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Guidelines on Overweight and Obesity: Electronic Textbook
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Assessment of Weight and Body Fat

Two measures important for assessing overweight and total body fat content are determining body mass index (BMI) and measuring waist circumference.

Body Mass Index: The BMI, which describes relative weight for height, is significantly correlated with total body fat content. The BMI should be used to assess overweight and obesity and to monitor changes in body weight. In addition, measurements of body weight alone can be used to determine efficacy of weight loss therapy. 

BMI is calculated as weight (kg)/height squared (m2). To estimate BMI using pounds and inches, use: [weight (pounds)/height (inches)2] x 703. Weight classifications by BMI, selected for use in this report, are shown in the table below.

Table IV-1: Classification of Overweight and Obesity by BMI*

 
Obesity Class
BMI (kg/m2)
Underweight 
 
<18.5
Normal
 
18.5-24.9
Overweight
 
25.0-29.9
Obesity
I
30.0-34.9
 
II
35.0-39.9
Extreme Obesity
III
greater than or equal to 40
* Pregnant women who, on the basis of their prepregnant weight, would be classified as obese may encounter certain obstetrical risks. However, the inappropriateness of weight reduction during pregnancy is well recognized (Thomas, 1995). Hence, this guideline specifically excludes pregnant women.
Source (adapted from): Preventing and Managing the Global Epidemic of Obesity. Report of the World Health Organization Consultation of Obesity. WHO, Geneva, June 1997.

Waist Circumference: The presence of excess fat in the abdomen out of proportion to total body fat is an independent predictor of risk factors and morbidity. Waist circumference is positively correlated with abdominal fat content. It provides a clinically acceptable measurement for assessing a patient's abdominal fat content before and during weight loss treatment.  The waist circumference at which there is an increased relative risk is defined as follows:

High Risk
Men: >102 cm ( >40 in.)
Women: >88 cm ( >35 in.)
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