Introduction

Weight loss, fat loss, who cares? They are essentially the same thing…right? While it might seem pedantic at first, the difference between the two terms is quite important, despite the fact that most people (myself included) tend to use the terms interchangeably—we most likely mean fat loss. If this does not make immediate sense, fear not! My plan today is to illuminate why the idea of weight loss, by itself, is not the entire story, and that fat loss is arguably the more important metric by which we should judge a “weight loss” diet. In doing so, I also aim to show you what the body is comprised of (i.e. body composition); a handful of different ways we can measure it; and, somewhat counterintuitively, why weight by itself actually is an important metric and how you can use it intelligently to achieve your dieting goals. What will become evident, as will be the case in most if not all of my posts, is that things are a little more complicated than they appear.    

First: a simple example

To begin to illustrate a point in the simplest way possible, envision the following scenario: You wake up in the morning, minimal clothing, stumble to the bathroom and weigh yourself on your scale. Let’s just say you weigh 150 lbs. (A weight chosen to equally offend everyone.) You then proceed to “decaffeinate” (as my former PhD advisor would say), as well as take part in the process of egestion. (You guessed it, the exact opposite of ingestion.) You then weigh yourself again: 147.5 lbs. Within minutes no less! Now, obviously you didn’t lose 2.5 lbs. of body fat. If only it were that easy…  

What we just illustrated in that ever-so-relatable example is the concept of body weight and the beginnings of the fact that your body’s weight is made up of more things than just fat. Which brings us to the concept of body composition.   

Body composition: what you are (quite literally) made of 

The absolute “Gold Standard” for evaluating body composition is, well…dissection. Obviously, this is not practical for evaluating the composition of the body in living people, but historically, these types of analyses have been done, and the body compartments have been categorized and inventoried.   

From the most superficial level of analysis, we can categorize your body into two compartments: fat and then everything else. This is literally called a two-compartment model, and many of the techniques used to estimate body composition in living humans are based on this fundamental concept [1].  

Thus, you are made up of fat mass (FM) and everything that is, by definition, not. Everything that is not fat is lumped together and termed fat-free mass (FFM). (How clever.) I should note that you will often hear people use the term lean body mass (LBM) interchangeably with FFM. For the purposes of this piece, I will consider the two equivalent, although, technically, they are not. (We have our pedantic plates full enough as it is!)

To place the two-compartment model in mathematical terms, your total body weight (TBW) is as follows:

TBW = FM + FFM

Simple enough. Well, sort of. In reality, the FFM portion can be delineated even further into everything that it is comprised of—i.e. body protein that makes up skeletal muscle and your organs; minerals that make up your bone; glycogen (the storage form of carbohydrate); and, as already illuminated at the beginning of this piece, body water. Additionally you have weight that comes from residual, undigested food in the GI tract (i.e. feces), but this is not a part of your body, per se. Thus, our simple two-compartment model can be expanded to incorporate more compartments that are better defined:

TBW = Fat + Protein + Bone + Water + Glycogen

And there you have it, a full breakdown of all the components of your body that contribute your body’s weight. For the average “Reference Man” this amounts to ~15% body fat, ~45% muscle mass, ~15% bone, and ~25% other (i.e. water and glycogen for the most part); and for the average “Reference Woman”, body composition amounts to roughly ~27% body fat, ~36% muscle, ~12% bone, and ~25% other [2]. Obviously, there is wide variation in these averages, but men tend to be bigger and have more lean tissue whereas women are on average smaller and have proportionally more fat tissue.   

Returning to our initial distinction between weight loss and fat loss, it now it obvious that a simple weight change on the scale can indicate a change in any one of these body compartments. Clearly, when people engage in a weight loss diet, their goal isn’t to lose body water—and certainly not muscle mass, organ weight or bone mineral density, despite all compartments decreasing during an energy deficit. Rather, the goal is, implicitly, to lose body fat. That is what people want.   

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