What is your body fat percentage? Is my body fat percentage correct? What an important societal debate! Today, body composition measurement plays a major role in any fitness assessment. I sometimes feel it’s the only measure that matters, judging by people’s reactions during fitness evaluations. Yes, they’re interested in their aerobic capacity, muscular strength, or flexibility, but don’t you dare forget to measure their body fat percentage!
But what exactly is this famous body fat percentage? Contrary to popular belief, it’s not the amount of fat, but rather the fraction of your weight that is made up of fat. Same thing, you say? Not quite. Body fat percentage is a fraction, not an absolute value. Your fat mass represents the total amount of fat in your body and is calculated using your body fat percentage (% body fat x total weight = fat mass). Why is the difference between fat mass and body fat percentage relevant? Let’s see…
In the fitness world, people often refer to a target body fat percentage (usually 12-15% for men and 15-18% for women) for an “ideal” body composition. However, since body fat percentage is a fraction, it’s possible to have different body compositions at the same percentage. Complicated? Let’s look at Table 1, which illustrates this perfectly.
Body fat percentage
% de gras (body fat)
Masse maigre (fat-free mass)
Masse grasse (Fat mass)
Indice de masse maigre (Fat-free mass index)
Indice de masse grasse (Fat mass index)
A quick look at the data in this table makes it clear that a given body fat percentage can represent many different body compositions. For example, two women with the same body fat percentage can have different amounts of fat and lean mass (muscle, organs, and skeleton). A woman weighing 80 kg with 18% body fat has 14.4 kg of fat mass, while a 66 kg woman has 11.9 kg of fat mass. Same percentage, different fat masses. You might say these differences aren’t very significant, but let me continue to add another perspective on this famous body fat percentage (which I dislike, by the way).
Most recommendations for optimal body composition are based on body fat percentage values. This is exactly where I think we go wrong. Why? Because it’s possible to have an “optimal” body fat percentage that’s actually associated with a problematic body composition. You can see in Table 1 that some values in the lean mass columns for women and men are in red. These indicate potentially insufficient lean mass (the target lean mass is based on the lean mass index[1], which is lean mass divided by height squared, just like BMI). For a given weight, you might have an “acceptable” fat mass according to body fat percentage (and the fat mass index[1], which is fat mass divided by height squared), but inadequate lean mass.
Let’s take a woman who just received her body composition results. She’s told her body fat percentage is 18% and congratulated for being in the “healthy” zone. Yet, that might not be the case. If our participant weighs 56 kg, her fat mass might be adequate, but her lean mass could be too low. This potential lean mass deficiency is usually characterized by insufficient muscle mass, which can lead to certain physical limitations (lack of strength, power, etc.).
Using body fat percentage as a measure of body composition has many weaknesses, yet it remains extremely popular (I can’t count how many times a client or athlete has told me they “need to be at X% body fat to be OK”). Many people aim for a certain body fat percentage as their ultimate training goal without really understanding what this measure represents, let alone the significant biases it involves. It’s much more relevant to use a body composition measure that takes body size into account, such as the lean mass index and fat mass index. This way, you can target an optimal body composition based on the individual, not on often obscure charts that are more confusing than helpful.
Worse still, I haven’t even addressed the variations and biases associated with the measurements themselves. But that’s for another post.
To answer the initial question about the ideal body fat percentage, I’ll answer using the fat mass index. For women, this value should be between 3 and 6 kg/m², and for men between 2 and 5 kg/m².
Happy calculating!
An interesting read…
- Bigaard, J, K Frederiksen, A Tjonneland, et al. Body fat and fat-free mass and all-cause mortality. Obes Res 2004; 12(7). 1042-9.
Let me know if you want a more concise or adapted version!