I frequently see misconceptions about body composition, both in the peer-reviewed literature and on good ol' social media. As such, I thought it may be helpful to share some basic information about some of the major body composition principles.⁣ [1/14]

Five levels of body composition have been proposed. These include the atomic, molecular, cellular, organ/tissue, and whole-body levels.⁣ [2/14]

*Nearly all common methods* use the molecular level.These include bioelectrical tests (BIA, BIS, etc.), density-based tests (Bod Pod, hydrostatic weighing, skinfolds, etc.), anthropometry-based tests (although they can't really "see" your molecules...), DXA, [3/14]

and more advanced multi-compartment (also called multi-component) models.⁣ As the name implies, the molecular level deals with the molecules of the body. [4/14]

So, fat mass at the molecular level means all the *molecules* of fat, wherever they may be. This means that "fat mass" is *not* the same as "adipose tissue," for example (more on that another day).⁣ [5/14]

This posts's image is a visualization of molecular-level body composition models ranging from one to six compartments. These data come from 170 individuals from my lab.⁣ [6/14]

The methods that most people encounter are 2-compartment (2C) methods that simply split body mass into fat mass (FM) and fat-free mass (FFM).⁣ [7/14]

As the name implies, FFM is all non-fat molecules. So, this can be broken down further by "pulling out" different groups of molecules from FFM. For example, FFM can be separated into water (W in the graph above) and "everything else," yielding a 3-compartment (3C) model.⁣[8/14]

This "everything else" is called residual (R) mass. For the 3C model depicted above, R primarily consists of protein, bone mineral, soft tissue mineral, and glycogen. These individual components can be sequentially "pulled out" of the residual mass, [9/14]

resulting in a greater number of compartments. ⁣For example, bone mineral could be assigned its own compartment, yielding a 4-compartment (4C) model. Then, soft tissue mineral could be pulled out, producing a 5-component (5C) model.⁣ [10/14]

Finally, we could get to the point where each major molecular constituent that we typically care about has its own compartment. This 6-compartment (6C) model splits body mass into: water, fat, protein, bone mineral, soft tissue mineral, and glycogen.⁣ [11/14]

While the 6C model is neat conceptually, quite a bit of data have demonstrated that adding compartments beyond the 4C model (and in many cases, beyond the 3C model) is largely unnecessary in terms of improved accuracy.⁣ [12/14]

So, the big-picture summary for now: 1. Molecular-level body composition models attempt to separate all the body's molecules into different compartments. 2. A limitation of most common methods is the fact that all non-fat molecules are lumped together as fat-free mass. [13/14]

3. More advanced models, constructed using multiple lab techniques, can split the body into a greater number of compartments, thereby reducing the number of assumptions necessary to estimate body composition. [14/14]