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

 

Last week I told you how body fat testing is a prediction, not a measurement.  I also talked about the 2-compartment model, its limitations, and how hydrostatic weighing is reasonably accurate for groups but can have an error rate of as high as 6% in individuals.

Hydrostatic weighing isn’t the only 2-compartment model out there.  Here I will discuss another 2-compartment model:  the Bod Pod.

Bod Pod = Bad

The Bod Pod is based on similar principles to underwater weighing; thus you would think it would have similar error rates.  In some instances, this is true; one study found error rates of about 2% on average, but errors of up to 6% in individuals.  However, other research on the Bod Pod has shown error rates higher than underwater weighing; one study found an average difference of 5.3% between the Bod Pod and a 4-compartment model, and an error rate of up to 15% in individuals!

One of the reasons the Bod Pod is worse than underwater weighing is because there are more variables that can affect the results.  For example, facial hair, body temperature, moisture, and the tightness of the spandex or swimsuit can all alter the results.

And, like underwater weighing, the Bod Pod can have errors when tracking changes over time.  In fact, one study showed the Bod Pod to be very poor at tracking changes over time.  Here’s a chart from that study:

Changes in % body fat, comparing Bod Pod to a 4-compartment model

The X-axis of this chart shows the change in % body fat for the Bod Pod, while the Y-axis shows the change for the 4-compartment model.  Each circle is an individual subject.  You can see the big disparity between the two method.  For example, one person gained 10% body fat but showed a loss of 1% with the Bod Pod.  Another person lost nearly 10% bodyfat but showed only a loss of 1% with the Bod Pod.  Another person lost about 5% body fat but showed a loss of 11% with the Bod Pod.

Do you see that “R squared = 0″ statement in the chart?  Do you also see the flat line in the chart?  That means there was no relationship between changes observed with the 4-compartment model and changes observed with the Bod Pod.  There was practically no agreement between the two methods.  This means the Bod Pod is horrible at assessing changes within individuals over time.  Now compare this chart to the one from last week on hydrostatic weighing.  You can see that hydrostatic weighing did much better; even though there’s some large error within certain people with hydrostatic weighing, there is at least reasonable agreement between hydrostatic weighing and a 4-compartment model in most people.

The Bod Pod:  The Verdict

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.

Underwater weighing and the Bod Pod are not as widely available as other techniques, such as bioelectrical impedance (BIA) or skinfolds.  Click here to read Part 4 of this series, where I discuss the accuracy, err, I mean inaccuracy, of BIA…

  33 Responses to “The Pitfalls of Body Fat “Measurement”, Part 3: Bod Pod”

  1. I wish I had read this before I wasted $45 to get a BodPod analysis done that was clearly ridiculous. I only did BodPod because it was more convenient than the dunk tank (which I don’t mind at all) and cheaper than a DEXA scan. But I never would have done it if I knew it’s accuracy was so far off compared to those methods.

      

    • MacMAdame, I wouldnt call it a waste. If it’s the best you’ve got then its the best you’ve got. I am finding the BodPod is more available than hydrostatic and we know its more accurate than BIA and calipers. You use the tools available to you. Personally, I know several people who use the BodPod regularly and a couple of them decided to seek out hydrostatic to compare and the results were very close give or take 1%. But if given a choice, hydrostatic is the most accurate. I did a bit of research before my first BodPod and during my research of the BodPod this is the only forum I’ve seen where someone has recommended against the use of a BodPod.

        

      • Eh, I could have done the dunk tank. There is one in my area. I was just being lazy because it’s harder to schedule with them via their web site than the BodPod place.

        I think part of the problem is that I had on a bathing suit that is reversible so it was double thickness from a normal suit and also it probably had air trapped between the two layers. No one told me that this would make a difference or I would have worn a different suit.

          

        • Test preparation should be reviewed before any test, but loose clothing, beyond what is recommended will indeed impact results.

          Conveniently this person leaves out the buoyancy error of the hydro-static and how many attempts it takes to get used to being shoved under water without any air in their lungs…..why NIH gave out grant for BOD POD per the Bod pod website.

          If it is so bad I wonder why is it used by so many on their user list which is quite impressive…all Mayo Clinics, NIH, Cleveland Clinic, NFL Combine, etc…. I wonder what this one person knows that all these elite facilities don’t?

            

          • things that make you go hmmm?

              

          • Matt,

            Conveniently this person leaves out the buoyancy error of the hydro-static and how many attempts it takes to get used to being shoved under water without any air in their lungs

            I did not leave out the buoyancy error for hydrostatic weighing, nor have I insinuated that hydrostatic weighing is perfect. You can read my post on hydrostatic weighing here. Also, these posts are about the inaccuracies in the methods…not about the challenges that people face when going through the methods.

            If it is so bad I wonder why is it used by so many on their user list which is quite impressive…all Mayo Clinics, NIH, Cleveland Clinic, NFL Combine, etc…. I wonder what this one person knows that all these elite facilities don’t?

            You are committing the appeal to popularity fallacy.. Simply because the method is used by some major organizations is no way a reflection of the accuracy of the method. Only scientific research can demonstrate accuracy…and the research clearly demonstrates that the Bod Pod is not very good on an individual level when compared to 4 compartment models. If you have any problems with the research cited here, then please offer your refutation of the studies. Otherwise you have no support for your statements.

              

  2. As with dee this is the first article that I have found that recommends against using the Bod Pod for testing. Some of the accuracy that is mentioned in the article can be controlled like any other scientific test, since your are only testing one thing, body fat, you have to remove any other variables that could alter the results. Your first test is your baseline and your future test will show your progress. This means for your next test you need to try and present yourself as close as possible as you did for your first test, that means wear the same bathing suit, fix your hair the same (if you had any styling product in your hair for the first test you need to have in in your hair for future tests), etc. Also, make sure you are using this for your own goal setting and not comparing to someone else since you don’t know any of their testing conditions.

    Unfortunately it doesn’t sound like your testing facility is following the correct guidelines regarding the Bod Pod testing. You should have been wearing either a speedo or compression shorts, a swimming cap (that would compress your hair), etc.

    Here is a pretty good web site that describes how the Bod Pod tests and the requirements for the test.
    http://floridafitnesstesting.com/

      

  3. The ease of use of the Bod Pod compared to underwater weighing is not a minor factor. #1- water weighing is a lot harder to find than even a Bod Pod, and if you’ve never done it, is not that comfortable of an experience.

    The key is consistency in readings, having done at least 5 tests myself over the course of a year, I’ve found it to be inline with the results I was seeing. Is it foolproof? No, and no method is, except for one… an autopsy.

    If someone is going to get that hung up on #’s perpaps they should just go by pictures

      

    • The key is consistency in readings, having done at least 5 tests myself over the course of a year, I’ve found it to be inline with the results I was seeing.

      That is your personal experience, but the data shows that readings can be very inconsistent within an individual whose weight is changing. I refer you to the study I discussed in this article where the Bod Pod was very poor in assessing changes in body fat over time compared to a 4-compartment model.

        

  4. Just got mine done today. I am a personal trainer/former fitness competitor and my body fat on the Bod Pod was 28% this morning. I weigh 143lbs, 5’6″ and I am very strong. I didn’t really believe it, especially because I have always been around 20% according to 7-site skinfold tests.

    My client on the other hand, had an accurate read out and RMR (using calculations from her diet and weightloss).

      

    • BOD POD & hydrostatic weighing SHOULD give a higher number than skinfolds, which only take into account the subcutaneous fat. Bod Pod and hydrostatic weighing are looking at intramuscular fat and fat surrounding organs as well. Also, consider that people carry subcutaneous fat weight in different areas of the body that even a seven-site skinfold can’t possibly estimate accurately. For example, a woman who has a large behind/sizeable breasts will have a higher reading with hydrostatic weighing/air displacement plethysmography because those areas are not sites used during skinfold tests.

      Just something to think about.

        

  5. I’m one of the statistical anomalies… If you’d like to read about my story and experience, I’ve shared it on my blog.

    No matter what, after changing my diet and physical training to be the best it can possibly be, I cannot allow a number that a machine spits out as the definition of my success in changing my health and performance for the better.

    http://www.painfreeposturemn.com/BLOG/bid/142525/Is-it-All-About-the-Numbers-My-CrossFit-Whole9-90-Day-Challenge

    I was encouraged to find this site.

      

    • Thanks for sharing your experience, Deb!

        

      • Thanks James. I appreciate hearing from you and your support. Hopefully by sharing my experience I can help someone else.

          

      • So when someone shares an anecdote that supports your side, you thank them, but when someone shares an anecdote that goes against your argument, you refer them to studies and discredit their experience. I’m confused by this inconsistency.

          

        • Frank,

          There is a difference between the two scenarios you present. When someone shares an anecdote that supposedly goes against my argument, they are trying to use that anecdote to refute the science. But anecdotes are not science, and thus are not valid arguments against it. It does not discredit their experience in and of itself; it discredits the idea that personal experience somehow should carry more weight than scientific data in a well controlled environment.

          If someone shares an anecdote that “supports my side”, I am doing nothing more than thanking them for sharing the experience. If you will note, I do NOT make the claim that the person’s experience somehow validates the science. It does not; the science stands on its own. That is the key difference between the two scenarios you present.

            

  6. Yes, I had a similar experience with the bodpod. I did a mobile hydrostatic test that came in at 15.9%. The guy who did it looked at me and said this seemed really high. I then went to my nutritionist who measured me at 10.7% using BMI electrodes. The bodpod test put me at 20% which was insane. I am incredibly lean, stronger than I’ve ever been, and really shocking to see such varying results. Definitely don’t rely on a number.

      

  7. I did the BodPod this weekend and it clocked me in at 25% bodyfat. Which absolutely blew my mind. I can’t understand how that’s possible. Other tests have always clocked me in at HALF of that. I’ve done calipers, infra-red test (http://www.futrex.com/62.html), even the BIA’s only clock me in at like 15-16% bodyfat. This picture is from 1 week ago…tell me it’s 25% bodyfat https://s3.amazonaws.com/static.fitocracy.com/site_media/user_images/stream/53978/vdvame_large.jpg

    So the BodPod gets a big thumbs down from me.

      

  8. One thing to consider with the reference to the 4-compartment model is that it to is not perfect. In fact in the 4 compartment model it may consider the following:

    1. DXA used to determine Bone Density (studies have shown statistically significant differences in bone density measurements depending on hardware and software used) – 1 source of potential error

    2. Bodpod or UWW used to determine Body Volume. Some potential source of small error

    3. Total Body water – through Deterium solution or some other method (BIA has been used here too) – some source of error

    4. Total Body Surface Area – mathematical estimation based on height / weight – or more recently 3D scanning technology – some source of error

    The above methodology can be used in University labs and obviously takes quite a bit of time and is therefore not commercially viable unless you want to pay $$$.

    But my point is this – take a small amount of error in four separate tests, put them together in another formula (with another potentially small error rate) and we have 5 x small error potential which as an exercise scientist myself creates more potential for repeatability error and is just statistically likely to lead to a greater source of error…..

    My suggestion, choose Bodpod or DXA with an informed and educated technician who can really help explain your changes to you. I am an academic, and definitely look at the literature, but you can find plenty of research to support or refute any method out there (or diet, or training method etc etc.) Choosing one method and sticking to it is the key to understand how your body is changing and ultimately let you know if you are on the right track or if you need to change something in your nutrition or training approach.

      

    • AG,

      While you make good points regarding the compounding of error, it should be noted that this has been addressed by scientists in the literature. In fact, 4-compartment models have been validated against cadavers, and they are still superior to other models, even with the compounding of the errors. The advantage of 4-compartment models is that, even with compounded error, they still address the shortcomings of the typical 2-compartment models.

      While I agree that it is important to stick with one method, that does not help eliminate problems. I have had clients who, despite significant weight loss and improvements in circumference measurements, have shown an increase in body fat % with the use of the Bod Pod. While these are the exceptions and not the rule, it is important to consider the major shortcomings of body composition estimation techniques. Sticking with a method doesn’t help if the method itself has major problems.

        

  9. I was tested by the BOD POD and my first test came out as 1% body fat. Given the fact that they say you need at least 3-5% essential body fat I called them up. When they retested me again I rang in at just 2.7%. I dont do any crazy diets, I just eat healthy and even cheat on the weekends and work out 1 1/2 hrs about 5-6 times a week. So in my case going back the same day and tested did prove to be fairly accurate.

      

  10. I am 120 lbs, 5 ft 4.5 inch female and it said I was obese. It said 33.4% body fat which I find absurd. I eat healthy and lift weight and do cardio 3- 4 times a week. I can do “real” push-ups and have visibly defined abs. My clothes are all size 0 or 2. How is it possible that I’m obese?

      

    • Yes Marina that is absurd. You seem very fit and healthy probably with a bodyfat % in the 18% – 20% range or less. Keep in mind that essential body fat % levels for women (10-12%) is higher than men (2-4%) with athletes around 14-20% for women and 6-13% for men so if you test out at 20-22% you are still very fit. Sounds like you have a good routine, keep knocking out those pushups!

        

  11. Excellent article!

    The US Navy circumference method has been tested accurate to within +/-1% of hydrostatic. I use it with all of my clients after getting consistently absurd results from electrical impedance scales and gimmicks like BodPod.

    There are several websites with free calculators. The formula uses age, height, weight, gender, and circumference of neck, waist, hips (women) to derive consistently accurate values. All you need is a tape measure and a scale.

      

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