Fundamentals
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Physiology
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Lecture Series
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Lecture 2
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Lecture 4
Lecture 5
Lecture 6
Lecture 7
Lecture 8

Lecture 5 page 1 2 3 4 5 6 7 8

Body Composition Component of Self-Assessment
As we have discussed, the Body Mass Index = Wt (kg) / Ht² (m²) gives a general idea of the degree of obesity or excess fat in populations. In an individual there may be a difference between estimated percent body fat and the measured body fat by bioelectrical impedance or other methods. "In special groups, such as old people and very muscular athletes, Quetelet's index will give an inaccurate indication of obesity, but these are not groups in whom obesity is an important clinical problem, and in any case, methods such as density, water, and potassium are also liable to give wrong estimates of fatness in such people."J.S. Garrow and Joan Webster "Quetelet's Index (W/H²) as a Measure of Fatness"Int. J. Obesity 9:147-153, 1985.

Garrow and Webster found that the regression of fat/H² on weight/H² was 0.955 for women and 0.943 for men. Prediction equations were developed based on these findings. Fat in kg can be calculated from weight and height as follows:

The greatest majority of patients seen in my clinical practice already know that they have increased body fat, and the degree of overfatness is of little practical utility to them. However, as detailed below obese subjects can be divided into three categories (see Table One below) of increased lean body mass, normal predicted lean body mass, and decreased lean body mass.

Increased lean mass as well as fat mass is seen in obese individuals. In 1964 Forbes reported that lean tissue in obese children was increased compared to non-obese peers (2). Drenick (3), using total body potassium, found increased lean tissue in obese adults (3). Webster et al. measured the body composition of 104 obese and normal weight women by densitometry (4). They reported that the excess body weight of the obese over non-obese women consisted of 22 to 30% lean and 70 to 78% fat tissue. Forbes and Welle (5) examined data on lean body mass in obese subjects collected in their laboratory or published in the literature. Their own data demonstrated that 75% of the obese population had a lean-to-height ratio that exceeded 1 standard deviation (SD) and that more than half exceeded 2 SD. A review of the literature supported these observations and determined that the lean body mass could account for approximately 29% of excess weight in obese patients. A proportionate increase of lean body mass of approximately 25% is considered normal. Deviations both above and below this amount of lean mass are observed on clinical grounds based on various etiologies listed below (see Table Two below). An example of data collected in the UCLA High Risk Breast Cancer Clinic is shown in Table Three below.

Body Mass and Percent Body Fat in Women at Increased Risk of Breast Cancer
(From Heber et. al. American Journal of Clinical Nutrition, 1996).

Lean body mass is clinically important for two reasons. First, lean body mass predicts energy expenditure and, thereby, the predicted rate of weight loss on a given calorically-restricted diet (10). Secondly, lean body mass can be used to diagnose increased or decreased lean body mass. In the first instance, the increased lean body mass can be used to calculate a more appropriate target weight than would be predicted from ideal body weight tables. In those subjects with reduced lean body mass, a program of aerobic and heavy resistance training can be initiated to provide for an increase in lean body mass and energy expenditure. In both markedly obese individuals and individuals with decreased lean body mass, there is linear relationship (Sterling-Pasmore Equation) of lean body mass to energy expenditure (ca. 13.8 Kcal/day/lb lean body mass). This represents approximately 90% of total energy expenditure in a sedentary obese individual, and provides a good clinical estimate of maintenance calories in my clinical experience.

The impedance meter is a simple electrical circuit with the following characteristics:

This type of circuit has a frequency-dependent impedance based on the resistance and capacitance (reactance) of the circuit elements which are fat and lean tissue in this case. As the frequency is increased the circuit acts more like a simple resistor, and electricity travels through the circuit easily. At low frequencies it acts more like a capacitor until at 0 Hz (cycles/sec) there is no circuit flow and the impedance approaches infinity. All bioimpedance analyzers use an equation such as the one shown below. The Biodynamics impedance analyzer in particular uses four sets of equations to be able to predict lean body mass with different constants for different body types.

LBM = (A X Ht²) + (B X Wt) + (C X Age) + (D X R) + E

Specific coefficients are proprietary information of Biodynamics, Inc.

Selection of Equations to be Used Based on Body Type

Data Provided By a Manufacturer on Correlation with Underwater Weighing (Bioanalogics, Inc.)

Potential Problems in the Clinical Use of Bioelectrical Impedance
During the first week of caloric restriction, there is a loss of body weight in excess of the loss of lean and fat tissue due to a diuresis. If patients are measured at their first visit and then weekly thereafter, it is possible to find that patients are apparently gaining fat as they lose weight using bioelectrical impedance. Since lean body mass is assessed based on both body water and muscle, the loss of water leads to an apparent decrease in lean body mass which in most cases exceeds the loss of fat in the first week of dieting leading to an increase in percent body fat (11). I have found the bioelectrical impedance measurement most useful at the first visit for assessing type of obesity (usual, decreased lean mass, increased lean mass, or fat maldistribution), and not useful for multiple serial determinations. In fact, I explain to patients that the machine is not accurate enough to pick up small changes, and delay repeating the measurement until the patient has reached a weight close to target weight.

A second potential problem is overemphasis on the quantitative accuracy of body fat estimation. Small changes cannot be measured using this device. It is important to stress this fact to patients. The changes observed in percent fat often don't impress patients as much as the ratio of the absolute change in fat mass in pounds compared to changes in lean mass.

Future Research
There should be standards set for calibrating machines from different manufacturers. There are a number of laboratories that have multiple methods for measuring body composition including total body potassium, underwater weighing, TOBEC, DEXA, and deuterium dilution. Such laboratories could develop standardized protocols similar to the CDC Lipid Standardization Program for calibrating machines on a national and regional basis. Such a program could also lead to the standardized use of formulas for determining percent body fat in specific ethnic and age populations of men and women.

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