Methodological standards for body composition-an expert-endorsed guide for research and clinical applications: levels, models, and terminology

Abstract

Body composition assessment is widely used in both research and clinical practice, yet confusion over basic concepts and terminology persists, leading to inaccurate assessments, comparisons, and interpretations. To address this concern, an international working group was formed to clarify basic concepts, standardize terminology, and provide guidance on the use and interpretation of body composition assessment. This initial publication addresses methodological standards, focusing on summarizing body composition levels and models, and introducing standardized terms and definitions. Body composition is organized into 5 distinct levels, ranging from atomic to whole-body, with each higher level encompassing the components of the preceding less complex levels. As a result, terms that describe components at different levels should not be used interchangeably. For example, the use of the molecular-level term "lean body mass" is discouraged because it inaccurately refers to fat-free mass (FFM), lean mass, or lean soft tissue (LST). FFM includes all compartments at the molecular level except fat (nonpolar lipids; mainly triglycerides), and FFM also contains nonfat (or polar) lipids. The term "lean mass" is equivalent to FFM, but not to LST, as FFM includes bone mineral content. Additionally, skeletal muscle is classified at the tissue-organ level and should not be confused with the molecular-level components FFM and LST. Likewise, fat mass and adipose tissue are different components: fat mass, mainly triglycerides, is assessed at the molecular level, whereas adipose tissue is measured at the tissue-organ level. Models are also specific to each level. It is crucial for researchers and clinicians to have a clear understanding of what each body component entails and to use accurate terminology to ensure precise assessment, reporting, and interpretation of body composition data.

Keywords: adipose tissue fat-free mass; body composition; definitions; fat mass; lean mass; lean soft tissue; skeletal muscle; terminology.

FIGURE 1

Selected functions of skeletal muscle, adipose tissue, and bone. Images retrieved from smart.servier.com.

FIGURE 2

The 5-level body composition organizational framework [3]. Note that fats mainly include nonpolar, or “neutral,” lipids in the form of triglycerides. Nonfat lipids, also termed polar lipids, include phospholipids and glycolipids primarily found in cell membranes [3].

FIGURE 3

(A) The main components of body composition at the molecular level and how they relate to adipose tissue at the tissue-organ level. Note that protein, mineral, and water are molecular components of adipose tissue, but they are neither fat nor nonfat lipids. Despite being synonymous of fat-free mass, the term “lean mass” should only be used to refer to nonfat or nonadipose tissue components in general, not to a singular body component. (B) Selected subdivisions of adipose tissue [9]. For specific descriptions/definitions and additional adipose tissue classification, refer to the text and Shen et al. [9]. ∗ Nonpolar lipids; ^† polar lipids. ^‡In the lower trunk and gluteal-thigh region, subcutaneous adipose tissue is divided into superficial and deep layers, separated by a fascial plane and differing in morphology and metabolism. AT, adipose tissue; ECW, extracellular water; ICW, intracellular water; SAT, subcutaneous adipose tissue; VAT, visceral adipose tissue.

FIGURE 4

Components of the most used models in body composition assessment. FFM, fat-free mass; FM, fat mass; LM, lean mass; TBW, total body water.

FIGURE 5

Body components assessed or estimated using common methods at the molecular, cellular, tissue-organ, and whole-body levels. Note that the figure highlights only the most conventionally assessed or estimated body component for each method and level. For example, although computerized tomography is typically used to assess skeletal muscle and adipose tissues, it can also be used to evaluate bone. Furthermore, dual-energy X-ray absorptiometry does not measure individual molecules or molecular groups but does measure a component of fat, which is triglycerides or nonpolar lipids. Images retrieved from smart.servier.com. 3DO, 3D optical; ADP, air displacement plethysmography; ALST, appendicular lean soft tissue; AT, adipose tissue; BIA, bioelectrical impedance analysis; BIS, bioimpedance spectroscopy; BMC, bone mineral content; CT, computerized tomography; D₃-Cr, D₃-creatine dilution; DXA, dual-energy X-ray absorptiometry; ECW, extracellular water; FFM, fat-free mass; FM, fat mass; ICW, intracellular water; LST, lean soft tissue; MRI, magnetic resonance imaging; TBW, total body water; US, ultrasound.

FIGURE 6

Precise terminology is recommended when describing fat-free mass (FFM) and lean soft tissue (LST), as they represent distinct body components. The term “lean mass” (LM) is synonymous with FFM but not with LST. “Lean mass” may be used as a general reference to nonfat or nonadipose tissue components but not to an actual, singular body component. The term “lean body mass” (LBM) should not be used due to its lack of accuracy [18]. Thus, FFM = LM = LST + bone mineral content (BMC). BMC, bone mineral content; FM, fat mass; FFM, fat-free mass; LBM, lean body mass; LM, lean mass; LST, lean soft tissue.

FIGURE 7

The term “lean mas” is inconsistently defined across dual-energy X-ray (DXA) manufacturers. In the report outputs of 2 manufacturers, “lean” and “lean mass” refer specifically to “lean soft tissue” (A, B), whereas “lean + BMC” represents fat-free mass (B). BMC, bone mineral content.