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A Guide to Body Composition

For far too many decades, dieters and diet books focused only on bodyweight.  If it went down, that was good.  If it didn’t or went up, that was bad.  And while body weight is more useful than some think, it doesn’t tell the entire story.  Rather, body composition, and altering it, is where the real focus should lie.

In the following guide I will address a number of topics related to body composition.  First, to ensure everyone is starting on the same page, let me define what it actually is.

Body Composition Basics: 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 does this matter?

Bodyweight vs. Body Composition

As I mentioned above, 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 start a new diet and exercise program.  A few weeks later you step on the scale and the number has gone done by a few pounds (or kilograms).  Clearly the program is working, right?

But here’s my question to you: What did you actually lose?

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 especially over the short-term.

But what was it?

Was it bodyfat, skeletal muscle, water?  Perhaps you just took a big poo and it was undigested food.

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 brings us to the issue of body composition.

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 what that means in another article but there are multiple “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.

Note: For all practical purposes, I will treat fat free mass (FFM) and lean body mass (LBM) as syonymous so don’t be surprised if I switch to LBM later in the article.

When you combine the two of them, it has to add up to your total bodyweight.

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.  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.   I’ll only say here that water loss tends to occur fairly rapidly, in the first few days of a diet and any FFM it represents can be ignored.

Measuring Body Composition

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.

Measuring body composition is discussed elsewhere on the site in some detail.  Here I’ll only mention that there are a number of methods which range from 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.

The importance of understanding body composition, tracking it and using to track diet and exercise results cannot be overstated.  Bodyweight is, at best, a crude indicator of what is going on in the body.  Tracking body composition is far superior since it can tell you what is or isn’t being gained or lost.

The Guide to Body Composition continues in Body Composition – Calculations.

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