Skinfolds are not equally compressible and skin thickness is not necessarily constant. More precise and technically sophisticated methods for determining body fat include underwater weighing, which requires total submersion of an individual and an accurate estimation of lung and abdominal gas, the assessment of total body water by dilution of tritiated water, and measurement of body fat by dilution of an inert fat-soluble gas such as Xenon. CT, ultrasound, and MRI have also been used to determine body fat. These measures are all expensive, difficult to do, time consuming and not applicable to office practice. Body impedance is measured when a small amount of current, applied through electrodes, flows through the body.
Resistance is inversely proportional to total body water through which the current travels.
Lean body mass and body fat are derived from total body water. Impedance measurements correlate quite well with total body water measured by other means. The major health risks of obesity increase in a curvilinear relationship, with prevalences increasing progressively and disproportionately with increasing weight. Weight increases beginning during adulthood and continuing for many years have the greatest adverse affects. With this degree of obesity, there is a minimum doubling of the prevalences of all causes of morbidity and mortality.