Overview :

Fat free mass can be estimated using several variables including bioelectrical impedance analysis. The precision can be enhanced by using equations specific for gender and fatness.

Parameters:

(1) age in years

(2) gender

(3) body weight in kilograms

(4) height in centimeters

(5) resistance from bioelectrical impedance in ohms (0-1000 ohm scale)

Bioelectrical impedance measurement:

(1) Total body resistivity measured with a 4 terminal impedance analyzer.

(2) The individual is lying down comfortably with limbs abducted.

(3) Current injector electrodes: (1) just below the phalangeal-metacarpal joint in the middle of the dorsal side of the right hand, (2) just below the transverse arch on the superior side of the right foot.

(4) Detector electrodes: (1) posterior side of the right wrist in the midline, with the prominent pisiform bone on the medial (fifth phalangeal) side, (2) ventrally across the medial ankle bone of the right ankle with foot semiflexed.

(5) Resistance to the flow of a 50 kHz (800 µA) injected current is measured on a 0-1000 ohm scale.

(6) Reactance is measured on a 0-200 ohm scale.

Criteria for separating normal from obese (anthropometric and independent of body density):

(1) men are considered obese if boyd fat is  >= 20%

(2) women are considered obese if body fat >= 30%

estimated lean body mass in kilograms =

= ((height variable) * ((height in cm)^2)) - ((resistance variable) * (resistance in ohms)) + ((weight variable) * (weight in kilograms)) - ((age variable) * (age in years)) + (intercept)

(Table 4, page 11, Segal et al: Set 1 + Set 2 variables)

Limitations:

• Variation in hydration can affect the accuracy of the equations.

Bussolotto M, Ceccon A, et al. Assessment of body composition in elderly: Accuracy of bioelectrical impedance analysis. Gerontology. 1999; 45: 39-43.

Segal KR, van Loan M, et al. Lean body mass estimation by bioelectrical impedance analysis: a four-site cross-validation study. Am J Clin Nutr. 1988; 47: 7-14.