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Measuring % body fat

Friday, April 01, 2011

My own personal philosophy is to

1) get the BMI below 30, and then
2) focus on dropping body fat.

The problem is, how do you actually *measure* body fat? It's elusive.

Here's a nice synopsis on Wikipedia:

Here's an interactive web tutorial on some methods:

(Calipers aren't mentioned in this one.)

Leigh Peele wrote a very nice post about the topic:

The pictures in there are especially helpful.

James Krieger wrote a really really in-depth series of 7 columns about the relative pros and cons of different methods for figuring out % body fat:

The Pitfalls of Body Fat Measurement: Part 1

The Pitfalls of Body Fat Measurement: Part 2

"The bottom line is that underwater weighing can give good results when looking at group averages, but not so good results when looking at individuals. The sad thing is that underwater weighing is actually the best method out of the 2-compartment models. Other methods, including the Bod Pod, BIA, and skinfolds, are significantly worse."

The Pitfalls of Body Fat Measurement, Part 3: Bod Pod

"The Bod Pod does OK when looking at group averages, with some studies showing error rates of around 2%; however, other studies have indicated average error rates of over 5%. The individual error rate for the Bod Pod can be unacceptably high in some individuals, and the Bod Pod is horrible for tracking change over time. For these reasons I would recommend against using the Bod Pod as a body composition assessment tool. Hydrostatic weighing, despite some of its problems, is much more reliable."

The Pitfalls of Bodyfat Measurement, Part 4: Bioelectrical Impedance (BIA)

"BIA can be problematic because it’s a prediction based off of a prediction, so the error gets compounded. When you look at group averages for BIA measurements, there tends to be bias, with BIA often underpredicting how much fat you have. As with other techniques, the individual error rates can get high, with some research showing error rates of around 8-9%. In fact, BIA doesn’t do much better than BMI at predicting body fat in some cases. When it comes to measuring change over time, BIA can often underpredict the amount of fat loss, and the estimated change can be off by up to 8%.

For all of these reasons, I am not a fan of BIA for measuring body composition in individuals. If you are going to use BIA for tracking body composition over time, I recommend very long time intervals between measurements (at least 3 months, but 6 months is probably better), as the error rate for BIA can be larger than the changes in body fat in you see. Whatever numbers you do get using BIA, always remember they are very rough predictions….and I emphasize very rough."

The Pitfalls of Body Fat Measurement, Part 5: Skinfolds

"Like BIA, skinfolds can be way off when it comes to determining body fat percentage in individuals. When it comes to tracking change over time in groups, then skinfolds do pretty well. However, errors for tracking change in individuals over time can be up to 3-5%. Thus, if you are going to use skinfolds for tracking a single person over time, I recommend very long time intervals between measurements (minimum of 3 months but 6 months is better); otherwise, the error rate is higher than the change that you can see. In fact, I recommend against even calculating a body fat percentage. If skinfold thicknesses are going down, then you are likely losing fat."

The Pitfalls of Body Fat “Measurement, Part 6: Dual-Energy X-Ray
Absorptiometry (DEXA)

"Despite the fact that DEXA represents a 3-compartment model, its error rates are no better than hydrostatic weighing, and in some cases is worse. Like other techniques, DEXA does well when looking at group averages, but not so well when looking at individuals. Individual error rates tend to hover around 5%, although some studies have shown error rates as high as 10%. When looking at change over time in individuals, error rates have hovered around 5% in some research, although other research has indicated DEXA to perform much more poorly. For these reasons, I do not recommend DEXA for tracking change over time in individuals. If you do use DEXA for tracking change over time, I recommend very long time periods between measurements (a minimum of 3-6 months), as you will need a minimum of a 5% change in body fat to reliably detect a true change in body fat in most people."

The Pitfalls of Body Fat Measurement, The Final Chapter

"Body fat testing isn’t useless, but you do need to be careful in how you interpret the results." "For extremely obese people, I recommend simple body weight and circumference measurements."


According to him, hydrostatic weighing (NOT Bod Pod) and DXA are the most accurate. For following trends in % body fat he recommends calipers or if you're extremely obese, a plain old measuring tape.

For those of us who have weighed over 300 lbs, our bones are likely to be extremely dense (we're weight-lifting all the time just by walking around), so DXA is probably a better bet than hydrostatic weighing if you want to go either of those routes. I had a DXA scan done Jan '10 and at that point they said my bones were 25% denser than average.

I checked my Tanita BIA scale against the DXA % body fat measurement and discovered that if the scale was set to "athlete" mode the readings matched exactly.

So for now I'm just going with what my scale says, although BIA is not supposed to be very accurate. I'm averaging the daily measurements over at physicsdiet.com.

Although DXA is the technology used on the Biggest Loser to assess % body fat, many people still do not know about it. Here is a nice PDF article by Mary Oates summarizing its use, with pictures:

So, where can you get a DXA scan done?

If you live west of the Continental Divide you can probably find a sports medicine or wellness center that will do it, if you just use Google Maps and search for "DXA" and "DEXA."

If you live in the East, it seems to generally be a bit trickier. You'll need to contact the radiology or imaging departments at a few local hospitals to find out if they 1) have the right machines (GE Lunar or Hologic) and 2) if they will even do a whole body scan (many places just use DXA for bone density testing and either don't know they can or don't choose to provide whole body composition analyses).

Once you've found a place that knows what you are talking about, you can bring the above Oats PDF article to your physician and ask for an order for the scan.

There aren't any lists anywhere of facilities that provide DXA body composition analysis, so I've started compiling one. The list of places that have responded so far is here:

If you know of a place that does it, or any lists where people collect that kind of information, please have them fill out this form here, so we can find more places:
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Member Comments About This Blog Post
    Thanks for directing me to your post. Your support is appreciated.

    1430 days ago
    Awesome info! The bod pod seems like such a good idea, shame it doesn't pan out. They could probably make it a lot more effective by making it more like a steel can, but that would probably freak people out.
    1825 days ago

    Comment edited on: 4/26/2013 11:33:04 AM
  • OOLALA53
    I've had the underwater weighing and DEXA ( at San Diego State University.) DEXA was a lot easier! But it's expensive. I have a Tanita scale, but I know it's way off. I also used calipers. Though they many not be accurate, they certainly seemed to match the changes that happened in my body over the year I used them. They are harder to use, but cheap and available.

    Don't you wonder how they can test the accuracy of the tests?

    2371 days ago
    Thanks for all the detailed info. Makes me wonder if a tape measure and a mirror isn't easier. emoticon
    2372 days ago
    Thank you for that detailed info I knew that all of the test are estimates I always thought the test in the water was most accurate learned something new today
    2464 days ago
    Great stuff here, gives me a new perspective on this type of measurement. I've been trying to lower my body fat % for years, and just seem stuck, and the tool of measurement is BIA (since that's what I have easy access to), I've also only been tracking infrequently, so maybe that's been a good thing. It's just not moving at all, but then again, I'm in maintenance, and my program isn't changed all that much either. I've recently upped my strength training so hopefully next time I get on that scale I'll see some difference.

    Thanks for all this info!
    2579 days ago
    My physical therapist has been measuring me monthly and it's proving to be a great motivational tool. I lost over 5 inches total last month, but the numbers on the scale are going down at a snail's pace now. I know my clothes are getting looser and I have bags more energy, so that's good enough for me. :-) Thanks for the post. Will check into the DXA technology here in England. Did it cost a lot to have it done in the States? Susan emoticon
    2580 days ago
    I know, right? Who would have thought (or even wanted to believe) that a measuring tape could be preferable than any of these high tech methods?

    Go figure.

    Like you said in the related thread, I'm going to continue using BIA because it's there. But I'm not going to believe it beyond an 8-9% margin of error.

    If you find any facilities around you that do this, please have them add themselves to the list! :-)
    2580 days ago

    Comment edited on: 4/2/2011 12:58:44 PM
    Actually, Krieger suggested not even converting the skinfold measurements to body fat percentage. He suggested using the sum of all the skinfold measurements as a figure of merit. I intend to shift to that and circumference measurements for tracking how my plan is working.

    I've debated about summing the skinfolds vs. using them individually, and I will see how they look once I start having them taken regularly again. I suppose summing them does eliminate some of the random error in individual measurements, and also the "fat comes off in sheets" rule of thumb would indicate that the sum of the skinfolds would be the best measurement of fat loss progress.

    However, all that only really applies to someone in a fairly normal range of body fat. For an obese person, circumference measurements are probably the best tracking method.

    None of which is, of course, what I'd have said a year ago.
    2580 days ago
    Interesting. Hologic is based here (I know one of their Directors). Maybe they have more machines here as well.

    Thanks for the info!
    2580 days ago

    According to Krieger the Bod Pod is not any better than calipers for accuracy and significantly worse for tracking changes in % body fat...
    2581 days ago
  • 250STRONG
    Great info! I just did a BodPod assessment yesterday. It was only $25 at my local university sports medicine dept. Definitely more accurate than calipers or bio-impedence (over 30% body fat).
    2581 days ago
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