Body Mass Index (BMI): Definition and Limitations

 

Definition

The Body Mass Index (BMI) or Quetelet index is a value derived from the weight (i.e. mass) and height of an individual. The BMI is defined as the body mass divided by the square of the body height, and is universally expressed in units of kg/m2, resulting from mass in kilograms and height in meters. 

 
 
 
 
 A graph of body mass index as a function of body mass and body height. The dashed lines represent subdivisions within a major class. Source: Wikipedia.

A graph of body mass index as a function of body mass and body height. The dashed lines represent subdivisions within a major class. Source: Wikipedia.

 

The BMI is an attempt to quantify the amount of tissue mass (muscle, fat, and bone) in an individual, and then categorize that person as underweightnormal weightoverweight, or obese based on that value. 

 
  ©  Fitnescity

© Fitnescity

 

 

History

The basis of the BMI was devised by Adolphe Quetelet, a Belgian astronomer, mathematician, statistician and sociologist, from 1830 to 1850 during which time he developed what he called "social physics." 

 
  Adolphe Quetelet created the BMI for measuring human body shape. It has prevailed for over 160 years

Adolphe Quetelet created the BMI for measuring human body shape. It has prevailed for over 160 years

 

The modern term "body mass index" (BMI) for the ratio of human body weight to squared height was coined in a paper published in the July 1972 edition of the Journal of Chronic Diseases by Ancel Keys. In this paper, Keys argued that what he termed the BMI was "...if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity.

 

 

Limitations of BMI as a Health Indicator

The medical establishment and statistical community have both highlighted the limitations of BMI.

BMI does not differentiate between body fat and lean mass.

Assumptions about the distribution between muscle mass and fat mass are inexact. BMI generally overestimates adiposity (i.e. body fat tissue) on those with more lean body mass, such as athletes, and underestimates excess body fat on those with less lean body mass.

 
 This graph shows the correlation between body mass index (BMI) and percent body fat (%BF) for 8550 men in NCHS' NHANES 1994 data. Data in the upper left and lower right quadrants suggest the limitations of BMI. [ Learn More ]

This graph shows the correlation between body mass index (BMI) and percent body fat (%BF) for 8550 men in NCHS' NHANES 1994 data. Data in the upper left and lower right quadrants suggest the limitations of BMI. [Learn More]

 

BMI does not indicate weight distribution.

Individuals with a similar BMI could have drastically different body shapes, and thus varying risk of disease and early mortality. It is now well established that individuals with higher levels of visceral fat (i.e. body fat that is concentrated around the midsection area,) are at much greater risk of disease and early mortality. 

(visit our Body Scanning section to learn more).

 

 

Notes