Got an email from somebody who wanted to know what that body percentage thing was all about–more specifically the fat part.
There are several ways for determining how much of your body is fat and not-fat. When people say "lean mass," that's not quite what they mean, because it includes stuff other than muscle–bones, organs, fluids, nervous and connective tissue, all like that. Not important here.
You can do pinch tests with calipers. There are tests in which you get dunked in water, electronic resistance measure, even MRIs, and you can buy a bathroom scale supposed to determine it if you stand on it barefoot. These vary as to accuracy. Again, the starting accuracy is less important than if it goes down or up.
The mirror test is still the best. You can't be fat and look lean. The abs are a rough measure you can use as a rule-of-thumb: Generally–not always, but generally–at fifteen percent body fat, you can't see 'em. At twelve percent, you can flex, and maybe see them a good overhead light, at least the top four and maybe the bottom pack. At ten percent, you can't miss them. At eight or nine percent, you don't need to flex them, you can see them whenever you look.
People who want to flash a six-pack (not a keg) do better to lose weight overall than to spend too much time working their abs. Doesn't matter how strong they are, if you are more than fifteen percent body fat, they won't show. Spot-reducing usually doesn't work in healthy people.
Ultra low body fat is not a measure of fitness per se. If you are anorexic or bulemic? Not so good. Too skinny can be as bad as too fat, despite the old adage than you can't be too rich or too thin. You can be at least one of those.
For the sake of an example, let's say that you weigh two hundred pounds and you have determined that your body fat percentage is, say, fifteen percent. What that means is that you have thirty pounds of fat and a hundred and seventy pounds of everything else. (There are formulas for separating out the everything else, but they don't concern us for the purpose of this discussion.)
Which means you can do ratios and determine how much you need to lose to get to what percentages. For this example: If you drop ten pounds and it's fat and not muscle, then you will weigh one-ninety, and twenty pounds of that will be fat. Divide it out, and you now carry a little over ten percent body fat.
If you stand in front of a mirror and flex your abs in the light, you will see them. Serious bodybuilders will try to come in at 6-8% body fat, and will look cut at those percentages. Below that, you start to look like a skinned chicken.
You can't get to zero and still walk around. A certain percentage is necessary for cushioning your organs, so a couple-three percent is the basement; you won't see this, it's not under the skin but usually in the thoracic cavity and in dribs and drabs in other spots.
There you you have the quick lesson in blubberology for the day ...