Body Fat Scanners


For my first post, I wanted to tackle something I know confuses a lot of people: home body fat scanners. Specifically, Sarah asked, “How exactly do those scales that tell you your body fat percentage work? Apparently I am 20% body fat and I am not quite sure how I feel about that considering I do not entirely trust the accuracy of the scale.” Well, Sarah, I too don’t have much trust in what these scanners have to say. Let’s do a bit of exploration into the topic to see if our skepticism is warranted.

To start, these home body fat measurement devices utilize an effect called “bioelectrical impedance” to work their magic. The concept of bioelectrical impedance maintains that different types of tissues in the body impede the flow of electricity differently, or have different resistances, allowing for their differentiation.  This current is then used to infer lean mass (the amount of non-fatty parts of you) from “total body water,” or the total amount of water in your body. Body fat, deposited throughout the body in adipose tissue, has very little water content; a vast majority of the body water lies outside of it.  This has to do with unfavorable molecular interactions between water and fat molecules, which we won’t get into here, that drive the water out.

Since non-pure water is an effective conductor of electricity, it stands to reason that the higher your “total body water,” the less bioelectrical impedance and thus the more lean mass you have. Once this lean mass amount is determined, it is simply converted to pounds and subtracted from your given body weight. This number then gives you the weight of fat in your body. Some quick division of that number by your total weight yields the body fat percentage. Simple!

This method can be applied in a variety of formats and the results can range from pretty accurate to wildly inaccurate. As you might imagine, accuracy and price go hand in hand on this one. Clinical impedance devices can cost thousands of dollars, while home devices can be found for less than fifty. Clinically, these devices were found in a study published in The Journal of Applied Physiology to have a margin of error of a little less than 3%, similar to other, more trusted body fat measurement systems. However, the systems used in that test were research grade and testing conditions were controlled as much as possible.

In general, one should not expect anywhere near that level of accuracy from even an expensive home unit. A wide variety of factors would need to be controlled, as they were in the study, such as recent food intake, exercise, and hydration levels. As reported by another article published in The American Journal of Clinical Nutrition, a change in hydration levels can have a dramatic effect on the reading. Specifically, dehydration can cause a net increase in the reported body fat percentage– again, the less total body water, the less lean mass, the more fat at your given body weight.

But, all is not lost for the home body fat scanner. Although not particularly accurate in comparison to other body fat measurement techniques, they do tend to be consistent in their measurements. They can be used over time to track changes in body fat percentage with better confidence than one could have in a single reading. Likewise, measurements taken over a span of time can be averaged to get a more accurate and useful figure.

In short, and to answer the question directly, the skepticism is valid if the device is used only once under uncontrolled conditions. At the least, one should use the home model in the manner most likely to present an accurate result (several readings taken on an empty stomach, fully hydrated, and several hours pre or post work out then averaged). If the result still seems off, exploring more proven but more involved methods such as densitometry (hydrostatic weighing), clinical bioelectrical impedance testing, or anthropometry (skin fold measurements) may be necessary.



  1. Mannat

    Great post Justin. At USC a class (EXSC 205Lxg) assignment required us to use skin fold measuring, bioelectrical impedance, and hydrostatic weighing. I found that the skin folds and hydrostatic weighing measurements were closer together and less than the bio-impedance measurement.

  2. Erin

    I actually took the same course at USC as the previous commenter mentioned (in addition to other courses covering similar material). I can’t remember my exact statistics, but I do recall that I was measured for body fat with the clinical bioelectrical impedance measure in addition to a home-use model similar to the one in the image at the top of the article. The home-use model reported that I had about 17% body fat whereas the clinical bioelectric impedance measure reported less than 10% body fat. The other measures fell between those two values. Needless to say, none of these methods are perfect for determining an exact amount of body fat; however, these measures can definitely be used to track progress and/or give a general idea of body composition

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