Hormonal alterations produced by stress responses

Body Composition Evaluation

BIA Method Comparison

Uses

BIA Testing

Bioelectrical Impedance Analysis

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Body Composition Evaluation

Hormonal alterations produced by stress responses, tumor production, and other disease-related conditions initially influence fluid shifts and maintenance of BCM. A typical response includes the reduction of potassium in intracellular spaces and an elevation in levels of extracellular water. The reduction of intracellular potassium leads to a reduction in cell membrane potential, an increase in intracellular sodium, and diminished electrical charge-holding capability. Hydration of lean tissues may alter from a normal 0.5-0.7 upward to 1.0 or greater. These edematous changes occur long before the serious clinical signs that would be found on physical exam. Obvious bone density changes may or may not occur, depending on weight loss and hormonal balance. If malnutrition comes into play, nutritional treatment is essential to reverse these detrimental changes.

Other methods of body composition evaluation have included air displacement, near-infrared interactance, biochemical evaluation, and bioelectrical impedance evaluation.

In the air displacement method, body volume is calculated using an enclosed portable container that determines the difference between empty chamber air volume and air volume with a person sitting in the container. This method assumes a relationship between body weight, body volume, and body density.[29]

Infrared interactance is based on the variations in chemical composition of fat and lean tissues of the body. Regional composition, specifically the biceps area, may be evaluated with this technology. However, whole body composition cannot be generalized from the calculation of a single region reading.¹⁸

Creatinine excretion from a 24-hour urine collection has been used to estimate FFM as shown in Figure 1. [30]

Figure 1. Fat-Free Mass Estimation by Creatinine Excretion

FFM (kg) = 22.2 x creatinine (grams/day) + 25.1 ± 8.8

(r=0.7; p=0.01)

Bioelectrical impedance evaluation, the main topic of this course (remember?), is based on the concept that the body acts as an electrical resistor and capacitor. Water content and other body chemical substances can conduct and hold electrical charge, while body structures resist or impede electrical charge. A small amount of electricity is introduced into the body and readings of resistance and reactance are captured at another site. The resistance reading is dependent upon electrical pathway length, volume, ionic composition, and current frequency. The estimations based on these properties must overcome the human body’s lack of uniformity for length, cross-sectional area, and ionic composition, making it difficult to accurately predict body compartment volumes.[31]

The accuracy and precision of the estimates derived from BIA measures depends greatly on the method used (hand to hand, foot to foot, or hand to foot), the individual’s position during measurement, any deviation from tested “norms” in body shape or health status, and the equations that are used. With additional information on sex, height and weight, regression equations are used to estimate lean and fat tissues. Data on tetrapolar BIA measures were gathered as a part of the NHANES III (National Health and Nutrition Examination Survey from 1988-1991 and 1991-1994). This data has been distributed to several body composition laboratories for ongoing evaluation and represents a total of 8,586 men and 9,209 women with wide ranges of body composition.[32]

This course will concentrate on the tetrapolar method of bioelectrical impedance evaluation because of its growing popularity and potential applications in a wide variety of settings and with a wide variety of populations ranging from persons who are healthy to those with acute and/or chronic disease. No claims are made about the "perfection" of this type of evaluation in this course. Indeed, there are many types of evaluations that may claim greater accuracy and/or precision. However, small hand-held single-frequency BIA machines are owned and operated in many clinical settings because of their ease and clinical use of the interpreted results. It is our hope that this course will assist these clinicians in making BIA a more useful tool in their practices.

Click here to go to BIA vs. Other Methods (next section)

References:

[28] Moore FD, Boyden CM. Body cell mass and limits of hydration: their relation to estimated skeletal weight. Ann NY Acad Sci. 1963;110:62-71.

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