Body Mass Index (BMI)

The Body mass index (BMI) is defined as a person’s body mass divided by the square of their height; usually expressed as: kg/m². The BMI was developed in the 1800’s by a statistician in Belgium. It is considered a reasonable tool for the indication of body fatness for most people. It is mostly used for screening purposes to put people into BMI risk categories for disease or illness based on obesity. The BMI was previously known as the Quetelet index and is occasionally still called that. It does have some limitations, but despite that, it is widely used.

The World Health Organisation categories for Adult BMI are:
Very severely underweight: < 15
Severely underweight: 15.0 to 16.0
Underweight: 16.0 to 18.5
Normal (healthy weight): 18.5 to 25
Overweight: 25 to 30
Obese Class I (Moderately obese): 30 to 35
Obese Class II (Severely obese): 35 to 40
Obese Class III(Very severely obese): > 40

Criticisms of BMI:

  • It only depends on height and weight and does not necessarily accurately reflect the amount of adipose tissue in all body types
  • It can be very inaccurate for people who are athletes, as the higher muscle mass can put them into the overweight category according to the BMI
  • It does not account for variation in body frame size
  • There are some selected studies that have shown that those with a normal BMI were at greater risk for obesity related health problems than those with a high BMI
  • There is no real physiological reason to square a person’s height in the calculation formula

See this and this for a more cynical criticism of the use of the BMI. Despite the problems with the BMI, it still does approximate health and is useful.

A paper in the journal Obesity Reviews in 2001 stated:

Body mass index (BMI) is the cornerstone of the current classification system for obesity and its advantages are widely exploited across disciplines ranging from international surveillance to individual patient assessment. However, like all anthropometric measurements, it is only a surrogate measure of body fatness. Obesity is defined as an excess accumulation of body fat, and it is the amount of this excess fat that correlates with ill-health. We propose therefore that much greater attention should be paid to the development of databases and standards based on the direct measurement of body fat in populations, rather than on surrogate measures. In support of this argument we illustrate a wide range of conditions in which surrogate anthropometric measures (especially BMI) provide misleading information about body fat content. These include: infancy and childhood; ageing; racial differences; athletes; military and civil forces personnel; weight loss with and without exercise; physical training; and special clinical circumstances. We argue that BMI continues to serve well for many purposes, but that the time is now right to initiate a gradual evolution beyond BMI towards standards based on actual measurements of body fat mass.

Alternatives to Body Mass Index (BMI):

  • Body adiposity index
  • Body shape index (ABSI)
  • Body fat percentage
  • Body volume index
  • Ponderal index
  • Rohrer’s index
  • Sagittal Abdominal Diameter (SAD)
  • Waist-hip ratio
  • Waist-to-height ratio

Related Topics:
Obesity | Obesity and the Foot

Page last updated: Jul 31, 2021 @ 11:04 pm

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