Body composition explained: Why it’s important for fitness trackers

It takes a lot of effort to stay healthy, but figuring out body composition can help
Body composition explained

Body composition isn't a metric typically found through fitness trackers, but you'll find several smart scales offering the feature as part of the tracking package.

It doesn't sound as exciting as VO2 Max or heart rate variability, but measuring body composition can yield significant results. Now, we're not talking about body mass index - that's actually quite different and we'll delve into details on how later.

So what exactly is body composition? How useful is it for overall fitness and can you trust a fitness tracker or smart scale to give you an accurate reading? Read on to find out.

Body composition versus body mass index

Body composition is quite literally measuring what your body is composed of. That means separating the water, protein, minerals and fat. You can figure out how physically fit you are by measuring body fat against everything else. This actually gives you a much better picture of your overall body fat percentage.

Body mass index or BMI is more commonly calculated because it's simpler and quicker to get results. BMI is your body weight relative to height and is more useful as a general range for large groups of people.

There are certainly pros and cons in figuring out both set of numbers. But if you want to know exactly what percentage of your body is good fat or bad fat then it's far better and more accurate to look at body composition.

How body composition is measured

There are several wacky methods that explain why body composition is rarely measured. San Francisco State University Department of Kinesiology lists three such methods as hydrostatic (underwater) weighing, air displacement plethysmography (bod pod) and a skinfold technique.

The first one is exactly what you think it is, and then some. Underwater weighing involves sitting on a special scale while submerged in water. The scale then takes various measurements and compares the calculations with what you weigh out of water, all while taking into account the pool's density, to finally tell you what your body fat percentage is.

Body composition explained: Why it's important for fitness trackers

It's not exactly something you can do at home sitting in a tub either. Heading to a university or fitness institution is your best bet to get these results.

The other methods like the Bod Pod involves climbing into well, a pod that measure air displacement. Georgia State University details even more ways that require you to hop into a contraption for calculations.

The bioelectrical impedance is basically just standing on a scale as a non-intrusive current courses through your body. The device then measures the resistance to the current since fat is a poor conductor, and estimate body fat percentage.

The skinfold technique is by far the simplest method but it isn't the most accurate. Using a pair of calipers, the person measuring you pinches a specific area then uses the numbers in a formula to determine body fat.

Depending on where you go, finding out your composition can be expensive especially if you go the large machine route. Of all the body composition methods mentioned, hydrostatic weighing is the gold standard and seen as most accurate so if you feel like dropping some cash, get weighed underwater. If cost is still a barrier, continue on to the next section.

Fitness trackers that measure body composition

Body composition explained: Why it's important and how fitness trackers are using it

Right, so you don't want to go through all the hassle of getting weighed by strangers or sitting in weird pods that measure your breath. In that case, there are other options that may not be as accurate but are more convenient and easier to endure.

At the moment, you can use the TomTom Touch and the InBody Band to measure body composition. Similar to the bioelectrical impedance machine, the fitness trackers use a low frequency electrical current sent throughout your body.

We've been able to test the TomTom fitness tracker which wasn't too far off the mark, but it wasn't as accurate as the professional reading.

There are more scales to choose from but not all of them are smart. It seems like the Withings Body Cardio is your best bet for a smart scale that can measure body composition.


In some ways, using a scale that employs the feature is more important than stepping on a dumb scale. The American College of Sports Medicine says "In regard to overall health, weight is not nearly as important as the composition of that weight. More important, rather than tracking weight, we should be aware of our body composition.

"Stepping on a weight scale simply tells us the combined weight of all our body's tissues. That weight may fluctuate throughout the day depending on the time of day, hydration status or what we are wearing."

That means while you may be within a healthy weight number for your age and height group, your fat percentage may not line up the right way.

Why is tracking body composition is important?

Body composition explained: Why it's important and how fitness trackers are using it

Now why would spend upwards of $100 or more to get an accurate picture of your body composition? Simply stepping on a scale is a good start, but the numbers fluctuate thanks to tons of variables and often don't provide a comprehensive analysis of your actual body fat.

The American College of Sports Medicine again says your body composition results could be used to "identify risks, personalize your exercise program or evaluate how well your current exercise and nutrition program is working for you…"

So while numbers will change, you could very well be in danger of diseases related to the amount of bad fats in your body. The American Heart Association notes that "If you have too much fat - especially if a lot of it is at your waist - you're at higher risk for such health problems as high blood pressure, high blood cholesterol and diabetes. That increases your risk for heart disease and stroke."


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