Why underwater weighing?

Comparison of methods

 

Underwater Weighing
Body Mass Index (BMI)
If you lose 5 lbs of fat and gain 5 lbs of muscle,
then your body will weigh more underwater.
Fat floats in water but muscle sinks.
If you lose 5 lbs of fat and gain 5 lbs of muscle,
your BMI does not change.
BMI can’t tell fat weight from muscle weight.

 

Underwater Weighing
Skin Folds
Results are valid for lean or fat subjects.
Different testers consistently get the same result.
Calculates actual body density from body volume.
Only possible on lean subjects with loose skin.
Different testers seldom get the same result.
Body density is estimated by prediction equations.

 

Underwater Weighing
Body Impedance Analyzer (BIA)
Body water content has little effect on results.
Different equipment => Similar results.
Fat % estimated directly from calculated density.
Strongly affected by body water content.
Different equipment => Different results.
Fat % is not directly related to BIA measurement.

 

Underwater Weighing
Air Displacement Plethysmography
Simple to calibrate in seconds with known weights.
Air temperature does not affect the results.
No gaskets to leak.
Calibration with test volume is time consuming.
Air temperature change in chamber => Error.
Leaky gasket => Large error.

 

Underwater Weighing
Dual Emission X-Ray Absorptiometry (DEXA)
Low cost for equipment acquisition.
Subjects are not exposed to X-Ray radiation.
Testers do not need a special certification or license.
Measures both subcutaneous & internal body fat.
High cost for equipment acquisition.
Subjects are exposed to X-Ray radiation.
Specialized training and certification required.
Does not measure internal body fat.