garbage in, garbage out

January 25, 2006 3:36 PM

A question about body weight, intake, and output. It requires an experiment on your part, so thanks in advance for your participation.

Experiment:
1. When you have a full bladder and bowel, weigh yourself.
2. Evacuate aforementioned fullness.
3. Weigh yourself again.
4. Drink 16 oz water
5. Weigh yourself again.

What, if any, is the difference in your weight? When either my husband or myself do this, using several different scales, our weights are always EXACTLY THE SAME, a phenomenon of which I must ponder, WTF yo? How is that possible?

posted by ferociouskitty to Health & Fitness (21 answers total)

The answer is most likely that your typical bathroom scale is not very accurate. The weight of the waste is likely less than the sensitivity of the scale.
posted by Justinian at 3:38 PM on January 25, 2006

Response by poster: *All of the scales that we've used have been digital scales measuring in 0.2 pound increments. Also, if the experiment is done HOLDING the 16 oz glass of water instead of consuming it, there is a predictable 1 lb weight gain. Somehow I always leave out important details when I ask questions 'round here.
posted by ferociouskitty at 3:42 PM on January 25, 2006

Just because a scale has a digital readout displaying 0.2 lb increments doesn't mean it's accurate to 0.2 lbs, or that it's any more or less accurate than a scale with an analog needle, it's just easier to read.
posted by TimeFactor at 3:53 PM on January 25, 2006

why don't you weigh yourself while drinking the water?
posted by borkencode at 4:01 PM on January 25, 2006

Er, the glass, if it's real glass, weighs non-zero amount. In terms of the question itself, The Merck Manual on Diarrhea says "In Western society, the stool weight of healthy adults is 100 to 300 g/day". That's between 3.5 and 10.5 oz. And based on other pages, it says that urine output should be between 500ml/day and 2.5l/day. It seems like, based on Volume of Human Bladder, that 300ml or so would be a good guess for a "full" bladder in most cases. 300ml is 10 fl. oz.

So that gives a range of 13 to 25 oz, more or less. Which is pretty close to 16 oz.
posted by skynxnex at 4:09 PM on January 25, 2006

Ferociouskitty: The weight of the glass added to the weight of the water is probably enough to register.

Try this: Void all waste. Weigh yourself. Get a 16oz glass of water. Hold it and weigh yourself. You'll probably get a reading of an extra pound. Drink the water and weigh yourself. From what you've said, you won't weigh an extra pound. NOW: Pick up the empty glass and weigh yourself. Presto, the extra pound comes back.

The weight of water+glass is probably enough to trip the sensitivity but the weight of the water alone is not.

(See TimeFactor's answer; the scale is likely not accurate to within 0.2 lbs despite having those increments. It's probably accurate to a pound or two).
posted by Justinian at 4:11 PM on January 25, 2006

Response by poster: Plastic cup. Weighs 2 oz, according to my food scale.
posted by ferociouskitty at 4:18 PM on January 25, 2006

Best answer: I can't find my source for this, but I once read that some digital scales cheat to give a false impression of accuracy: if you weigh yourself three times in a short space of time you get back the same result each time -- literally the same result. ie. multiple readings within 1lb or 2lb or whatever are all reported as the same.
To clear this 'cheat mode' you would need to weigh something of a different weight -- eg weigh yourself, get your husband to stand on the scales and reset them while you drink your water, and then get back on.
Let us know if this works!
posted by nowonmai at 4:33 PM on January 25, 2006

well, i dont have a scale that registers in 10ths, but i have seen 1lb difference between full/empty bladder and full/empty colon...
posted by joeblough at 4:38 PM on January 25, 2006

I think nowonmai is right about digital scales having a "memory". Sometimes I weigh myself then, say, pee or take off my jeans or whatever and then reweigh myself and see that the weight hasn't changed. To get around this, I pick up one of my fat cats (~15lbs), step on, and the differential seems to be enough to knock out the previous reading. Put down the cat, step on again, and the weight will indeed have gone down.
posted by ch1x0r at 4:39 PM on January 25, 2006

The basic problem is what you're looking for is way beyond the accuracy of most (any?) home consumer level scale. Even if the scale reads to XXX.2 doesn't mean it is actually accuracte to +/- 0.2 pounds.
posted by 6550 at 4:45 PM on January 25, 2006

And to answer your question, my scale, which reads to XXX.5 showed me weighing the exact same number without holding the glass and with holding the glass with 16 oz of water. I didn't bother to drink it and weigh again without holding the glass.
posted by 6550 at 4:49 PM on January 25, 2006

Yeah, do what borkencode suggests.

Weigh yourself while holding the glass of water. Now, without stepping off the scale, drink the glass of water. Keep holding the glass. Did you magically lose a pound by drinking the water in the glass?

Now do another experiment, just in case the scale just isn't that accurate: Weigh yourself. Now drink 72 ounces of Dr. Pepper and weigh yourself again. Difference? Now pee out the Dr. Pepper (or induce vomiting for quicker results). Weigh yourself. Difference?

Also, a lot of digital home scales work this way: When you turn them on, they calibrate to zero. When you step onto them, they weigh you and then lock onto the weight. The weight they display will not change unless you step off and restart the scale. If yours is one of these, that could be your problem.
posted by JekPorkins at 5:01 PM on January 25, 2006

Response by poster: Actually, the scale I use most frequently was about $120 and claims to be accurate to half a pound. I checked nowonmai's answer as best because the other 2 scales we have do have this effect. Still a mystery though.

Can't try borkencode's experiment, b/c all the scales calibrate to zero when turned on.

So, uh... did any of you actually try this before commenting? Just curious.
posted by ferociouskitty at 6:20 PM on January 25, 2006

I performed a similar experiment less than a month ago.
posted by JekPorkins at 6:31 PM on January 25, 2006

This certainly doesn't address the water discrepancy, but it's quite possible that emptying certain gaseous contents of your bowel, which are lighter than air, leaves you heavier than before.
posted by Eater at 6:35 PM on January 25, 2006

I do this for intellectual curiousity in the morning. The scale I use (reads to .5 lb) seems to be relatively reasonable for reconciling output loss, but I have stepped on it, gotten one reading and then after a short pause stepped on it again and gotten something different by up to 1.5 pounds.

Interestingly enough, its deltas are accurate because in the past 10 days, it's shows a delta of -2.5 pounds which was corroborated by being weghed on the same scale in a doctor's office in the same interval.
posted by plinth at 8:25 PM on January 25, 2006

Eater: Are you suggesting that unreleased gas has the potential to "lift" the body ever so slightly!?

This must be investigated! How much gas would it take to generate a pound of lift? How bloated would one have to be to shed (temporarily, of course) 10 lbs? Is it even possible, or would you rupture before that point?

Fascinating.

This is going to keep me up at night.
posted by aladfar at 9:31 PM on January 25, 2006

Hmm. A liter of helium has about a gram of lift under standard conditions. Methane is the lighter-than-air component of digestive gas -- according to the back of this envelope, it should provide 1/4 as much lift as helium. So you'd need to accumulate four liters of it in your tummy to take off a gram of body weight, or 1800 liters to take off a pound. Highly impractical as a weight loss strategy -- you'd be better off adding regular helium supplements to your diet, or, better, hydrogen.
posted by Eater at 6:52 AM on January 26, 2006

Upon waking: 222.2 lbs.
After urinating: 218.8 lb.s
After drinking ~16 oz. of water: 219.6 lbs.
After showering (damp short hair): still 219.6 lbs.

I used a Tanita digital bodyfat monitoring scale and I'm known to have a fairly large bladder. :)
posted by xiojason at 9:05 AM on January 26, 2006

The first three weighings were within a span of two minutes. The last two weighings were about 10 minutes apart.
posted by xiojason at 9:06 AM on January 26, 2006