Yesterday, I was reading the label on a bottle of water.* It claimed that the human body is 66% water. I am naturally skeptical of precise claims made by bottled water labels, especially since I know for a fact that Calvin (of Hobbes fame) said that the human body was something like 90% water.
Granted, Calvin is wrong; I mean, he’s a cartoon kid, so how much can he know? So I poked around a little to see who was closer.
Seems like it’s probably the bottled water label – but I get a number of different answers. Depending on how I google, I see claims that the human body is anywhere from 50% to 75% water. Something tells me I could find higher claims if I looked for them
What’s the Straight Dope? What percent of Joe or Jane Average is made of water?
Depending on who you’re talking about, it could be anywhere in that range. Men and women, fat and skinny, short and tall – each factor contributes differently.
The problem is that there are (at least) two ways to count this. First, you could count up the amount of hydrogen and the amount of oxygen present in the various chemicals in the body, and then figure out how much water you could make from those H and O atoms. I think that this gives you about 90%. On the other hand, you could also just count up how much of that H and O is actually in water molecules. In this case, I think that the answer is closer to 70%. The difference arises because a protein molecule, say, contains both hydrogen and oxygen atoms, but the protein itself does not contain water.
Both numbers in this post are the product of my unreliable memory, but the principle should be correct.
I had a feeling that 66% was too specific to be meaningful.
If we assume that we are talking about water molecules, as opposed to hydrogen and oxygen atoms – what’s the point? Is there anything useful to be gained by knowing what percent of your body is water, or should you just remember to drink when you’re thirsty or when your pee is really yellow?
Oh, that’s easy. Consider that each atom is (in a bizarre quantum mechanical way) basically space with a minute amount of stuff thrown in. The human body is made up of atoms, hence it is 99.99% empty space.
Interrobang!?, they were probably approximating 2/3 by saying 66%. So I wouldn’t discount it as being too exact
Myrr21, I think that is what he was after, but you’d better add some 9’s to the end there. IIRC, if the nucleus of a hydrogen atom were the size of a basketball, the electron cloud would be about the size of a football field.
This depends on how you define empty space- the wavefunctions of the electrons aren’t localized, so there’s a non-zero probability of an electon being at any given location in the body. By this standard, 0%.
This is at least as meaningful (which is to say, not very) as the 99.99% answer.
Back to the OP which I don’t think was ever answered accurately.
Women are ~ 50% water by weight.
Men are ~ 60% water.
Someone asked if this was useful information. For me, yes. I use these factoids on an almost daily basis to assist in calculating fluid and electrolyte replacement for my patients.
If you’re allowed to count individual atoms and then reconfigure them into any particular molecule you wish, then you do get a high percentage of water.
But arrange them differently and you could get equally high percentages for alcohol, sugar, diamond or even chocolate chip cookie bake mixture.
Yeah, as useless as knowing the exact body composition of water may be to many of us, it seems really odd to make the existance of water up. I mean, its not like we have a great shortage of it inside, right?
I’m pretty sure the Calvin & Hobbes strip that that was in contained one of Calvin’s fantasies in which he became 90% water and thus liquefied. I think it had to do with him not being able to go to the bathroom/drinking too much water on a car trip, IIRC.
Wow, this New Yorker is being really useful for Straight Dope things. This article is about an influential surgeon named Dr. Francis Moore, and it just happens to say that he calculated that on average, an adult male is 55% water, an adult female 50%. A very skinny person, it goes on to note, can be as much as 70% agua, so you have a rough idea of the distribution there as well.
The best way to answer this is to phrase it differently - why is a woman made up of of more fat than a man?
Women have more fat, hence less water. The reason they have more fat (on average) is due largely to the effect of estrogen to promote fat synthesis (and conversely, their relative lack of testosterone to promote muscle growth). Muscle “holds” water, fat doesn’t.
