In my textbook "Physiology" by 'Linda S. Costanzo' its written that total body water is the amount of fluid present in the body. So my question is that is it just the water content it means by "amount of fluid" or it means amount of liquid content within the body (I.e. water, oils etc.)

Secondly, i have no idea that why is it inversely proportional to fat content. Is it because the decomposition of fat release relatively lots of water molecules and thats why its inversely proportional.


1 Answer 1


Total body water is the total amount or percent of one's body that is comprised of water. It isn't the same as total body fluids, but on careful examination, it's close. For example, the average adult has a blood volume of about 5 liters (there are formulas to calculate more accurate numbers.) Blood is a fluid. It is normally about 55% plasma with the rest being mostly red blood cells. Plasma (also a fluid) is ~90% water and red blood cells are about 70% water (estimates vary slightly), So while blood is mostly water, blood (fluid) is not equivalent to water.

Most cells have a fairly high percentage of water. The exception is fat cells/adipose tissue. Whereas muscle is ~70% water, adipose tissue is only ~10% water.

Therefore, if, say, 70 pounds of lean human tissue is (say) 70% water, and 70 pounds of (white) fat contains only 10% water, a 140 pound lean/muscular athletic male with 10% body mass comprised of fat will have more total body water (TBW) than a sedentary male of the same weight but with more adipose tissue. That's why the more fat your body is composed of, the lower the TBW.


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