Question: I recently had my body fat measured in a hydrostatic tank. I was surprised that the percent fat from this measurement was so different from the percent fat using electrical impedance that was done at a local health club. Which is the more accurate? Could you explain how you can determine percentage of fat from weighing a person in water? Thank you.
ANSWER: The body fat measurements we get from electrical impedance and from hydrostatic weighing are quite similar under the carefully controlled conditions in our lab. However, hydrostatic weighing is the "gold standard" and, when done properly, is the most accurate of the techniques normally used in the health and fitness setting to determine body fat. In fact, the formula used to determine body fat with electrical impedance is generated from comparisons with hydrostatic weighing measurements.The technique for determining body fat from hydrostatic (underwater) weighing is based on Archimedes Principle for determining density. Some 2,000 years ago, allegedly while bathing, Archimedes deduced that, when an object heavier than water is weighed in water, it will be lighter than its out-of-water weight by exactly the weight of the water it displaces. Since density is equal to the mass (or weight) of an object divided by its volume, we can calculate the volume by weighing this object in water.
For instance, what if you wanted to determine the density of a shot put (or other solid object) that weighed 7.0 kilograms in the air? If the weight of the object in water was 4.0 kg, it must have displaced 3 kg of water. Since a liter of water weighs 1 kg (at 4 degree celsius), the volume of the shot put is 3 liters. The density, then, is 7 kg (the weight of the object in air) divided by 3 liters (the volume determined by underwater weighing) or 2.33 kg/liter.
The same technique can be used with people. If a person weighs 70 kg in the air and only 2 kg in the water, he/she must have displaced 68 kg of (or liters) of water. The density of this person would be 70 kg divided by 68 liters or 1.03 kg/liter (much less than a shot put). Of course, weighing people is not quite as simple as weighing shot puts because we don't like to be weighed in cold water so the density of the water is never exactly 1, and there is always air in the lungs and in the digestive tract that makes the volume of the body larger than it really is.
However, when adjustments are made for these factors, the body density can be accurately calculated. The problem lies in converting the body density to percent body fat. The formulae for these conversions have been generated from the careful chemical analysis of about 10 to 15 cadavers. However, people are so different that these formulae don't exactly match any specific person, so there may be error in predicting the amount of fat from the density measurements.
We have recently done some interesting work in this area using the MRI at LDS Hospital to see if we could generate better conversion formulae. It is easy to see both the external and internal body fat with the MRI, but there are still some technical problems to overcome before new and more accurate formulae can be determined.
Merry Christmas!
Garth Fisher is director of the Human Performance Research Center at Brigham Young University.