About the "Medical Recommendation" of Ideal Weight

By Steven B. Halls, MD

The medical profession likes simplicity. Researchers have tested if being overweight causes health risks, using a body mass index (BMI) of 25 kg/m2 as the definition of overweight. This is obviously too simple, but it is a common method used in data analysis. Numerous studies have confirmed that having a body mass index over 25 kg/m2 is associated with increased risk of disease (like diabetes, high blood pressure, heart attacks, etc), and risk of a shortened lifespan3.

It is important to point out that this simple definition of "overweight" by medical researchers, is not representative of how people regard each other, nor is it how a doctor would judge an individual patient, nor is it used by most nutritionists.

Furthermore, the BMI of 25 kg/m2 definition of overweight is a "unisex" threshold, that suits neither men or women particularly well. Nevertheless, a BMI of 25 to 29.9 is defined as "overweight" by some important institutions1.

The definition of "underweight" at a BMI below 19 kg/m2 is also arbitrary. It may be somewhat suitable for women, but is quite unsuitable for men.

A better definition of weight categories2 is shown in the table below. It suggests that an "overweight" BMI is actually 27.3 for women, and 27.8 for men.

Adults Women Men
anorexia < 17.5
underweight <19.1 <20.7
in normal range ideal weight 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
severely obese 35 - 40
morbidly obese 40 - 50
super obese 50 - 60 kg/m2

 

Another problem with all of the above definitions, is that they only apply to adults. For children under age 18, the "normal" BMI changes with age. Thus, to judge a child's Body Mass Index, you should refer to the boys BMI chart or girls BMI chart instead. In my opinion, children whose BMI is somewhere inbetween the 5th to 85th percentiles, should be considered normal, and not overweight.

 

Discussion

There are MANY websites on the internet that offer a simple "body mass index calculator" function. The vast majority of these websites use the too-simple definition of overweight at 25 kg/m2. You should be wary of these types of websites, not only because of their simplicity, but also because they don't compensate for Gender, or for Kids, or for different Ages of adults. The halls.md Body Mass Index Calculator does all of these things. It is a versatile tool, which also computes the "percentile" statistics of the individual compared to the equivalent population.

References

  1. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults.  National Heart, Lung and Blood Institute. June 17, 1998
  2. Standards Committee, American Society of Obesity Surgery; Obesity Surgery. Dec. 1997; 7:523
  3. Calle EE, Thun MJ et al, Body mass index and mortaility in a prospective cohort of US adults. N Engl J Med 1999; 341:1097-105.

 

Last modified 10-November-2003 by Steven B. Halls, MD

Medical BMI ideal weight recommendation issues