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On Bodyfat Measurements: Three Methods of Note

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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.

Bioelectrical Impedance

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.

How Impedance Technically Works

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.

How They Really Work

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:

  1. Go pee-pee immediately upon rising.
  2. Drink 1000ml (2 ‘regular’ sized bottles).
  3. Wait until you go pee-pee again.
  4. Wait 5 more minutes.
  5. Take measurement.

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.

Hydrostatic Weighing – the “Gold Standard”?

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 = (MAMW)/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:

  • Brozek formula: BF = (4.57/ρ − 4.142) × 100[1]
  • Siri formula: BF = (4.95/ρ − 4.50) × 100[2]

(Note: I prefer the Brozek formula because it has the word “bro” in there. True story. Take that, bro-science haters!)

Science Winz?!

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

Holy Crap that was Long, How about a Summary? Sure!

How Hydrostatic Weighing is done

  1. Weight is measured on land with as little clothes as possible to assure accuracy
  2. Subject gets into a tank of water with a specially designed scale
  3. Subject expels as much air as possible
  4. Subject is completely submerged in water
  5. The underwater weight is measured

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.

The Bod Pod

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.

 

Next Time…

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.

Does this change your idea about which method is best for you? Next, I’ll detail my PERSONAL method for testing bf% with my clients!

About the Author

John Romaniello is a level 70 orc wizard who spends his days lifting heavy shit and his nights fighting crime. When not doing that, he serves as the Chief Bro King of the Roman Empire and Executive Editor here on RFS. You can read his articles here, and rants on Facebook.

  • Antonio Talluri

    Hi
    I think that the problem is shooting at a wrong target.
    Body fat should not be taken into consideration. Life and health derives from cells and water.
    Bodycomp goals should be purported to identify hydration states and body cell mass adequacy.
    Impedance can provide reliable indexes. ( Refer to BIVA)
    But who wants to learn more?

  • Reka

    Why do the scales ask for gender and age? They should give you the estimate based on your body alone, otherwise they are just guessing / tampering the info. I find this ridiculous and I can’t trust those scales. Definitely not reasonable, especially for athletes.

  • Chris

    No DEXA love?

  • Christian

    Real excited about the next post. I personally use calipers and while they are kind of a pain in the… Donkey, they are by far the cheapest way

  • Noel

    Thanks for the info. Curious to what you think about BMI? And school children BMI or body fat measurement as part of PE/Wellness?

  • Felix

    Very interesting I look forward to hearing what you think is the best way to measure body fat

  • Joseph Piscitelli

    Roman, I used to use the calipers, until about 2 years ago. It’s interesting, the calipers tell me that I’m about 9%, while the bio-impedance register at 15%. I think that I am somewhat in the middle of the two, as I have a well defined six-pack.
    I don’t quite know what to believe, so I just keep an eye on the numbers.
    P.S.
    I try to keep my water at about 60% on the bio-impedance.

  • Joseph Irons

    WOW! You are wicked smart… Help me obiwan you are our only hope

  • Ryan Graczkowski

    Well, it doesn’t change my mind so much, but it does change my approach. I was using a bio-impedance device to measure, but I’ve really been struggling with getting consistent results. I think your way will really give me something to work with as opposed to trying to get an average measure of some kind.

    • bodymass12

      Thanks for sharing your information.

      If you want to measure your body for knowning that either you have gain the weight or loss the weight then use the Body Mass Index, It give the accurate result.

      http://healthoffers24.com/body-mass-index

  • James

    Hey John, you recently posted that you were working with Uriah Hall as a coach (nutrition?) – I was thinking it’d be very interesting if you write an article about nutrition principles for high level athletes with weight restrictions, high workload etc… cheers :)

  • Noelle

    Bod pod makes me happy, the tanita at my local health food store makes me sad…. ;)

  • Roman – any thoughts on BodyMetrix? (the one Tim recommends in 4HB)

    • Phil

      Have been using BodyMetrix for a while. It is very good and less invasive then calipers (although always have my Harpenden with me in case). All my clients prefer it. I have a few hardcore client for which I do the scan of the muscle so that we can actually see the evolution over time.

  • Reese

    Good info, although did you know your new blog posts aren’t showing up on your ‘blog’ page? The newest one shown is still the back-squat post. I had to search from Google to find this post.

  • John Balash

    I have the electrical device and it doesn't seem all that accurate. My bodyfat is consistently about 13% according to this even though I know I've lost fat and gained muscle. I could've saved about $20 and bought the calipers.

  • Darren

    Had issues commenting on the 1st part of the blog but i use the skinfold calipers and measure the lilac crest, supraspinale and abdominal areas as i carry most of my fat there. Other areas are a lot thinner, so my bodyfat can range from 11% to 23% in measurement…so learning the proper way to do it would help greatly!

  • Lloyd Alaimalo

    Great post Roman! We use the tape measurement method in the Army: neck and waist for males, neck, waist, and hips for females. Three measurements are made, the average is calculated, then these measurements are plugged into an electronic army form that automatically calculates body fat %. Like you said, I try to use the same person to take my measurements. When I dont, the numbers fluctuate. I also use the mirror too.

  • Brain dead Phillip posting again. Can't remember if I posted on the previous blog on body fat testing but I use two methods to test my BF. Warrior calipers and Impedance, also have some straight forward skin fold type calipers. The “Acue-measurement” calipers chart does not go low enough for my age category (63) so is useless unless I take measurements and then go to the web for a formula that will do the calculations. My gym used a 3 location skin fold formula and the result was 11%. About the same time as gym results I bought a “weightwatchers” scale and it's calculation was 22%. As a result of this difference I usually consider the scale results to be twice what the skinfold testing says. Now a year later than the above results I have a “Warrior calipers (digital) and measure 6.7% BF and my weightwatchers scale measures about 14% so my original observation is holding true. I had not realized that first thing in the AM was not the best time to measure with BIA so I'll do some testing at another time as I do my businesses in the bathroom before I step on the scale. I believe you made a YouTube video on using a scale and remember that you wet your feet and I have tried that and it did make a difference.

    Bottom line is that if I don't think the 2 above methods are telling me the correct story on my BF the mirror is what I use and a pinch with my own fingers. If the flesh on my abs seems mostly skin then I'm a happy camper!

  • Deb

    Interesting information Roman. Personally I probably couldn't afford money- or time-wise to do any of these tests!

    I have a set of calipers and do a skin-fold measurement at the suprailliac. It is gradually decreasing so I figure I'm making progress, but I am skeptical about whether the percentage is correct. I am very lean on the top half of my body, and like most other women, tend to store fat in my butt and hips. The calipers say I'm only 16% bodyfat, but I think it's inaccurate! What are your thoughts here? Is a multiple site reading better?

  • Marie

    Thanks for the info. I'd like to give a BodPod a try one day, just to see how it works (and I could fantasize I'm going into Outer Space to thwart evil thanks to your awesome Super Hero workout).

    ;)

  • Rob

    Haven't checked into a bod pod for a couple years. Last I checked they were still not all that common. I live in Harrisburg PA and the closest one available at that time was at a university in Philadelphia. I like what you said about the accuracy of the numbers not being as important as seeing a change in the numbers. I don't always trust that the numbers on some of those pieces of equipment at the gym are really weight in pounds but it doesn't really matter if they're increasing. It's not about bragging, it's about improving.

  • Nat Jeffery

    I find that the mirror gives me a great evaluation. (As long as it is the same mirror.)

  • Clare

    Are my pants tight?

    That's my method!

  • EGA

    Great post; thanks for all the research!

  • Good basic information Roman, but I want to clarify a few things. The BodPod uses the same concept as underwater weighing but uses Boyle's law which is the relationship between volume and pressure. Archimedes principle gives us body volume and body density but the Brozek and Siri equations give us body fat from body density.

    ALL body fat equations, such as the Brozek and Siri, or skinfold equations are either based on cadaver research (reference man or women) or have been generated from a population using a criterion technique such as underwater weighing.

    Most skinfold and bioelectrical impedance devices use equations generated from underwater weighing.

    The BodPod and underwater weighing both assume a constant hydration status just like BIA (73%) and constant bone mineral content.

    Because the largest portion of our fat free mass is water, this 73% assumption gives underwater weighing and the BODPOD the same error as good BIA or skinfold equations, which is typically around +/- 3.0 to 4 percent in groups, but as large as 5-6% for individuals. Meaning if underwater weighing or the BodPod gave you value of 20%, your true percent fat is actually between 15 and 25%, but most likely between 17 and 23%.

    If you use the wrong equation or a cheap body fat BIA scale you can almost double these values. Meaning the scale you picked up at Walmart for $30 showing 20% fat is off on average as much as 5 to 6% but can be as bad as 8 to 10% or more.

    DXA also assumes water to be 73% of your fat free mass, it cannot detect water, thus the DXA has the same errors as the BodPod and underwater weighing. Not to mention the DXA was never designed to estimate fat.

    Which is another point, we cannot “measure” body fat unless we kill you and dissect you, we can only “estimate”.

    The best methods are multiple compartment models that estimate body water in addition to body volume. 3C, 4C, 5C, 6C, but that is heavy science stuff.

    Tracking changes is another problem because most devices are not sensitive enough to detect small changes, in fact, if you have not lost at least 5 to 10 pounds, most devices or techniques will not be accurate because you have not exceeded the technical error of the device, which is in addition to validity error. This is called reliability, which can add another 2-8% error when tracking changes.

    There are many things to consider when estimating body fat, but at the end of the day, what most people have access to are methods just ass good as underwater weighing or the BodPod, such as a good BIA and equation and a well done skinfold test and appropriate equation.

    I hope to see more Roman, I would like to see you go deeper into this.

    I will end with a suggestion. Why do we need to estimate %fat? We know if we are fat or not, why not just use raw values like the ones you get from a tape measure and skinfolds? Everyone want to plug raw values into an equation which just gives you an approximation of total body fat.

    To get a true fat analysis costs hundreds and hundreds of dollars and even then you are still a few % off from your real fat.

    If you measure your biceps and it is 35 cm and a biceps skinfold is 15 mm, you train for 4 weeks and measure again and your circumference has changed to 36 cm and your skinfold is now 12 mm, guess what? you just added muscle and lost fat at your biceps and it cost you less than $50. You can do this all over your body, which is way better than general total body %fat.

    Dr. Moon, PhD

    Body Composition Researcher

  • IT

    Hi Roman

    Your example of the the guy who had diarrhoea where the %Body fat drop from 14% to 9% was because he was very dehydrated after diarrhoea. This is not true. With BIA, the more dehydrated you are, the higher the reading for %Body fat as you mentioned less water give more impedance the the reading.

    BTW, the best time to measure is not in the morning but late afternoon, 1 hour after drinking 1 glass of water,

    Cheers

    IT

  • Frank A

    Hi Roman,

    I conducted hydrostatic weighings while at college. Water temperature makes a difference also the formula doesn't consider bone size and density. And n ot many people really like to expel that much air.

    There was a report that an athlete – Herschel Walker? measured at a negative body fat. Don't think so… So there are pros and cons to any method. I'll comment on the skinfold caliper method next time.

    I like looking in the mirror and seeing how much muscle definition shows!

  • John,

    This is great and useful information for someone super serious about their fat loss goals. Thank you for sharing these tools.

  • Thanks for sharing!

  • @Dean: I agree with Roman. In the morning, you're in the fasted state. No volume of food will screw around with the reading. Also, you're probably not hydrated, so the drinking and peeing will stabilize the readings nicely. Finally, it's all about the timing of the measurements. If you measure your bodyfat levels at 6am every Monday morning, having just woken up, you'll probably get consistent readings. Once again, Roman talks about measuring more to gauge progress than to actually know where you're at, as there's always error.

    @roman: I still think visual comparisons are the best. To each his own, my friend! :-)

  • Neil

    Hi Roman.

    The content of this discussion is something I'm finding really interesting.

    I've been using a bio-impedence scale for years at home and have wondered how accurate it is. my scales read between 23 and 25% but if i use the tanita monitor at my gym it can be something between 18 and 23%.

    And then when i use estimators on the net they're totally different. e.g. i used one i downloaded with a fitness program it suggests my bf is 14%!!!

    none of the methods make sense to me and i have absolutely no idea what my bf actually is!

    i tend to use the mirror. if i look a bit soft i start fat loss mode. if i think i'm looking a bit scrawny, then its time to start eating more calories again.

    i think im going to give your body comp blitz program in the new year.

    i love your blog by the way, its very informative and very entertaining. not as dry as some others.

    keep the good work up. thanks

    Neil

  • @Dean – I have to disagree. Unless you're eating the exact same meals, or at least very similar meals at the same times, your hydration levels will have too much variance day to day.

  • Dean

    Roman, testing bodyfat% in the morning is not the way to go, Roman man. You'll be dehydrated which will give you a whacked out reading. Best time is 2 hrs. after your last meal of the day. And no alcohol on that day for you drinkers out there.

    Peace and chicken grease.

  • Ylwa

    Both Fred and I use bio impedance, since it's the only opition available at our gym. We do have one of those scales as well (not Tnita though, it's nnot available here), but have come to not trust it. For me, it's pretty consistent, but a few percentage units over what my BI reading says. Fred on the other hand, comes out at 17% compared to 9% on his BI. My parents have the same model and he gets the same results there.

    I've been looking at skin calphers more and more (problem is I don't have the education for it and noone else at my gym does either), so I'm looking forward to reading your next blog, since I've understood that they're quite a few error sources as well.

  • Ted

    Honestly, makes me not trust anything. lol.

  • Gerry

    Hey Roman, I always enjoy your posts. I'm keen to hear your view on the DEXA method in the upcoming post. I'm going for mine tomorrow and then for a second one at the end of January to get a view on the absolute numbers. In the meantime, I plan to track differences via my skinfold calipers which are in the post right now.