Measuring and Tracking Body Composition

Continuing from the last blog, let's look at some of the practical aspects involved in both measuring and tracking changes in body composition or body fat percentage (BF%). I will be discussing a number of methods but it's important to realize that none provide more than an estimate. Every method has pros and cons.

From a tracking standpoint, changes are more important than absolute numbers. Since they can also be relevant, I will also be looking at some non-body composition methods of tracking progress in this blog and will make recommendations for combinations of those methods that can be used.

True Body Composition Measurements

The first set of methods I want to discuss are true body composition measurements in that they measure (or estimate) some aspect of actual body composition. They vary in their accuracy, difficulty of use and availability and I will describe them more or less in order from least to most complex.

Body-Mass Index (BMI)

The BMI is a fairly old measurement which relates an individual's body weight to their height. More technically BMI is defined as weight in kilograms divided by height in meters squared. For decades BMI has been used to indicate general health or some kind of ideal weight and insurance companies use some version of it to determine how much to charge you per month.

A high BMI tends to correlate with health risk and a BMI greater than 25 kg/m2 is defined as overweight and a BMI greater than 30 kg/m2 is considered obese. Very low values are equally problematic with a value below 18.5 kg/m2 is considered unhealthy or malnourished (possibly indicating an eating disorder or wasting disease). Between 18.5 and 25 is considered optimal. It's critical to note that these are only averages and it's been established that individuals with a high BMI can be healthy while those in the optimal range may be unhealthy.

Part of the reason for this is that BMI is not strictly speaking a measurement of BF% and doesn't indicate body composition or how much fat or LBM someone is carrying. Two females who are 5'7" tall and who weigh 150 pounds have the same BMI. If one is an athlete with 20% body fat and the other is inactive at 35% body fat, not only is their body composition different but so are their relatively health risks. It's also not uncommon for active individuals, generally males, to be told that they are overweight due to a high BMI score although they are relatively lean and simply carry more muscle mass. This leads to active individuals to often suggest that BMI should be thrown out for being useless but this is an over reaction. BMI was never meant to be used in an athletic population.

In the general public, it is simply not that common to find people with high BMI who also have a low BF% although people with a low BMI often have a fairly high BF% (they are often called skinny fat). It's also possible to have a high BMI and be metabolically healthy or a low BMI and be unhealthy. But no body composition method is perfect and BMI is not useless, its limitations simply have to be acknowledged. For that same group, BMI will give at least a rough indicator of general health risk along with giving a fairly easy way to track changes from diet and exercise (technically since height is not changing, tracking body weight would provide the same information).

And while BMI has primarily been used to track overall health trends, it turns out that it can give a rough estimate of BF%. A calculator to determine BMI can be found here:

That BMI value can be used to estimate BF% here:

While the above links may be inappropriate for athletic individuals, I think it is probably the easiest approach for people first starting out. It's quick and easy, provides a good starting point and can be used to track changes over time. Once someone has been working out consistently for 6+ months, I would not consider BMI to be accurate and they should use another of the described methods.