T Nation

Jason N.: More On Impedence Devices

Jason: Just some clarification. As I understand what you’ve posted in the past, impedence devices (e.g. the Tanita) measure the FAT-FREE compartment of our body (water, muscle and bone). They do not DIRECTLY measure fat per se, is that correct? Two other questions:

  1. If they do not DIRECTLY measure fat, does that mean that they extrapolate our fat percentage from the difference (roughly) between our fat-free compartment mass and our TOTAL weight? (All impedence devices that I’ve seen either weigh TOTAL weight directly (Tanita) or you plug your total weight in (Omron).
  2. Any theory as to why they appear to become more accurate and reproducable (at least in my case) the closer you come to your “ideal” weight and/or as you lose fat?

Just need to bump this up so it doesn’t go into the “Testosterone Black Hole…”

Mufasa - Bioelectrical impedance devices measure the body’s resistance to an electrical current. If you look at a Tanita scale, you see 4 metallic electrodes. Two of those electrodes generate a voltage and two of the elctrodes measure the residual voltage which is left over after passing through the body. The ratio between the starting and ending voltages is impedance. To figure out how to convert impedance values into a percent body fat, validation studies were done with Hydrostatic weighing or preferably multi-compartment body composition models. Thus teh impedance value along with things like weight, height, and sex were used to create a coplicated algorithm which converts all these factors into a %body fat. OK, that may sound reasonable, but here are some problems that we run into when we use these scales.

The problems are primarily affected by changes in total body water (TBW). TBW is assumed to be about 73% of your fat free mass (FFM). Any changes to this quantity of water or to its electrolyte content will change your impedance and thus change your value for %body fat. So you answer to question #1 is that these devices really don't do either of your suggestions, but they clearly do not measure fat directly. In reality, it is much more like the second answer because most of the changes that cause the machines to vary occur in the FFM compartment. Basically if you assume that this device is measuring your TBW, and that TBW is 73% of your FFM, it calculates FFM and subtracts that value from your body weight leaving you with a measure of your fat mass (FM). FM/Body weight = %body fat.

Impedance can also be altered by changes in skin temperature, skin moisture, body posture and much more. So to get the most reliability, you need to find the time of day where your skin temperature, TBW, skin moisture is the most consistent. If you don't work out in the morning, then it is pretty easy. If you have a Tanita scale, wake up, go to the bathroom and hop on with no clothes on or with the same type of clothing on every single time. Clothes add to your weight and thus could artificially change the values. After you have this measure, get another measure after you have taken a shower in the morning and have completely dried off (hair also). Track the reliability over a couple of weeks and then just use the one with less variability.

In response to question #2, the more fat you lose, the more homogenous your body becomes. Impendance works great when measuring simple homogenous solutions which are evenly distributed. Humans are clearly not an evenly distributed homogenous solution. Thus if you get rid of fat and increase muscle mass and body water, you move towards a more evenly distributed homogenous structure. Just an idea. Let me know if you have other questions and if you are looking for a way to figure out when to use the scale, let me know a few details about your training schedule and diet.

Jason: As always, your explanations are OUTSTANDING! It all makes sense. I think that with the Tanita (or any impedence device), you really have to be more consistent with water intake, time of day, etc. when you utilize the device. Like any technology, I would think that the impedence device manufacturers are always looking at how to improve their devices (remember the Home Computers of the 80’s compared to now?)


Without sounding like I’m beating a dead horse; I utilize these devices to monitor TRENDS. However, from what you’ve pointed out, with a little dilligence on our part, we can improve the accuracy and precision of the instruments dramatically.


By the way…I’m in the middle of a New Year’s diet and workout revision. (I like to look at where I’ve been and where I would like to be at this time next year). However…I’ll keep you updated on how things go with my impedence devices (Tanita and Omron), ESPECIALLY as I continue to lose bodyfat. Again; THANKS!

You can’t improve the accuracy of these instruments, the accuracy was determined by the manufacturer and is only possible if you calibrate the scale as tight as possible. What you can do is increase the repeatability, making it useful for tracking trends.

I think I worded what I wanted to say badly. I’m not suggesting that we can change the accurracy and precision of the instrument, but can obtain more reproducable and “closer to true” measurements by being more consistent with WHEN we measure, under what CONDITIONS we measure, and at simlar levels of hydration. (I GUESS we’re saying the same thing???).


Well…hopefully everyone gets the point!