Underwater weighing and Bod Pod apply the same general principle of densitometry (density measurement) to estimate body fat. However, to do so, the former uses water and the second uses air displacement to estimate body volume. In both situations a person's weight and volume is used to determine their density (density = mass/volume), which in turn is used to estimate percent body fat.
Underwater weighing has been done at universities for decades and is still considered one of the "gold standards." Since our body is about 60 percent water, this weight would be negated when we are submerged in a tank of water. After removing as much air from the lungs as possible, the remaining body weight underwater is largely attributed to the relative amounts of body fat and nonfat or lean body mass (LBM). A person with a higher percentage of body fat will be less dense and thus a little more buoyant than a leaner person who weighs the same. Thus the person with the higher body fat level would actually weigh less underwater than a leaner individual of the same body weight.
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