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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Mike
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Mike

Thank you James for your excellent article on bodyfat measurement methods. I used 2 DEXA scans on me in one week. Both on Mondays using the same routine each time(exersice, food, time of the exam and I believe hydration) and with the same machine operated by the same technician. My bodyweight in my first exam was 117kg with 180cm height and my BF% according to the DEXA scan was 42,1%. On the following Monday my measurements where: 114kg with no lean mass loss but with increased BF% 42,6% ! In this week I followed two vey popular weight loss systems.… Read more »

Rob
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Rob

Thank you for the thorough, well-researched summary. This is the most valuable collection of information on the subject available. Regarding your advice to take measurements less frequently when the error rate is high, my intuition would tell me to do just the opposite. Whether a measurement has errors due to inconsistency or a consistent bias, I would expect that more frequent measurements, plotted on a chart, would be the best way to show the true trend, and allow you to make adjustments to your diet and exercise. What I do is use my body fat scale every day and plot… Read more »

Judith
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Judith

I have always sensed that weight loss measured on the scale tends to lag fat loss as inferred from changes in my body circumferences and skin fold thicknesses. The calipers are a useful tool. I would like to invent electronic underwear that gives me the answer more quickly and accurately.

Leo
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Leo

Thanks james

Leo
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Leo

What are your thoughts on ultrasound testing? They just use a wand similar to what they use for pregnant women on up to seven skin fold sites.

Pierre
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Pierre

I’m somewhat curious that you suggest that the only way to accurately tell your body fat percentage is to strip your body down and weigh each of the component parts. How then can you tell how much error there is in the methods you examined, considering there is no true baseline to compare it to? I’ve had a few DEXAs done over the past couple of years, and the total body weight the DEXA machine gave for me has always been less than 1% variation of the scales. For a tiny beam of x-ray to be that close, surely that… Read more »

Matt
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Matt

When you say 4-5% error rate, does that mean 4-5% BF or 4-5% of the total?

For example I had DXA put me at 8.1% BF. Would it then be 7.6-8.5% (4-5% either way) OR

3.1-13.1% BF?

Thanks!

Tjun Kiat Teo
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Tjun Kiat Teo

I am curious. Has there been any study that established sort of a working conversion between waist hip ratio and body fat percentage. For example a waist hip ratio of 0.8 for male is equivalent to ?certain level of bodyfat percentage

Denise
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I want to thank you for saving me $49 ! I had a DEXA scan done in May 2009 and was pretty disappointed to be told that I was 29.7% fat. The likely hood of my building 10 lbs of muscle are pretty slim! And it is highly unlikely that I will lose 10 pounds of PURE FAT and then maintain a BMI of 21. It was depressing to realize that a body fat percentage of nearly 30% (thus practically “obese”) was my lot in life. I’ve been toying with the idea of having another (since I think, MAYBE I’ve… Read more »

tom
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tom

I disagree with #3. I have developed a measurement method that I perform on myself every month or so and it is very sensitive to changes in body density. I am talking about changes only here, not the absolute fat% value. I agree with you about the percentage fat being inaccurate and that number is of less value to me anyway. I measure my own weight hydrostatically. I practiced exhaling to my minimum residual lung volume, until it became very consistent (many sessions over a month long period). Then, I used empty bottles of various sizes as buoys and determine… Read more »