“Knowing the limitations are knowing the things better”

A lot of myths and confusions have been revolving around this innocent looking number – Body Mass Index (BMI) since it has started becoming a daily use tool to analyze your weight with respect to your height.

In the article below, the author tries to break through the truths and fallacies of these myths and questions:

How serious is BMI in representing our body composition(body fat)?

It is rather true that BMI cannot be considered as a complete measure of body fat, but it is indeed an indirect measure of obesity.

Looking into certain limitations of BMI will help us use this tool more efficiently. So here it is:

Firstly, the most significant of this index is that it doesn't differentiate between fat mass and the non-fat mass of the body. The non-fat mass include the muscles, bones etc. Thus BMI always will have a tendancy to over-estimate fat content on those with more lean body mass (e.g., athletes) and underestimates excess fat on those with less lean body mass.

Thus BMI can prove inaccurate for people who are very fit or athletic, as their high muscle mass can classify them in the overweight category by BMI, even though their body fat percentages frequently fall below that of a more sedentary person of average build who may have a normal BMI number.

Secondly, as seen in the BMI Calculation formula, the height factor is squared and hence any difference in height will have a larger say on the number than a difference in weight. Thus BMI tends to give a larger BMI for tall people and a lesser number for short people. So short people are misled into thinking that they are thinner than they are, and tall people are misled into thinking they are fatter.

Then, an equally confusing issue is the variations that occur from time to time in the categorisation based on BMIs. The range in the BMI scale that can be deemed as normal has been subjected to changes many times by many world health bodies and institutions. So has the threshold between overweight and obese been changing.

BMI Recommendations - NIH, WHO

In addition to the above limitations, BMI fails to consider other significant variables such as the changes in body composition that occur with age, gender, ethnic group, and leg length, which are important as far as body fat calculation and risk prediction due to it are concerned .

What should we conclude about BMI?

BMI is a reasonable indicator of body fat for both adults and children. Because BMI does not measure body fat directly, it should not be used as a diagnostic tool. Instead, BMI should be used as a measure to track weight status in populations and as a screening tool to identify potential weight problems in individuals.

Along with BMI there are certain other indexes as well which can help us in the trip to a healthier body. We call them alternatives here:

BMI Alternatives

A. BMI Prime

BMI Prime, a modification of the BMI system, is the ratio of actual BMI to upper limit optimal BMI (25 kg/m2).

Individuals with BMI Prime less than 0.74 are underweight; those between 0.74 and 1.00 have optimal weight; and those at 1.00 or greater are overweight.

BMI Prime is useful clinically because it shows by what ratio (e.g. 1.36) or percentage (e.g. 136%, or 36% above) a person deviates from the maximum optimal BMI.

B. Waist circumference

Waist circumference is indicative of visceral fat (body fat stored within the abdominal cavity and therefore accumulated around a number of important internal organs such as the liver, pancreas and intestines). And it is this fat which poses more risk than fat elsewhere. According to NIH, waist circumference in excess of 102 cm (40 in) for men and 88 cm (35 in) for (non-pregnant) women is considered to be high risk for type 2 diabetes, hypertension, and CVD.

In April 2016, Intermountain Medical Centre concluded that Waist circumference was a stronger predictor of heart disease than BMI. Their study shows that abdominal obesity than total body weight or BMI (weight to height ratio), is a strong predictor of left ventricle dysfunction.

Other than BMI and the above alternatives, there are more specialized methods, such as DEXA bone density scanning or computed tomography (CT) that can directly measure total and regional body fat, but these methods are expensive and more time-consuming.

WHO adds more cut-off points in BMI categorization to include ethnicity factor (2004)

In 2004, due to the increasing evidence that the associations between BMI, percentage of body fat, and body fat distribution differ across populations, WHO added extra cut-off points in categorisations based on BMI for various populations.

Underweight BMI range
Normal healthy BMI range
Obese BMI range

It was noted after ethnicity-based studies that the proportion of Asian people at a risk of type 2 diabetes and cardiovascular disease is substantial at BMIs lesser than the global cut-off for overweight, 25kg/m2. WHO thus determined the cut-off for observed risk from 22 to 25kg/m2 and for high risk, from 26 to 31 kg/m2, with regard to Asian population.