Why does my Tanita body fat monitor consistently show that I've lost approximately one percent of body fat after I shower?

January 9, 2008 9:54 AM

Why does my Tanita body fat monitor consistently show that I've lost approximately one percent of body fat after I shower?

First, the model I have is a combination scale and body fat monitor. It operates by standing on the scale. (This is only important to note that this is not a hand held unit and that the measurement is coming from my feet, not my hands. This fact may or may not impact the explanation I seek.)

I understand how the measurements are made and all of the caveats as to their accuracy. I am guessing that the fact that I am taking long, hot showers is one of the primary causes of the observed behavior - I just can't work out the details of why. Yesterday, for example, I stepped on the scale to measure my body fat percentage. The read out was 14%, which is on the plus side of what it should read, but within a reasonable margin of error. I then proceeded to take a long, hot shower and measured my body fat percentage again. Like clockwork, it read 13%.

There are other possible contributing factors to the readings. I generally weigh in after a workout, but before consuming any liquids or food. Intuitively, one might expect a lower percentage of body fat, but it is always higher than expected. The general correlation is that the longer the workout, the higher the percentage of body fat the monitor reads. After a long, hot shower, before consuming any calories or fluids, the longer the workout yields a larger drop in measured body fat.

I've repeated this test for months, even done several measurements before and after a shower to get a sense of the accuracy of the scale. The results are always as I have described here. What do you think the cause of this behavior is?

posted by sequential to health & fitness (11 comments total)

I'm assuming that this device via some kind of resistance check between the two terminals you are standing on. Since showering both removes oils and such from your skin, as well as allows you to absorb some water I'd imagine that it would change the resistance you present to the device.
posted by frieze at 9:58 AM on January 9, 2008 [1 favorite has favorites]

Man, that's some good soap you use.

Seriously, your body fat takes a long time to change. One day isn't going to do anything. One shower certainly isn't doing anything.

First: It's not terribly accurate. It can be useful to show trends over time.

But, you have to measure the same way every time.

What you're doing here is fiddling with the way it makes its measurements. The amount of moisture on the surface of your skin is skewing the results.
posted by cmiller at 9:59 AM on January 9, 2008

Second frieze on the resistance theory. According to Tanita, the scale sends an electrical impulse through your body, and the signal travels faster through lean muscle. After taking a shower, something (possibly water absorbed into your body) is making the signal travel faster, which is tricking the scale into thinking you have more lean muscle.
posted by burnmp3s at 10:19 AM on January 9, 2008

Bioelectrical impedance is fickle to all sorts of factors. Skin temperature can affect the measure of resistance (Caton, Med Sci Sports Exerc, 1988 ).

Poke around on google scholar for "bioelectrical impedance."

On a related note, what is the reliability of the scale? If you measure yourself 10 times in a row, do you get the exact same value each time?
posted by tiburon at 10:26 AM on January 9, 2008

stick with hydrostatic water testing for accurate measurements...
posted by dawdle at 10:27 AM on January 9, 2008

The general correlation is that the longer the workout, the higher the percentage of body fat the monitor reads.

Your hydration status, which is influenced by your workout intensity, will affect the impedance reading.

These scales work by taking an impedance reading and then using an equation that considers your sex, height, and maybe fitness level to estimate total body water, which can be translated into your fat free mass. With fat free mass and your total weight, it then calculates your % body fat.
posted by tiburon at 10:31 AM on January 9, 2008

For what it's worth, I have found lately (because I have been trying to monitor my weight) that I weigh a fraction less after my morning shower than before it. I presume that our results may be related.
posted by sueinnyc at 12:23 PM on January 9, 2008

When I want to get flatteringly low body fat percentage readings on my Tanita scale, I tense my legs, or hold a squat position. Give it a try!

The Tanita scale registers body fat by measuring electrical signals five times during the test and averaging the results. Tensing your muscles will create lower resistance. After a shower, I often suspect my clean feet reduce my body fat reading, too.

The Tanita scales are useful as an indicator of whether you're losing or gaining body fat, but the actual measurement itself may vary in quality from person to person. That's why there's a "youth", "adult", and "athlete" setting. This is also true of caliper body fat measuring: test practitioners will come up with different results on the same person due to exact location of caliper testing and the tension applied to the calipers. The gold standard in body fat measurement is the dunk tank, which can still be challenging for some who can't exhale completely and stay calm under water.
posted by lothar at 1:01 PM on January 9, 2008

Could it merely be due to loss of water b/c of minor sweating in a hot shower? I can lose nearly half a pound, seriously, in my gym's steam room in five minutes.
posted by Camofrog at 2:43 PM on January 9, 2008

The tanita scale works by sending a signal through your body and seeing how fast it comes back. The signal moves slower through water than through lean tissue, and since fat cells are mostly water, a slower signal means more fat.

You probably drink a lot of water during long/hard workouts; when you weigh yourself right afterward, your body fat percentage would appear to be up because of all the liquid you ingested. Then, if you're like me, you probably pee before you take a shower. Your measurement after your shower would reflect that loss of liquid, and your fat percentage would appear lower.
posted by Kololo at 3:05 PM on January 9, 2008

you know how when you lick a 9 volt, you feel a shock? (Part of) the reason is that your tongue is moist (and also more sensitive, too) and conducts electricity much better than your normal skin.

When you shower, your skin absorbs a lot of water and it will therefore conduct electricity better, throwing off the measurement your scale takes.

For what its worth, I have heard that measuring BMI using this method is notoriously inaccurate. the 1% difference is well within the margin of error. Even a much more involved test only yields 2 1/2% accuracy:


That number provides a guide to body fat with an error of 2 or 2 1/2 percentage points. But exercise and liquid intake before the test can skew results.
posted by jpdoane at 7:52 PM on January 9, 2008