Fill in the blank: The human body is __% water.

[Ethilrist]

Adds new meaning to “you are what you eat.”

[Epimetheus]

*Originally posted by KarlGauss *
**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. **

I’m not being obtuse here or anything, but I was under the impression that most of the water in a human body was cellular water- water within cells. Do fat cells not contain any water? (I wouldn’t be surprised if not, but if you have a clarification or a description I would love to hear it, my knowledge of this area is obviously very fuzzy)

[barbitu8]

Other than skin calipers, there is another simple, quick (but not very accurate - more accurate than calipers, however) method to determine the % of body fat: bioimpedance. The basis for this method is that fat does not contain the water than lean tissue (muscle does). One of the drawbacks of this method is that if you are dehydrated (after a long run, for instance), you will get a higher reading than after you are well hydrated, giving a higher fat % than you actually have.

[KarlGauss]

*Originally posted by Epimetheus *
**I’m not being obtuse here or anything, but I was under the impression that most of the water in a human body was cellular water- water within cells. Do fat cells not contain any water? (I wouldn’t be surprised if not, but if you have a clarification or a description I would love to hear it, my knowledge of this area is obviously very fuzzy) **

You are absolutely correct. About 2/3 of the total body water is intracellular. Of the 1/3 that is extracellular, 3/4 is “interstitial” (surrounding the cells) and only 1/4 is intravascular. In other words, only 1/12 of your body water is in the blood.

Getting back to your question, fat cells do contain some water, sure, but it pales in comparison to the size of the entire, fat-filled cell. So, for a given body weight, the person with more fat will have less cellualr volume available for intracellular water to accumulate/distribute.

This is one reason why women have lower thresholds for getting drunk. Alcohol is water-soluble. For a given amount of ingested alcohol, then, women will have less water soluble space for it to distribute into (by virtue of having more fat cells). The net effect is a higher concentratation of alcohol in the (water-soluble) blood.

[Interrobang]

Originally posted by Epimetheus
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.

I should have been more specific and less snarky, although it’s interesting to know what that info can be used for. I was really wondering if there’s any practical reason that I, Joe Water Consumer, need to know how much water is in the average human. The bottled water label pretty much said, in essence, “The human body is 66% water. This is water. Have some.”

I’m sure that there are a number of real reasons for some people to know body water percentages – but I doubt that they’re as interested in the global average. (Mayhap they are, though, I dunno.)

[Interrobang]

Originally posted by KarlGauss, not Epimetheus
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.

I should have been more specific and less snarky, although it’s interesting to know what that info can be used for. I was really wondering if there’s any practical reason that I, Joe Water Consumer, need to know how much water is in the average human. The bottled water label pretty much said, in essence, “The human body is 66% water. This is water. Have some.”

I’m sure that there are a number of real reasons for some people to know body water percentages – but I doubt that they’re as interested in the global average. (Mayhap they are, though, I dunno.)

If this posted twice, it’s 'cause I was correcting the quote attribution.

[bonzer]

*Originally posted by Marley23 *
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.

To amplify on this a bit, Marley23 is refering to the article by Atul Gawande in the May 5th edition of the New Yorker. The overall thrust of the piece is that Moore was a pioneer in multiple fields of medicine by dint of the attitude that, in cases where the patient was likely to die anyway, any procedure, however experimental, was worth trying. His greatest acheivement was probably the first truely successful organ transplant: transfering a kidney between identical twins. Overall, the article struck me as a fascinating one about a period in the history of medicine about which I knew less than I thought I did.
Where the issue of the percentage of water in the human body comes in is that, relatively early in his career, he took a sabbatical to study in the infant field of nuclear medicine, i.e. the use of radioisotopes for diagnostic purposes. One idea he came up with was measuring the percentage of water in the body using heavy water. The test experiment was injecting a small known amount of heavy water into rabbits, letting it circulate for a few hours, then taking a blood sample. The amount of heavy water in the sample then enabled him to estimate the total amount of water in the rabbit. As a crosscheck, they killed the rabbits and desiccated them, to find out how much water they actually did contain. Having confirmed that the two methods agreed, they extended the deuterium experiment to humans, producing the numbers mentioned by Marley23.
One can no doubt quibble with the methodology, but Gawande makes a plausible case that this was the start of a significant field of biomedical research.
For all this progress, the overall tone of the article is distinctly tragic. An interesting read.