Definition
Clinical obesity is defined as a Body Mass Index (BMI) value ≥ 30 kg/m2. A BMI between 25 and 29.9 is considered overweight, but not obese 2. Normal weight BMI ranges from 18.5 - 24.9 and underweight is ≤ 18.5 2. Obesity has been further divided into three classes 2. Class I: BMI 30.0 - 34.9, class II: BMI 35.0 - 39.9, and class III: BMI ≥ 40.0.
Body mass index indicates overweight for height, but does not differentiate between fat mass and fat-free mass 43. For this reason, BMI may not be an accurate measure of obesity. BMI is a calculation that examines whether an individual is weight-height proportionate. There is debate on whether classifying people as weight-height proportionate or non-weight height proportion is meaningful or appropriate. Individuals that are relatively muscular may score overweight on a BMI despite having low body fat and being and/or appearing healthy.
Measurement of abdominal circumference and BMI are interrelated, however circumference measurements can be used independently to predict increased disease risk . An abdominal circumerfence measurement greater than 88cm for females and 102cm for males places an individual at increased risk for developing type II diabetes, hypertension, dyslipidemia, and cardiovascular disease (CVD).
The following table represents the classification of overweight and obesity by BMI and Abdominal circumference and the associated disease risk.
Table IV-2: Classification of Overweight and Obesity by BMI, Waist Circumference, and Associated Disease Risk*
Disease Risk* Relative to Normal
Weight and Waist Circumference
| |
BMI (kg/m2)
|
Obesity Class
|
Men > 102 cm
(> 40 in.)
Women > 88 cm
(> 35 in.)
|
Men >102 cm
( >40 in.)
Women >88 cm
( >35 in.) |
| Underweight |
18.5
|
|
-----
|
----- |
| Normal+ |
18.5 - 24.9
|
|
-----
|
----- |
| Overweight |
25.0 - 29.9
|
|
Increased
|
High |
| Obesity |
30.0 - 34.9
|
I
|
High
|
Very High |
| |
35.0 - 39.9
|
II
|
Very High
|
Very High |
| Extreme Obesity |
 40
|
III
|
Extremely High
|
Extremely High |
|
* Disease risk for type 2 diabetes, hypertension, and CVD.
+Increased waist circumference can also be a marker for increased risk even in persons of normal weight
Table: http://www.nhlbi.nih.gov
Use of BMI
Body mass index is commonly used in obesity research to examine obesity 43. Determining BMI involves a quick and simple equation using weight and height while other methods tend to be more time consuming, may require special equipment, and are typically more invasive. There is not clear agreement in the literature on how to classify obesity by other methods. |
Limitations of BMI
Body mass index is not an accurate measure of obesity across all populations. Individuals that have relatively large amounts of muscle mass may be classified as overweight or obese, while individuals that are considered underweight by BMI (≤ 18.5) may have a relatively large amount of body fat. In effect, BMI does not discriminate between fat mass and fat-free body mass 43 . In studies where BMI has been used in attempt to estimate body fat, the standard of error is high. |
Body Composition Tests, an alternative to BMI
Body composition asses the relative percentage of body weight that is fat and fat-free tissue 1. Body composition tests are able to estimate the actual percentage as well as amount of body fat in an individual. |
Body Composition Assessments
Body composition can be estimated by a variety of techniques. These include densitometry methods such as hydrostatic (underwater) weighing, and plethysmography (air displacement), and anthropometric methods such as skinfold measurement 1. Other methods include bioelectrical impedance analysis (BIA), dual energy X-ray absorptiometry (DXA), and near-infrared interactance (NIR) 1.
Skinfold measurement must involve a skilled technician to be as accurate as possible. In this technique, the technician takes skinfolds with a caliper device at standardized cites on the subject that is being tested. The caliper device measures the different skinfold sites in millimeters. The sum of folds is put into a regression equation to estimate body fat percentage 1. Skinfold measurement offers a quick, non-expensive method for body composition assessment.
Hydrostatic weighing estimates body composition based on Archimedes’ principle. This principle states that a body immersed in water is buoyed by a counterforce equal to the weight of water displaced. In other words, the closer a body’s weight in water is to the body’s dry weight (weight on land), the higher the percentage of fat-free mass on the body. An individual with more fat free mass for the same dry body weight weighs more in water, therefore has higher body density and lower percentage body fat 1.
See Heyward’s Applied Body Composition Assessment for an overview of plethysmography, BIA, DXA, and NIR and other assessments such as waist-to-hip circumference. (link) |
| |
Resources for Body Composition Assessment (Local)
Bellingham, Washington:
Exercise Physiology Laboratory
Western Washington University
Bellingham, WA 98225-9076
Ph: 360-650-2851
|
Calculating BMI
To calculate BMI, take body weight in kilograms and divide it by height in meters squared:
BMI = weight (kg)/height (m2)
Alternatively, BMI can be calculated in standard units by taking weight in pounds multiplied by 705 and dividing the resultant number by height in inches squared:
BMI = weight (lb) * 705 / height (in2)
BMI may be calculated using the BMI chart below or the online BMI Calculator |
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