The Useless Body Mass Index (BMI) Body Fat and the Body Mass Index Figure 1: the Useless Body Mass Index (BMI)

The Useless Body Mass Index (BMI) Body Fat and the Body Mass Index Figure 1: The Useless Body Mass Index (BMI)

This chart was created by InvictaHOG, and has been released into the public domain. It may be found at: http://en.wikipedia.org/wiki/File:Body_mass_index_chart.svg .

I am including a discussion of the BMI for completeness, although I do not recommend its use. From my point of view with respect to use for individuals the Body Mass Index (BMI) is an artefact that should have disappeared along with button-up-boots!

The BMI was devised somewhere between 1830 and 1850 by Belgian polymath 1 Adolphe Quetelet. For that reason it has historically been known as the Quetelet index . It is meant to reflect a person’s relative thickness or thinness . The formula for calculating the BMI is weight in kilograms divided by the square of the height in metres . That is: Weight (kg) / [Height (m)] 2 For a given height, as weight increases the index increases. So one would expect that a large BMI would reflect a weight problem: specifically an excess fat problem. But it often does not ; the BMI has some in-built problems because the original concept was faulty.

Faults in the BMI 1. Firstly , although the index relates weight to height, it takes no account of the proportions of lean and fat body tissue. A lean bodybuilder who is ripped down to a body fat level below 5% may have the same BMI as a sedentary individual who carries very little muscle and whose body consists of a high proportion of fat. This clearly makes the index inaccurate for individuals . 2. Secondly the index takes no account of natural bone structure. People with heavier bone structure would be expected to carry more weight than those with lighter bone structure, yet a straight reading of the BMI alone will predict heavily boned individuals as carrying more fat than their more lightly boned peers, even if that extra weight were composed entirely of lean tissue. To remind yourself of the importance of bone structure in determining suitable goal weight see the document “Sensible Goal Weight ”. 3. Thirdly if you take a block of wood and measure its weight, and then you compare that to another block of wood whose dimensions are double those of the first, you’ll find that the weight of the larger block is eight times that of the smaller block, not two or even four times the weight. a. That’s because weight doesn’t increase simply by the ratio of dimensions or even by the square of the ratio of dimensions . It increases according to the cube of the ratio of the dimensions! That is, double the dimensions and weight increases by eight fold. (2 cubed = 8. i.e. 2 x 2 x 2 = 8) b. The impact of this on the BMI is that taller individuals with exactly the same body shape and body composition as shorter individuals always have a larger BMI. If the index properly reflected body composition, for a given body shape the index would be the same regardless of height. In summary, the BMI is virtually useless as an indicator of how you are going with your fat loss, or as a measure of your body fat percentage! The Ponderal Index: Is it any Better? There was an attempt to improve on the BMI. It is called the Ponderal Index . But that too was a failure. In light of the fact that weight increases according to the cube of dimensions, an index that would make more sense than the BMI would be one which calculates the ratio of the weight with the cube of the height rather than the square of it. That is: Weight (kg) / [Height (m)] 3 This index was in fact proposed in 1921 by F. Rohrer . It is called the Ponderal Index (PI), although it is also known as the Rohrer Index after its creator. This index does have value, especially for very short or very tall people, and as such it is used in the field of paediatrics . The PI was designed to be a Corpulence Index . In old fashioned language, a corpulent individual is a fat individual. So it was designed to be an index that measures fatness . In addition overweight people are generally also more ponderous (slow) in their movements than lighter people: hence the term Ponderal Index . But this index also has limitations. It doesn’t work properly because it assumes that the body shape of people does not vary with height. This is clearly not true. Taller people are usually not the same shape as shorter people (although they sometimes are). Most taller people are naturally thinner : by both bone structure and muscularity. So this new index falls down in terms of its usability for all people of all heights.

In fact an index which takes into account the change in natural build as height increases would need the weight compared with the height raised to a power somewhere between 2 and 3. And even with this adjustment, like the BMI before it, s till no account is taken regarding the proportions of lean to fat tissue within an individual. The attempt to create a meaningful index is so fraught with problems that ANY index of this type is actually quite useless to you. Neither the BMI nor the Ponderal Index is of much value in determining your body-fat percentage. They may or may not reflect how much body fat you carry, so unless they are measured and interpreted by an expert in the field of biometrics (measurement of body parameters) you will never know whether or not you are one of the people for whom one of these indices is of value or not : whether they do reflect your relative level of body fat or not . And trust me when I tell you that the average doctor has absolutely no clue about this. Do you even know how much training your GP has had in this area? Forget this index. It is virtually useless to you.

1 A polymath is a person whose expertise spans many (Greek poly ) subject areas. Another well-known polymath was Leonardo da Vinci .