Q: I have a question concerning the terminology 'body fat' that we use. Say that a woman weighs 100 pounds (for the sake of an easy number) and she is 20% body fat. That means that she has 20 pounds of body fat on her...what about the other 80%? Also, is there a way to show you how much muscle one has??

A: The other 80% is bones, muscles, organs, water, blood and other fluids, tendons, ligaments, etc. etc. There's no precise way to know how much actual muscle mass one has, so what people usually do is talk in terms of body fat (the 20% in your example) and lean body mass (the other 80%). Sometimes the mistake is made in assuming that all of the other 80% is muscle, which of course is not true.

We do know that the majority of long-term weight loss or gain is a mix of lost or added fat and muscle (water weight changes are only short-term and changes in bone and organ weight is relatively minor compared to fat + muscle) which allows us to be able to judge how successful one's weight loss or weight gain is by comparing the changes in fat and muscle mass.

For example, suppose a man starts at 240 lbs and 25% body fat (60 pounds of fat total), loses weight, and ends up 180 lbs and 20% body fat (36 pounds of fat total). His total fat loss is then 60 - 36 = 24 pounds. But he lost 60 total pounds of weight, which means that he lost 60 - 24 = 36 pounds of muscle (roughly, of course. This discounts changes in bone, organ, etc. weights). I would consider this weight loss to be not very successful since he lost more muscle than fat. For somebody overweight like this man was, he should be able to have 70% or more of lost weight being fat.

I do want to reiterate that all these measurements are quite imprecise. The golden standard of body fat measurement, the underwater weighing, has 1% statistical error. The caliper measurements can easily be off by 2-3% even when done by a pro and various bioimpedance devices (scales and handhelds) are almost a complete crapshoot. Water weight can also vary GREATLY in short-term -- hence all the 'miracle diets' which 'lose' 10 pounds in a week. Those, by the way, are just about the worst thing you can do to yourself. So one must keep all these facts in mind when talking about body fat measures.