Is it lean soft tissue or fat-free mass? I get that question a lot when individuals are talking about body composition. The definitions for total body mass and fat mass are well agreed upon and most individuals have little difficulty with these terms.  On the other hand, the definitions for skeletal muscle mass are a little more complicated. Skeletal muscle mass is sometimes referred to as either fat-free mass or lean soft tissue. These two terms are often used interchangeably to refer to skeletal muscle, which only adds to the confusion surrounding the two terms. It is important to realize the two terms do not mean the same thing, and the figure below demonstrates the difference between them. Lean soft tissue is the sum of body water, total body protein, carbohydrates, non-fat lipids and soft tissue mineral (Prado and Heymsfield 2014). Conversely, fat-free mass includes bone as well as skeletal muscle, organs, and connective tissue (Prado and Heymsfield 2014). The main difference between the two centers on how bone mass is handled.  If bone mass or bone density cannot be measured it has to be calculated with the skeletal muscle mass and you have fat-free mass.  On the other hand, if you can measure bone mass or bone density you can separate it out from skeletal mass and you now have lean soft tissue and bone mass.

Therefore, the use of the term fat-free mass or lean soft tissue is ultimately dependent upon the methodology used to measure skeletal muscle.  Determination of body composition using a 2-component model (e.g., skinfolds, underwater weighing, etc.) (Figure) divides the body into either fat mass or fat-free mass. On the other hand, determining body composition using a 3-component model (e.g., dual x-ray absorptiometry [DXA]) can differentiate between fat mass, bone mass and lean soft tissue mass and divides the body up into these three distinctly different components (Figure). 

The term fat-free mass and lean soft tissue also indicate to some extent the accuracy of the measurement of skeletal muscle mass. Body composition measurement methods using a 2-component model make the following two assumptions: 1) the composition of an individual’s total body mass is either fat-free mass or fat mass and 2) the density of these two components is constant throughout the body. The flaws in these assumptions explain the inaccuracy of the 2-component model since the inclusion of bone mass with lean soft tissue leads to an overestimation of skeletal muscle mass, and the density of these two components can differ in regions of the body. In addition, there is a progressive loss of bone mineral with aging that leads to a decrease in body density over time (Shephard, 1991), making the use of a 2-component model less accurate in populations that may have different body densities than the normal population (i.e., athletes, older adults, etc.). Several methods are available to measure skeletal muscle mass using a 2-component model, ranging from simple, inexpensive field methods (e.g., skinfold thickness, bioelectrical impedance, etc.) to more complicated and expensive laboratory methods (e.g., air displacement, underwater weighing, etc.).

DXA, which measures bone density, is the most common 3-component method of measuring body composition.  By measuring bone density, DXA is able to eliminate the assumptions that 2-component methods make regarding bone density.  This ultimately improves the accuracy of the DXA especially in athletic and older populations whose bone density varies from the bone density in the average population.  Since the bone mass is being measured we are able to subtract it from fat-free mass and are now left with lean soft tissue.

The bottom line is one can be correct in using the term fat-free mass or lean soft tissue, but only in the context of knowing which method was used to measure skeletal muscle.  The use of lean soft tissue would also imply a greater degree of accuracy in the determination of skeletal muscle mass since a 3-component method was used to determine this measure of skeletal muscle mass.

References

Heymsfield, S.B., Gonzalez, M.C., Lu, J., Ja, G., and Zhreng, J. 2015. Skeletal muscle mass and quality: Evolution of modern measurement concepts in the context of sarcopenia.  Proceedings of the Nutritional Society 74, no. 4:355-366

Prado, C.M.M, and Heymsfield, S.B. 2014. Lean tissue imaging: A new era for nutritional assessment and intervention.Journal of Parenteral and Enteral Nutrition 38, no. 8:940-953.

Shephard, R.J. 1991. Body Composition in Biological Anthropology. Cambridge, Great Britain: Cambridge University Press.

About the Author
Donald Dengel, Ph.D., is a Professor in the School of Kinesiology at the University of Minnesota and is a co-founder of Dexalytics. He serves as the Director of the Laboratory of Integrative Human Physiology, which provides clinical vascular, metabolic, exercise and body composition testing for researchers across the University of Minnesota.