If you’re concerned about good health, your sheer poundage isn’t nearly as important as finding out how much of you is made up of fat. In other words, you need to spend less time thinking about your weight and BMI, and more time concentrating on your body-fat percentage—the weight of your fat compared to the total weight of your body.
Everyone has a certain amount of essential fat stored in small amounts in organs, bone marrow, muscles, and the nervous system. This fat supports the normal functioning of these systems. Women have a bit more fat in the breast, pelvis, hips, and thighs, which is a prerequisite to making babies. Along with this bare minimum, it’s important to have at least a little more fat to use as an energy reserve, so you don’t collapse the next time you skip breakfast.
Unfortunately, the only place you can get a foolproof measure of your body-fat percentage is on an autopsy table. Some other techniques are nearly as good, but require the work of professionals and expensive hospital equipment. A few are much less accurate, but can be carried out at home. Here’s a quick roundup of the ways to measure your body fat:
• Imaging. An MRI (magnetic resonance imaging) machine can peer under your skin to create shocking scans that show the amount and distribution of your body fat.
• Hydrostatic weighing. Some specialized laboratories (and a few health clubs) have water tanks that are designed for underwater weighing. This technique works because fat isn’t nearly as dense as muscle and bone (which is why well-padded people float more easily). Using a bit of math, you can combine your underwater weight with your normal weight to get a fairly accurate measurement of your body-fat percentage.
• Skinfold measurements. To perform this test, a professional painstakingly pinches the folds of fat in various places on your body using calipers. It’s not particularly accurate, and it doesn’t sound like anybody’s idea of a fun Friday night. However, it works well if you just want to monitor changes in your body fat, as long as you get the same person to administer the measurement each time.
• Electrical conductance. This test sends a faint current through your body, calculates your body’s electrical resistance, and uses the result to estimate your body-fat percentage. This works because muscle contains a lot of water, and so conducts electricity quite well, while fat does not. This technique is used in hospital-grade equipment that costs tens of thousands of dollars, and in cheapie department-store versions, which look like ordinary digital scales.
Source of Information : Oreilly - Your Body Missing Manual