Applied nutritional investigationLimits of body mass index to detect obesity and predict body composition
Introduction
Obesity is a common condition in many societies, and its incidence is rising.1, 2 Numerous chronic health conditions (e.g., diabetes, hypertension, heart disease, sleep apnea, and degenerative joint disease) are associated with obesity, and mortality rates increase with increasing body mass.3, 4, 5, 6 Consequently, diagnosis and treatment of obesity is a major health issue.
Obesity is defined as an excess of body fat. Body fat is difficult to measure. However, increased body fat is usually accompanied by increased total body mass, so indices of relative weight are commonly used to diagnose obesity and to track progress in the treatment of the obese person.1, 2, 3, 4, 5
One of the most commonly used indices of relative weight is the body mass index (BMI; sometimes referred to as the Quetelet index).7 BMI (body mass in kilograms divided by height, or meters, squared) was not originally intended as an index of obesity but is now commonly employed as such in epidemiologic studies, where it accurately predicts obesity-related morbidity and mortality.3, 4, 5 A BMI of 30 kg/m2 is considered the threshold of obesity.2 However, the accuracy of BMI as a body-composition marker is controversial.8, 9, 10, 11, 12 Some have suggested that BMI inadequately predicts percentage of body fat,8, 9, 10 whereas others have suggested that BMI may be useful to predict body fat indexed to height but not to predict percentage of body fat.11, 12 The issue of whether indices of overweight predict body composition is important because body composition (i.e., adiposity) rather than excess body mass is the more important health risk.6 The current study was undertaken to determine whether BMI accurately identifies individuals as obese or not obese and to investigate the ability of BMI to predict body-fat content measured by bioelectrical impedance.
Section snippets
Methods
Our institutional review board approved the current study. Subjects were recruited by advertisements in a medical center (consisting of a hospital, clinics, a medical college and biomedical research building, hospital-associated fitness center, and an obesity clinic). Participation in the study was not limited by race, sex, or body size. The study sample was restricted to ambulatory adults (age > 18 y). Amputees were excluded. Informed consent was obtained from each subject before the study.
Results
Fifty-three men and 88 women participated in the study. The study sample population was largely white (98%; 4% of whom were Australian or European students studying in the United States, 3% of whom were of Hispanic ethnicity). A wide range of age and body size was represented in the sample (Table I). The men tended to be larger and leaner than the women, but age (mean and range) was similar.
Impedance values for men were as follows: resistance = 423 ± 11 Ω, reactance = 51 ± 2 Ω. Corresponding
Discussion
In the current study, all subjects who were obese by the BMI criterion (BMI ≥ 30 kg/m2) were also obese by the bioelectrical-impedance criterion (i.e., had body fat of at least 25% in men and 30% in women). However, 30% of the men and 46% of the women who were not obese by the BMI criterion (BMI < 30 kg/m2) actually had obese levels of body fat and therefore would have had their obesity status misclassified based solely on BMI. Thus, BMI alone does not appear to accurately identify all cases of
Summary
With impedance-derived body fat as the criterion method, obesity can be confidently presumed at a BMI of at least 30 kg/m2. However, obesity levels of body fat commonly occur below this threshold, and BMI alone cannot detect it. Body composition in individuals is difficult to predict accurately from BMI alone, but this is less true for body fat/height2 than for percentage of body fat. Indexing body fat to body height may be more appropriate than indexing body fat to body mass, but measurement
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