The Pitfalls of Body Fat “Measurement”, The Final Chapter


If you've been reading my series on body fat testing, you have now learned that even the best techniques have larger error rates than most people realize.

Does this mean body fat testing is useless for individuals? No, but it does mean you should exercise caution when using these techniques, and be aware of the large error rates. Here are some important tips to keep in mind:

  1. Always remember that any number you get from a body fat test is a VERY rough estimate, and could be significantly off.  Thus, don't put too much faith in the specific number.
  2. Point #1 holds true even when measuring change over time.  I've heard many people say things like, "I lost 4 pounds of fat and gained 1 pound of lean muscle."  The numbers are never even close to being this precise.  Given the error rate for measuring change over time, there's a good chance those numbers are way off.
  3. Even the best techniques have a 4-5% error rate when measuring change over time.  This means, to accurately detect a decrease in body fat in most people, the body fat percentage needs to drop by a minimum of 4-5%.  This means you should take long periods of time between measurements.  I would say a bare minimum of 3 months, but 6 months is probably better.  I see too many people taking measurements as often as every 4 weeks or so.  That's too frequent and unreliable.
  4. Remember that fat-free mass and muscle are not the same thing.  So just because your fat-free mass increases, doesn't mean you had an increase in muscle.
  5. You don't need to have your body fat tested.  A combination of body weight and circumference measurements (like waist circumference) will give you a very good gauge of whether you're losing body fat.  If your circumference measurements are decreasing, you are likely losing fat.
  6. If you are going to try to track change in body fat over time, then I recommend hydrostatic weighing or skinfolds.  These methods have shown the best accuracy rates for measuring change over time.  However, these techniques are difficult to do with extremely obese people.  For extremely obese people, I recommend simple body weight and circumference measurements.
  7. When using skinfolds to track change over time, you don't even need to bother calculating a body fat percentage.  If the sum of your total skinfolds is decreasing, then you are likely losing body fat.
  8. Whatever technique you choose, keep the conditions as identical as possible between measurements.  That means having the same technician perform the measurement on you, using the same equipment, and at the same time of day.

I hope you enjoyed this series on body fat testing, and I hope it opened some eyes regarding the techniques that are out there.  Again, body fat testing isn't useless, but you do need to be careful in how you interpret the results.

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3 years ago

Omron body composition monitor shows 6% body fat on me which is impossible. My abs don’t show at the bottom nor do I look ripped. I looked at pictures similar to me and they were more near 15% body fat at least. I look great but I have seen better abs on prisoners war people than me ( I don’t recommend that way because their caloric restriction included malnourishment, looking good but unhealthy ). I am 5 foot 9 inches (69 inches) weighing in at 140 lbs. According to the government I can be as low as 126 lbs and… Read more »

4 years ago

“And how else are you to know when you should stop losing (or gaining) weight?”

How about a mirror? You mention an “aesthetic” amount of body fat, but judging by what you wrote, you don’t seem to know what that word means.

4 years ago

I know this is an old post, but I just wanted to thank you for putting together such a well researched, informative article. I was trying to decide between getting a bod pod or spending the big bucks on DEXA to track my weight loss progress. Having read your article, I’ll be perfectly happy to have the same trainer at my gym check me each week. Now I know enough to capture the measurements as well as the final number. Knowing what I know now, I won’t get hung up on the “predicted” body-fat % number, I’ll be focusing on… Read more »

6 years ago

I have read all your articles. Fantastic!

6 years ago

In point 5 you say that you don’t need to have your body fat tested. You say that body weight and circumference measurements are enough. I disagree with this. I think that knowing your absolute body fat percentage is very important. How else are you to know if you should lose (or gain) weight in the first place? And how else are you to know when you should stop losing (or gaining) weight? Consider the following scenario. A man has a BMI of 26. He thinks he’s muscular and lean–thus weight loss is not on his radar. But because of… Read more »

6 years ago

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7 years ago

I forgot to add today was my second DEXA scan, the last one was 6 weeks ago, when I definitely had much more body fat and was not feeling as strong (the proof being I now lift heavier weights, squat deeper, run faster, have a lower RHR, etc. )

7 years ago

I found this article relieving to read. In the last 6 weeks, I have lost about 4kg. However, I measure 7cm less on my waist and 5cm less in my hips. I do HIIT training as I am fit despite a 10kg weight gain in the last year. Weights are sacred to me because I value being strong to the point where I would rather be strong and a bit chubby than skinny and weak. I got a DEXA scan today and it claimed I lost up to 3kg of lean body mass and gained a whopping 3% body fat,… Read more »

7 years ago

How do you reply to the researchers that have conducted extensive body composition research using a DXA and declaring it a “Gold Standard” means of assessing body composotion? There are dozens of journal articles that conclude DXA’s being a “Gold Standard” method (the vast majority are not industry sponsored).

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