Health and Wellness

ID #5401

Body Mass Index and Benchmarks, Alan McIver, Cape Town, South Africa.

A widely quoted measure of body mass (weight) is the Body Mass Index (BMI), the formula for which is:

BMI          =    M/H2

where M is one’s mass in kg, and H is one’s height in meters. Benchmark BMI's are:

•     20 or less -- Malnourished

•     24 --   Ideal, and 

•     28 and above -- Obese

However, since humans are 3 rather than 2-dimensional, the above formula is incorrect and such guidelines are misleading because the formula should be M/H3 rather than M/H2. Why do I say this? Apart from common sense, I have a nephew who is 6ft 4 inches tall, in peak physical condition and doesnt carry an ounce of excess fat yet his BMI is 28. Another example: Calculate the BMI's for Springbok rugby lock forwards. In spite of being in superb physical condition, their BMI's are typically 29. I have thus created what I term the Corrected BMI (CBMI), the formula for which is as follows:

CBMI        =    M/H3

For which the revised guidelines (see the attached spreadsheet for the calculations) are:

•    12.5 or less -- Malnourished

•    15.0 -- Ideal, and

•    17.5 or more -- Obese

A table in which values of the CBMI are presented as a function of height and body mass is attached. Note that, while results are identical at the midpoint of the range, differences between the BMI and the CBMI are large at extremes of the range. There are two noteworthy points here:

(a) A correct measure of body mass is important because it is demotivating to be told that you are obese, especially when carrying no fat and thus convinced that this is not the case. Note that this is most likely true in the case of men, who are for the most part taller than women. 

(b) When using BMI, body mass estimates are too low for tall people (25%@2-m) and too high for short people (40%@1-m). Such errors are so large that BMI's are of limited value insofar as weight guidelines are concerned.

I would like to demonstrate that the CBMI can be used to provide useful guidelines for the body mass of anyone between 1-m and 2-m tall, in which case it would be most valuable. If you have any information I might use to check the values of CBMI's please don't hesitate to contact me. 

Alan McIver, Dubai, January 2013

PS I am amazed that the BMI, which is 150 years old, is still in use today. One wonders how many diagnoses have been made on the basis of a formula that is clearly incorrect. 


attached files: Comparison of Acceptable Body Mass using BMI and CBMI.xls, Values of CBMI by Mass and Height.xls, CBMI Guidelines.xls

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Last update: 2014-03-20 21:53
Author: Alan McIver
Revision: 1.43

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