Cut the Waist

Cut the Waist

Illustrative photo for 'Cut the Waist'
The philosophy of Cut the Waist is to prevent obesity related ill health through education resources

The origins and limitations of BMI

The origins of BMI

The concept of BMI - a simple ratio of weight in relation to height, was the work of a Belgian statistician, Adolphe Quetelet who published his "Quetelet Index" in 1832. It is important to emphasise that Quetelet had no interest in studying obesity when he developed this index.

It was Quetelet's interest in applying probability calculus to human physical characteristics which led him to develop an index of relative weight. He used this index to study the growth of normal man, having established that during normal growth, weight tends to increase in relation to height in meters squared1.

How BMI describes various levels of body fat

After World War II, following reports of increased mortality and morbidity of overweight and obese life insurance policy holders, the validity of the Quetelet Index was confirmed as a practical index of relative body weight. Renamed the Body Mass Index, it was adopted by the World Health Organization in 1995 as a tool to quickly and easily determine level of obesity.

The limitations of BMI

Body Mass Index (BMI) has proved an invaluable tool for identifying and tracking population trends in weight. BMI data continues to provide extremely important information regarding the rate of acceleration of the obesity epidemic over the last thirty years.

Although BMI is a useful measurement across populations, it is increasingly apparent that BMI has significant limitations in the assessment of the individual as it does not take into account the distribution of body fat. BMI measurement does not provide any information regarding where body fat is stored2.

Thus two people with exactly the same BMI can have very different patterns of body fat distribution and thus very different risk of cardiovascular disease and risk of developing type 2 diabetes:

Men matched for BMI and total body fat: Differing 'adiposity phenotypes'

The above CT scans show cross sections through the abdomens of two men who have exactly the same weight and BMI. The black area within the white body outline circumference represents fat stored under the skin, whereas the white central area represents storage of high risk internal body fat.

The scan at the top of the slide demonstrates that this man has stored most of his excess fat under the skin (larger black area). The scan below this demonstrates that this individual stores much more of this body fat as internal, high risk intra-abdominal or visceral fat (larger white area). BMI does not distinguish between these two very different patterns of body fat partitioning.

References

1. Eknoyan G. Adolphe Quetelet (1796-1874) - the average man and indices of obesity. Nephrol Dial Transplant 2008; 23: 47-51

2. Chan DC, Watts GF, Barrett PHR, Burke V. Waist circumference, waist-to-hip ratio and body mass index as predictors of adipose tissue compartments in men. Q Med 2003; 96: 441-447