Overweight definition using body mass index
by Steven B. Halls, MD FRCPC
Defining "Overweight" is a sensitive topic. This discussion of obesity and body mass index is here to give you background information about the Body mass index (BMI) levels that categorize fatness. But BMI can potentially misclassify people as fat, even though their percentage of body fat is not excessive. Therefore, trust your own judgement about your own body because BMI-based body descriptions can be wrong.
First, lets discuss the current standard definitions of overweight and obesity used by the Center for Disease Control (CDC) and World Health Organization (WHO).1,2
- For adults, a body mass index of 25 or more is considered "overweight" and a BMI of 30 or more is considered "obese".
- For children, the charts of Body-Mass-Index for Age are used, where a BMI greater than the 85th percentile is considered "at risk of overweight" and a BMI greater than the 95th percentile is considered "obese". ( See the Boys BMI chart and Girls BMI chart for more detail).
Combined, the Child and Adult criteria form a chart like this below:
and here is the Obesity criteria:
Is there room for improvement of these standard definitions? Naturally yes.
- Body Mass Index changes with Age, obviously in Children but also in Adults.
- Men and Women are different, so why should adult men and women have exactly the same BMI?
- Short adult women have higher BMI than taller women.
- Race/ethicity and nationality affect body composition and BMI.
- Muscular people, atheletes and bodybuilders particularly, have high BMI values, but are not fat.
These potential improvements are discussed on the next page, but first, here is some background information:
From the NHANES II survey ( USA 1976-1980 )3, these definitions of Overweight and Obesity were used until year 2000. ( You can still use them, if you like.)
|underweight||< 19.1||< 20.7|
|in normal range||19.1 – 25.8||20.7 – 26.4|
|marginally overweight||25.8 – 27.3||26.4 – 27.8|
|overweight||27.3 – 32.3||27.8 – 31.1|
|very overweight or obese||> 32.3||> 31.1|
These NHANES II definitions had several nice features:
- Men and Women had different overweight thresholds ( BMI of 27.3 for women, 27.8 for men).
- The concept of "Marginally overweight" is a nice sounding phrase. and "marginally overweight" began at 25.8 for women, 26.4 for men.
But the current WHO and CDC definition of Overweight for adults, at BMI of 25 kg/m2 was chosen to be suitable for international standardization and simple enough to remember and calculate with a pencil and paper.
Next, Click here to read about the halls.md v2 classification of BMI for body description. You may want to use it instead.
- Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. National Heart, Lung and Blood Institute. June 17, 1998
- Preventing and Managing the Global Epidemic of Obesity. Report of the World Health Organization Consultation of Obesity. WHO, Geneva, June 1997
- NHANES II. 1976-1980 data from USA.
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