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Is Body Mass Index (BMI) the best way to calculate obesity?

What's the best way to decide if you are a healthy weight?

The measure most commonly used by doctors is the Body Mass Index (BMI); a simple measure of weight relative to height, calculated as follows: BMI = weight (kg)/height (m) 2 . It was devised in 1832 by a Belgian mathematician called Adolphe Quetelet (1796-1874) and a healthy BMI is said to be less than 25kg/m 2 whilst obesity starts at a BMI of 30. If you have a BMI of 40 kg/m 2 or above, you are said to be “morbidly” obese, an unhelpful and rather pejorative description which would be better replaced by the term “extreme”.  Nowadays GPs and hospital trusts use BMI as means of deciding who is eligible for obesity surgery, with some areas of the country requiring a BMI of at least 50 kg/m 2 before even being considered. 

BMI is, however, a deeply flawed measure and, therefore, a hopeless means of deciding who would benefit from surgery. This is because it does not discriminate between fat and muscle and so provides no information whatever about the relative proportions of fat to lean (muscle) mass. More importantly, it says nothing about the distribution of body fat – the key factor in determining  obesity-associated health risks such as diabetes, high blood pressure, heart disease and certain forms of cancer. Studies have shown that BMI is actually a very poor predictor of long-term health and that a better estimate is an individual’s waist measurement. This is because fat stores around the waist (central fat) are strongly associated with fatty deposits inside the abdominal cavity (visceral fat) and it is the latter which increase weight-associated health risks. A recent study of more than 225,000 US men and women showed that increased waist circumference was a consistently better predictor of death due to any cause, including heart and lung disease, than BMI.

So, in simple terms, you have a healthy weight if you have a healthy waist circumference.  But what is a safe waist measurement? Different criteria apply at different ages and with different ethnic origins, but as a general rule if you are male and you are more than 40” (102cms) around the middle, you should start worrying. For women the upper limit waist measurement is no more than 35” (90cms). If you are Chinese or South Asian then the upper cut-off should be much lower, probably no more than 31.5” (80cms). To measure your waist circumference apply a cloth tape snugly around the middle at the level of the umbilicus (belly button). No cheating, relax and breathe out! 

Dr David Ashton

8th November 2011

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*Weight loss surgery results and benefits vary and are different for each individual. As such, Healthier Weight cannot guarantee specific weight loss goals.