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Guidelines on Overweight and Obesity: Electronic Textbook
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Classification of Overweight and Obesity

According to BMI

The primary classification of obesity is based on the measurement of BMI. This classification is designed to relate BMI to risk of disease. It should be noted that the relation between BMI and disease risk varies among individuals and among different populations. Therefore, the classification must be viewed as a broad generalization. Individuals who are very muscular may have a BMI placing them in an overweight category when they are not overly fat. Also, very short persons (under 5 feet) may have high BMIs that may not reflect overweight or fatness. In addition, susceptibility to risk factors at a given weight varies among individuals. Some individuals may have multiple risk factors with mild obesity, whereas others may have fewer risk factors with more severe obesity. It should also be noted that the risk levels shown for each increment in risk are relative risks; that is, relative to risk at normal weight. They should not be equated with absolute risk which is determined by a summation of risk factors. No randomized controlled trial studies exist that support a specific system for classification that establishes the degree of disease risk for patients during weight loss or weight maintenance. The classification is based on observational and prospective epidemiological studies.

Recommendation: The BMI* should be used to classify overweight and obesity and to estimate relative risk for disease compared to normal weight. Evidence Category C.

Source (adaped from): Preventing and Managing the Global Epidemic of Obesity. Report of the World Health Organization Consultation of Obesity.WHO, Geneva, June 1997 (65).

*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 (44).  Hence, this guideline specifically excludes pregnant women.

Rationale: Overweight and obesity are classified by BMI (see Table IV-1). Calculating BMI is simple, rapid, and inexpensive. The classification can be applied generally to adults. See Appendix II for comments about obesity and its classification in children. The basis for this BMI classification scheme stems from observational and epidemiologic studies which relate BMI to risk of morbidity and mortality (11, 81-83, 131, 155, 278, 539-541).  For example, relative risk for CVD increases in a graded fashion with increasing BMI in all population groups, although absolute risk in obese versus nonobese persons depends on the summed contribution of all risk factors.


Evidence Statement: The same BMI cutpoints (see Table IV-1) can be used to classify the level of overweight and obesity for adult men and adult nonpregnant women, and generally for all racial/ethnic groups. Evidence Category C.

Rationale: Clinical judgment must be used in interpreting BMI in situations in which it may not be an accurate indicator of total body fat. Examples are the presence of edema, high muscularity, muscle wasting, or for very short people. The relationship between BMI and body fat content varies somewhat with age, sex, and possibly ethnicity because of differences in factors such as composition of lean tissue, sitting height, and hydration state (526, 542).  For example, older persons often have lost muscle mass and have more fat for a given BMI than younger persons, women may have more fat for a given BMI than men, and persons with clinical edema may have less fat for a given BMI than persons without edema. Nevertheless, these differences generally do not markedly influence the validity of BMI cutpoints either for classifying individuals into broad categories of overweight and obesity or for monitoring weight status of individuals in clinical settings (526).

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

Source (adaped from): Preventing and Managing the Global Epidemic of Obesity. Report of the World Health Organization Consultation of Obesity.WHO, Geneva, June 1997 (65).

* 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, et al, 1987) Hence, this guideline specifically excludes pregnant women.

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