Medical Water content of the human body

[CRGreathouse]

I was wondering how the total amount of water in a typical human varies by weight and sex.

I saw a site claiming that the typical male/female water percentages by mass were 62% for men and 51% for women; fair enough, though I've seen higher figures. (I'm more concerned with the ratio between the two than the absolute level, in both cases.)

It also says (understandably) that the water percentage decreases as BMI rises. The particular figures it gives, though, are hard for me to swallow. Consider a range of people with identical height (or a single person gaining or losing weight), so that their weights are proportional to their BMI. At the obesity threshold, the water weight would be 30 * 48% = 14.4 units; at the morbidly obese theshold, the water weight would be 40 * 36% = 14.4 units. That would suggest that gaining weight (at least past a certain point) does not increase the amount of water in the body, which seems questionable to me.

Does anyone have better figures? Or know of a database that records appropriate information to determine the answer?

 

[Borek]

My gut feeling would be the lower the BMI the lower the water percentage, as water resides mainly in soft tissues (is that an English term? Thats how these things are called in Polish).

 

[Andy Resnick]

You are asking a good question- the water content of various tissues has been measured (simply weighing and drying) way back in the 1930's.. I have a moldy book around here somewhere with the data... I did find this on wiki which is close to what I recall:

"Regarding specific tissues: Lean muscle tissue contains about 75% water by weight. Blood contains almost 70% water, body fat contains 10% water and bone has 22% water. Skin also contains much water. The human body is about 60% water in adult males and 55% in adult females."

Aside from whether or not the BMI is an accurate index of body fat composition, it's clear that increasing the amount of adipose tissue has a very small effect on body water.

But it may not be as simple as that- adipose tissue requires a blood supply as well. IIRC, losing a pound of weight corresponds to 75% fat and 25% blood/support tissue, but I can't find a reference that supports that. This abstract

http://jap.physiology.org/cgi/pdf_extract/23/5/621

points out that the ratio of adipose tissue to blood supply is not constant, and for obese subjects, the relative amount of blood supply is very small.

 

[Borek]

body fat contains 10% water

Surprise

Now I feel like if my BWI (Brain Water Index) was above average.

 

[CRGreathouse]

You are asking a good question- the water content of various tissues has been measured (simply weighing and drying) way back in the 1930's.. I have a moldy book around here somewhere with the data... I did find this on wiki which is close to what I recall:

"Regarding specific tissues: Lean muscle tissue contains about 75% water by weight. Blood contains almost 70% water, body fat contains 10% water and bone has 22% water. Skin also contains much water. The human body is about 60% water in adult males and 55% in adult females."

Aside from whether or not the BMI is an accurate index of body fat composition, it's clear that increasing the amount of adipose tissue has a very small effect on body water.

But it may not be as simple as that- adipose tissue requires a blood supply as well. IIRC, losing a pound of weight corresponds to 75% fat and 25% blood/support tissue, but I can't find a reference that supports that. This abstract

http://jap.physiology.org/cgi/pdf_extract/23/5/621

points out that the ratio of adipose tissue to blood supply is not constant, and for obese subjects, the relative amount of blood supply is very small.

Great, thanks for the info! That actually makes sense -- much more than the page I was looking at, at least.

So for every additional mass unit of adipose tissue, the total water content should increase by 10% (1*0.1) to 25% (0.75*0.1 + 0.25*0.7) of a unit. A 150-pound male who is 60% water would have to gain 90 to 225 (!) pounds to increase his constituent water by a quarter.