P30DK072476, P30DK040561)
Topics
  • Health Sciences › Medicine › Physiology › Body Composition Measurement Techniques
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Cell Biology › Muscle metabolism and nutrition
  • Health Sciences › Medicine › Physiology › Nutrition and Health in Aging
Keywords
lean body mass, fat-free mass, structural lipids, polar lipids, non-polar lipids, triglycerides, body density, total body water, lipid extraction, lean soft tissue mass, lean muscle mass
MeSH terms
Adipose Tissue, Animals, Body Composition, Body Water, Humans, Lipids

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