Clinical Assessment: Body Size, Shape, and CompositionSaturday, October 1, 2011
Today’s post is another excerpt from “Best Weight: A Practical Guide to Office-Based Weight Management“, recently published by the Canadian Obesity Network.
This guide is meant for health professionals dealing with obese clients and is NOT a self-management tool or weight-loss program. However, I assume that even general readers may find some of this material of interest.
BODY SIZE, SHAPE AND COMPOSITION
BMI, calculated by dividing weight in kilograms by height in metres squared, provides a good measure of weight-related statistical risk of co-morbidities in population-based studies. On an individual level, however, the usefulness of BMI in ascertaining risk decreases as the body composition of people with similar BMIs can vary widely, especially at lower weights. For example, an athlete with a BMI of 32 and 15% body fat clearly does not have an indication for obesity treatment and likely faces minimal or no weight-related medical risk. On the other hand, a sedentary individual with a BMI of 27 and 30% body fat is likely at increased weight-related risk for co-morbidities. While easy to measure and entirely reproducible, BMI fails to take into account differences in race, sex, body frame, musculature, and age, all of which affect the ability to ascribe risk on an individual basis.
Other methods of assessing body composition include electrical bio-impedance analysis (BIA), dual X-ray absorptiometry (DEXA) and air-displacement plethysmography (BodPod®). While DEXA is the more accurate measure, repeated tests are expensive and expose patients to radiation. In contrast, BIA is cheap and accessible and, despite its imprecision (measures vary dramatically with the patient’s level of hydration), is often used by weight-management centres to monitor changes in body composition during weight loss. While the diagnostic and prognostic utility of measuring body composition remains debatable, some clinicians find it helpful for counselling and motivating select patients. The risk with BIA is that body fat percentage changes quite slowly and, coupled with the inaccuracy of office BIA analysis, this can mean that body fat percentage measurements can change barely from one visit to the next. The patient who has lost a medically significant amount of weight may find this discouraging.
More important than the absolute amount of body fat is its location. Intra-abdominal or visceral adipose tissue is a better predictor than BMI of cardio-metabolic risk factors, respiratory impairment, fatty liver, and reflux disease. The easiest way to assess abdominal adiposity in clinical practice is by measuring waist circumference. This is best done by positioning the patient in front of the seated examiner, identifying the bony landmark (upper edge of the iliac crest), and asking the patient to hold onto the tape and turn around, thereby bringing the tape around the abdomen. After ensuring that the tape is snug and parallel to the ground, the circumference is measured at the end of a regular expiration using the cross-arm technique. Table 7 on page 40 details the waist circumference cutoffs for identifying central obesity for different ethnic groups.
© Copyright 2010 by Dr. Arya M. Sharma and Dr. Yoni Freedhoff. All rights reserved.
The opinions in this book are those of the authors and do not represent those of the Canadian Obesity Network.
Members of the Canadian Obesity Network can download Best Weight for free.