What Does Body Composition Mean?

At long last, more and more people are finally becoming aware of the concept of body composition and that it somehow differs from simply focusing on changes in body weight.

However, this is all too commonly leading to confusion as people ask “What does body composition mean?“. They know that it’s important but aren’t quite sure why it’s important or what the concept is about. I want to help take some of the confusion out of the topic here.

What Are You Made Of?

I’m not talking here about the scientifically proven fact that little girls are made of sugar and spice and everything nice while boys are made of slugs, and snails and puppy dog tails; rather, I want to talk about what the human body is composed of in biological terms.

Let’s imagine that I could magically (and hopefully painlessly) separate your body into all of its different components and put them on a slab somewhere (putting you back together might be a problem). What would we find? Well, there’d be some skeletal muscle, some fat cells (possibly too many fat cells), your bones, your organs, your brain, a whole bunch of different minerals, your blood, some water and probably a few other minor components that make up the totality of what makes you you

Depending on which type of tissue we’re talking about, we’d find massively varying amounts. Skeletal muscle can make up 25-40% of someone’s total weight, fat can range from less than 10% of the total in extremely lean individuals to 40-50% in the morbidly obese. Everything else I listed accounts for some proportion of your weight as well. The average brain is about three pounds, organs take up some space, blood weighs so much, you get the idea. If you add up the weights of all of these individual parts, you would end up with the total weight of your body. When you get on the scale, that’s what it’s telling you, the sum total of every different bit of your body and what it weighs.

So why is this relevant to the question of “What does body composition mean?”


Body Weight vs. Body Composition

When you talk about dieting and diet books (or even weight gain for those who are trying to increase rather than reduce weight), it’s safe to say that the majority of information out there focuses on weight loss. People want to see the scale drop, the faster the better. Diet books talk about weight loss, quick weight loss centers try to get the scale to go down as quickly as possible, even the TV show The Biggest Loser, which should be doing more to educate (and less to try and kill its contestants) focuses only on the weekly weigh-in to determine success or failure. It’s all about weight.

Why is this a problem?

Let’s say you step on the scale after dieting or exercising for a few weeks. And, happy days, the number has gone down by a few pounds (or kilos for my foreign readers).

Here’s my question to you: What did you lose?

Now, unless something very strange is going on, odds are it wasn’t bits of your brain or organs, it’s not likely to be bone either. But was it body fat? Was it skeletal muscle? Was it just water? Did you just have a rather large bowel movement that morning and that’s why you weigh less?

The typical bathroom scale that only measures weight can’t answer those questions. All a typical scale can tell you is whether or not your weight has gone down or up (if that’s the case). It can’t tell you what type of tissue (e.g. muscle, fat, water) was gained or lost.

That’s where body composition comes in.


Models of Body Composition

Recall from above how I listed a whole bunch of different tissues in your body that comprise your total body weight. Well, researchers, depending on how difficult they want to be, will group those organs in various ways and use that to develop body composition models. There are a number of different ones ranging from simple 2-component models to far more complex models involving 4 or more components.

Thankfully, for the majority of non-research applications (e.g. dieters or athletes), the 2-component models are just fine. In that model, the body is divided rather simply into:

  • Fat Mass: This is the sum total of all of the fat in your body. I’ll discuss this in detail in another article but there are three or four different ‘types’ of fat in the human body. All of it goes under fat mass.
  • Fat Free Mass: This is simply everything else. Everything that isn’t fat mass, including muscle, bone, organs, minerals, blood, etc. is fat free mass (often abbreviated FFM). I’d note that both glycogen (carbohydrate stored in the muscle) and water count as FFM; I’ll explain why in a second.

Put simply: Total body weight = Fat Mass + Fat Free Mass.

And even that simple 2-component model gives dieters and athletes the tools that they need to far more accurately track what’s happening in their body. As I mentioned above, it would generally be rare for people to be losing bone, brain or organs in any significant amount. So if someone is losing weight and they are not losing fat free mass, that means that what is being lost is fat mass (body fat). That’s good.

However, in some situations (including diets with insufficient dietary protein, or without the right type of exercise), it is possible to lose fat free mass; and since brain, organs, etc. aren’t likely to be going down, a decrease in FFM often means a loss of muscle mass.

This is generally (but not always) a bad thing, for reasons beyond the scope of this article. I would note that water loss can show up as fat free mass on certain types of diets and this can cause athletes and lean dieters to get very concerned; they think they are losing skeletal muscle mass but they really aren’t. This is a topic I’ll discuss in more detail in a later article.

I’d note that measuring body composition can also be useful when someone is trying to gain weight. An athlete usually wants to be gaining muscle mass, not body fat. By tracking body composition while in a gaining phase, they can determine what is actually being gained.

You might be wondering how body composition is actually measured. There are a number of methods available ranging from very low- to very high-tech and low- to high-cost. Many gyms will use skinfold calipers (small pinching devices which measure fat thickness), there are also handheld monitors and specific scales (such as Tanita) that use body water to estimate body composition. I’d note that, for the most part, I don’t find Tanita scales terribly useful. Other methods such as DEXA scans (a very high-tech method) and others exist. This will be the topic of a forthcoming article with specific recommendations.

In any case, by looking at changes in body composition, rather than just changes in weight, it becomes possible to tell what is actually changing in the body. Is muscle being gained or lost? Is fat being gained or lost? Are the changes just water being shifted on and off the body? While only looking at weight can’t tell you any of that, measuring and tracking changes in body composition can.

In a future article, I’ll go into more details about body composition, what some typical values are, how it’s measured, etc. Of course, most of the articles on this site and all of my books are aimed at improving body composition (e.g. more muscle with less fat) so you can read more there in the meantime.

But now you know the answer to the question “What Does Body Composition Mean?



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