The body-fat percentage lie

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Dr Jordan R. Moon is the chief science office at FitTrace and Colorado State Director of the National Strength and Conditioning Association (NSCA).

If you asked most lifters, regular exercisers, personal trainers, strength coaches, sports coaches, athletic trainers, nutritionists and doctors why they asses body-composition levels, they’d give some or all of the following reasons: to identify health risks associated with excessively low or high body fat value; to educate clients and athletes about the risks of low or high body fat values; to monitor changes in body composition; to estimate healthy body weight goals; to formulate dietary and exercise recommendations; to monitor growth, development, and age-related changes.

While nearly everyone is using body composition data for the above reasons, what they may not understand is the data they are getting is widely variable and even the so called ‘gold standard’ methods have large individual errors. In reality, measure body composition is virtually impossible, regardless of the method or technique you use.

Shredded stats
We have all heard it at some point or have even been guilty of it ourselves. ‘I was shredded and down to 2% fat’ or some similar exaggeration regarding the state of their physique. Today some websites let users post their body fat as part of their profile and most people are excessively generous with their numbers. I am not questioning the leanness of people or arguing that they are not ‘shredded’. What I am questioning is the accuracy of these body-fat percentages, because it is impossible to know exactly what your percentage is.

Let us start by first trying to understand what a body composition measurement is compared to other health and performance measurements. Everyone has had their blood taken at some point, or had their blood pressure taken. The data from these tests is a true measurement, not an estimation. For example, your blood pressure is a measurement in millimetres of mercury and your blood glucose is in milligrams per decilitre. A max bench press or squat is measured by the amount of weight you are lifting, specifically, the mass of the object (bar and weights) moving against gravity. Even your body weight on scale is a true measurement of your mass in the presence of gravity and won’t change unless there is less gravity, such as on the moon. Your height is a true measurement since all you are measuring is the length of your body in inches or centimetres. So what does this have to do with body fat, muscle, lean mass, and fat-free mass?

Body of evidence
The reality is that all body-composition measurements are estimates. The only true ‘measurement’ of body composition must be done using cadaver analysis techniques. So you can get a true measurement of body composition if you want, but you would need to be dead to get it. In fact, nearly all of the methods used today that have been considered gold standards use data from cadavers to predict your body-fat percentage. And in case you were wondering, measuring body composition of cadavers is extremely difficult because ever component of the cadaver needs to be broken down individually. That means every single lipid in your entire body is broken down and measured.

This is so difficult to do that only a handful of studies have ever been done on cadavers and most of the data we use to predict body composition comes from cadavers analysed from 1945 to 1986. This means that when you do a BodPod or underwater weighing test your actual fat percentage is based on a couple of cadavers measured several decades ago. This is one important reason why you can’t say your body composition was actually measured.

Assumed errors
Knowing that your percentage is only ever an estimate and not a measurement allows us to now interpret the data with assumed error. What does this mean? No matter what method you use, so long as you are alive, there will be assumed errors in your percentage value. To understand how large these errors are we need to first know the variability in data from cadavers. Specifically, we need to know the differences between cadavers in order to know differences between living people.

One component of body composition that is assumed is the amount of water in muscle, adipose tissue, and your fat-free mass. While methods such as BodPod, underwater weighing, and DXA assume a constant amount of water in your fat-free mass of around 73.7%, we know that this value can range from 68% to 81%. So if the water in your fat-free mass is below or above the 73.7% your data will be less accurate from any of these methods.

Constant changes
Every test you use to measure your body composition assumes that this number is constant when we know that is always changing, especially with changes in exercise and diet. What we don’t know is by how much they are changing, or if these water amounts are changing the same with everyone. So again, we are forced to assume a constant and interpret our data understanding these assumptions. While this may seem difficult to comprehend, what this actually means to you and your body composition data is that errors will always be present.

So what do these errors mean to you and your data? To put it bluntly, we don’t know and here is why. Depending on your body type, exercise status, age, sex, and ethnicity, each method may give different results. The best thing we can do is use complicated methods that measure body water such as deuterium oxide along with other more advanced techniques. Multi-compartmental models are now considered the only true gold standards because they measure as many of the assumed variables as possible. Once such model is the Six-Compartment model that breaks the body down into fat, calcium, sodium, potassium, chlorine, nitrogen, and water. But even they won’t provide the level of complete accuracy.


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