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When we measure body fat we usually measure a percentage of fat of the total bodyweight but to me this doesn't seem right and here's why.

Say my total bodyweight is 80kg and I measure my body fat percentage to be 10%, that is I have 8kg of fat. Now suppose I gain 5 kg of muscle only, now my total bodyweight is 85kg and my fat hasn't changed so 8kg of 85 is still fat.

This means that my body fat percentage now is 8/85*100%=9.4%.

So my body fat percentage has dropped but I did not lose fat, I believe this can be misleading.

If I am correct, why do we actually prefer measuring body fat percentage and not body fat in kg?

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Well not sure if my answer will follow the rules since it will not be backed up by scientifics evidence but maybe it will help anyway.

In your example, you take two cases with two bodyweight close to each other so the difference might not be perceptible but lets take an other one. If you take a person that wheigh 80kg and has 16 kg of body fat, it would amount of 20% his total bodyweight. His body fat would most likely be visible at certain spots (depending of the repartition I guess). Now if you take a 130kg person that still have 20kg of body fat (so around 12%) he would look way much leaner.

My point is that an healthy (and/or aestethic) quantity of body fat should not be calculated with absolute value because it will totally depends on the total bodyweight. 8kg of body fat might be a lot for thinner/smaller people (like children) but unhealthy/to little for higher weight/taller people.

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    This is a good answer (+1) on why expressing body fat as a percentage of body weight generally is better, especially for aesthetic purposes or when comparing different persons. But it doesn't quite answer how it matters to an individual. Having an excess of body fat has negative health implications; does adding more muscle tissue offset these? Is 15% BF at BMI 18.5 equivalent to 15% BF at BMI 15/20/25/30? Probably not exactly, so how do we know the optimum? I imagine there aren't any exact answers to be had (and probably depends on more than just BF/muscle ratio), but it's quite interesting.
    – gustafc
    Feb 20, 2023 at 9:22
  • I would think that if health issues is the concern, BMI and BF% would not be sufficient in themself to assess them correctly, better have analysis from a medical professional. For me these two datas are more an indication of general fitness/lifestyle, excess of body fat has negative health implication as you say but so does lack of it. And about the "optimum" I would assume that it would depend solely on the person point of view as long as it does not bring health issue. I'm not knowlegable yet to affirm anything though so if anyone has more information, it is welcome ! Feb 21, 2023 at 9:13
  • I have the impression that BF% is more important than BMI for health issues, as people with high amounts of lean tissue can have the same BMI as obese people even though both have very different fat % levels.
    – Luciano
    Mar 3, 2023 at 13:57

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