In my last post, I gave a basic overview of body fat testing, and why it’s important.
More interestingly, I talked about the fact that absolute accuracy is not as important as consistency with methodology, frequency, and timing.
I also asked you, my readers, to drop in on the comments section tell me which method you were using.
The breakdown was about as I expected: about half the commenters were using bioelectrical impedance scales, just under 25% were using skinfold measurements (calipers), and the remainder were using everything ranging from eyeballing it to DEXA readings.
And so, today I want to focus a bit more on the various methods of body fat testing that are the most popular among my readership, as determined by the comments posted in the previous blog.
For the purposes of this post, I’ll focus on the methods you’ve either specifically said that you’re using, or that you are liable to have access to.
According to reader comments on both the blog and my Facebook page, the most popular method of body fat testing (by a significant margin) is with the use of bioelectrical impedance devices, such as Tanita scales and hand held analyzers. The popularity of these devices is simple to explain: they’re easy, convenient, and relatively cheap.
Anyway, the main thing is that unlike methods where you get measured by a pro, bioelectical devices have no on-going cost inherent inconvenience: once you have a scale, you can use it whenever you like, from the comfort of your own home.
So here’s the deal: bioelectrical impedance devices work by sending an imperceptible electrical shock through you.
All such devices work by employing at least two points of contact in the form of electrodes; half of those electrodes generate an itty-bitty voltage and the other half measure the residual voltage that remains after the current passes through the body.
To put it most basically: certain tissue (muscle) has a high water volume and so is electrically conductive; the tiny shock passes through it quickly without impeding or degrading the voltage. Other tissue, like fat, has a lower water volume and impedes/slows down the shock.
All of this factors into the ratio between the starting and ending voltages, or your impedance value, which is determined by how long it took the shock to travel through your body and the remaining voltage at the end of the journey.
Depending on the type of machine, the impedance value is then combined with other tidbits of information like weight, height, Zodiac sign, age, favorite color, sex, etc. in a complicated algorithm that converts all these factors into a body fat percentage score.
As with any algorithm, there are both variables and constants; the variables are things that are more of less unique to the client. The constants are the parts of the equation that are considered to be absolute and applicable to every person who uses the scale.
This is where we run into the biggest problem with impedance scales: hydration.
One of the aspects of the formula that is considered to be absolute, and therefore factored in an identical way to all clients, is total-body water (TBW). TBW is assumed to be about 73% of your FFM. Which means that this value (73%) is factored in for ALL people on ALL scales at ALL times.
You can see the issue here: TBW is treated like an absolute while any one with a functioning brain can see that it’s a variable. However, because it’s assumed to be absolute and the value never changes, all changes to this quantity of water or to its electrolyte content will influence your impedance and thus change your what the machine calculates your BF % to be.
The important thing to understand that although these devices are used to determine body fat, realize that what the machine is really measuring is the amount of water in your body.
After measuring your TBW, which is assumed to be 73% of your Fat Free Mass, the device calculates your FFM. Your FFM is then subtracted from your total body weight, and the difference between those two values is, of course, the amount of fat on your body, or your FM.
Once the machine knows your fat mass, it divides that number by your total body weight, and gives the result in the form of a percentage—that is, your assumed body fat percentage.
All of which is obviously leading me to my point: your level of hydration is going to have an extreme impact on the measurement the device takes, which will have a potentially huge impact on the reading.
Here’s an example of how your hydration can radically effect your measurement. One of our commenters pointed out that he had been tested with a bioelectrical impedance scale, and given a reading of 14%. However, three days later—after an unfortunate bout of diarrhea—he stepped on the scale and read out at 19%.
Obviously, he didn’t gain 5% body fat in 3 days. So what happened? Simple: losing water for several days will dehydrate you to the point where you have had an extreme change in TBW, and the scale picks that up. Further, as the reader notes, the contents of your bowels will be factored into any reading (and feces contains water).
This is an extreme case, of course, and if you happen to be stricken with an illness, I trust your health is the priority, not your body fat; still, it’s an interesting story that illustrates the point quite nicely.
For non-illness related tests, try this…
JUST FOR KICKS: If you have one of these devices, take a measurement right now. Then, drink 2-3 bottles of water, wait 30 minutes, and measure again. Your results are going to be quite different.
Impedance measurements can also be altered by changes in skin temperature, skin moisture, body posture and host of other factors—so really the readings are always “iffy.”
Does this mean that using bioelectrical impedance scales is a bad idea, or that these devices are useless? Absolutely not. As has been mentioned, they’re convenient and fairly inexpensive.
Plus, as I keep stressing, the actual measurement is not the end-all-be-all as long as you’re making progress; however, to ensure you are making progress and not just getting helter-skelter readings, you need to do everything you can to make the readings count.
As I mentioned in my previous post, to maximize reliability, you need to make measurement conditions as consistent as possible. To that end, I always recommend taking your measurements at the SAME TIME on the SAME DAY, each week.
Taking it a step further, you need to do your best to make your hydration as consistent as possible. To that end, I recommend doing the following:
In summary, bioelectrical impedance may not be the best choice, but with a little diligence, it can work very well. Even if it’s not overly accurate, it can give you an accurate reflection of your progress.
While not used by many of our readers/commenters, a few people brought this method up in the way that I expected; that is, Hydrostatic Weighing (HW) is often considered the “Gold Standard” of body fat measuring—usually, people like to say it’s the best and most reliable method. I’d have to say that I have no problems with that statement—of all of the tests most available hydrostatic weighing is theoretically the most accurate.
I do take issue, though, when people claim that hydrostatic weighing is 100% accurate—which is just flat out isn’t.
I’ll touch on that below, but first–I personally think HW is fun-tastic because it relies on a scientific rule from one of my favorite people of antiquity: Archimedes.
Personally, I’m a fan of him because of his use of the sun and parabolic mirrors to create what has become known as “Archimedes Death Ray” and burn down an armada of ships off the coast of the Sicily during the Siege of Syracuse.
More relevant to hydrostatic weighing, however, is Archimedes’ Principle, which states:
“The volume of an object submerged in water is equal to the volume of the water that the object displaces” and “a body immersed in fluid is acted upon by a buoyancy force, made evident by a loss of weight equal to the weight of the displaced fluid”
In this case, we’re using this idea to measure body volume. The volume of a body is expressed as (Vb).
To get there, we need to figure a few things out.
For hydrostatic weight, the difference between a person’s mass in air (MA) and mass in water (MW) is the weight of the water displaced. The volume of this displaced water is obtained by dividing the mass of the displaced water by the density of the water (DW), which is based on the temperature of the water.
The equation for Vb is as follows:
Vb = (MA – MW)/DW
Next, an additional volume that contributes to Vb must be taken into account when using HW. This additional volume, which does not contribute to overall tissue density, is the amount of air in the lungs after a maximal expiration—this is residual volume, expressed as (VR).
The residual volume must be measured and subtracted from the Vb to calculate density, which is expressed (Db).
Db = MA/(Vb – VR)
The densityis then plugged into an equation used to predict percent body fat (%BF). There are two equations that are commonly used:
(Note: I prefer the Brozek formula because it has the word “bro” in there. True story. Take that, bro-science haters!)
So, I gave you a bunch of formulas, talked all about some principles, and therefore this HAS to be super accurate, right?
Not always. There are a few issues.
Like bioelectrical impedance, hydrostatic weighing depends on formulas which aren’t quite as perfect as we’d like them to be. Once again, there have to be variables, and there have to be constants.
The variable are all based on the subject (you), but the constants are based on data collected significant period that isn’t necessarily applicable to all people.
One of these constants is called “the reference body.” This is a value we assume is “fixed”, consisting of average and aggregate numbers based on measurements from over 50 cadavers taken over the past 150 or so years.
This creates a sort of strange baseline which isn’t going to be appropriate for the measurements of a good number of people. Notwithstanding the fact that the average body size and dimensions for “average” individuals are almost never really applicable to weight training people, it’s also true that those dimensions have changed over the past century and a half.
If that wasn’t enough, consider also that the “reference body” was comprised of people who were pretty old. It should go without saying that values based on aggregated measurements of sedentary elderly people aren’t going to translate too well to people who are active.
This is particularly relevant, because the vast majority of people who tend to measure body fat happen to be active. That’s a pretty big hole in hydrostatic weighing.
The other main problem with this technique is, once again, hydration. Essentially, if you hop into the tank having recently had a lot of fluid, your measurements will all be different—sometimes radically so.
We can’t do anything about the first problem. I’ve reached out to the scientific community, but it seems that they are unwilling to adjust the formula based on 150 weight trained cadavers between the ages of 25-50 anytime soon.
As for the second problem, the the hydration issue, once again, it’s the same deal: BE CONSISTENT
This process is completed three times and the weights are then averaged.
The average number is then placed into a special equation to determine body fat percentage.
I would say that despite the issues above, if you have access to hydrostatic weighing and don’t mind getting it done once a month, it’s a nice way to keep track.
Moving on, we have the Bod Pod, which is a strange looking, egg-shaped device that will help to calculate body fat percentage in much the same way hydrostatic weighing does: by using body volume as a metric.
While only a few people mentioned in the comments that they hit up the Bod Pod for their measurements, it’s a pretty common method of tracking. It’s also available at good number of gyms, and a most universities, so it’s worth covering.
As soon as you get inside what appears to be an escape pod, air pressure changes are used to determine the body volume of the subject contained in the unit.
Which is to say, before you get in, there is a premeasured amount of air, and you being inside displaces some of that are and changes the air pressure.
That gives the machine your body volume. From there, Body weight is divided by body volume, giving you body density.
Body density is then plugged into equations similar to those that hydrostatic weighing uses. Whereas HSW uses Archimedes’ Principle, the Bod Pod uses Boyle’s Law, which is the relationship between volume and pressure.
Of course, that means that in terms of the equations…well, we run into all of the “reference body” problems that were seen with hydrostatic weighing.
It’s also important to note that the Bod Pod is also sensitive to changes in ambient air temperature and air pressure; in order to work most accurately, the Bod Pod should be in it’s own temperature and pressure stable room. Generally, gyms and universities that are equipped with tech like the Bod Pod is likely to be air conditioned, but it’s something to keep in mind.
I like them because they tend to give pretty consistent readings; because of that, Bod Pods have worked their way into the athletic world and are used in a number of professional sporting environments. In fact, the Bod Pod is currently the main method of body fat testing used at the NFL Combine.
The BodPod is pretty common and relatively inexpensive (usually 25 bucks for a session). If you’re using this method, make sure you get the reading done on the SAME DAY of every week, preferably at the same time. And, again, watch your hydration levels.
I’ll talk about the method that I consider to be the best: the Skinfold measurements. Okay, maybe not “best” but certainly my favorite.
I know, I know…sooo many people hate on skinfold testing, but I’ll explain my reasoning as well.