What is obesity?

John DiTraglia M.D.


Both the American and Canadian Medical Associations have now declared obesity to be a chronic disease, like type 2 diabetes and hypertension. This is a good thing if for no other reason than that it reduces the stigma and discrimination on many victims of obesity. Then what is obesity? The World Health Organization’s definition of obesity is “abnormal or excessive fat accumulation that may impair health.” “To use this as a definition of an actual ‘disease,’ it would be prudent to remove the word ‘may’”, as pointed out in an editorial in April’s issue of the journal Obesity (1).

Most studies of obesity use the body mass index (BMI=weight in kilograms divided by height in meters squared) as the definition of it. The body mass is composed of fat mass but also muscle mass, bones and other soft tissues. Of these fat mass is only a small part. Lean mass makes up the vast majority of body mass. The BMI might be a reasonable compromise in studying collections of a population but it is a particularly poor way to decide if an individual has a disease.

We need to decide if an individual customer has fat that is impairing his health. A fat but fit person would not benefit from weight loss, whereas a person with fat in his liver and belly and small muscle mass and a BMI well below the usual 30 kg/m2 cut off would benefit from losing that fat. However a fat but fit individual who is struggling with depression and social anxiety due to excess body fat does have the disease after all.

In kids we have even more problems with BMI as a definer of obesity because their BMI is constantly normally changing. So we use charts and the BMI z-score to try and deal with this. Your BMI z-score is a measure of how far your BMI is from the average BMI using percentiles for the whole group of same sex, same age kids. But it gets more and more inaccurate as you get into the higher BMI percentiles. BMI z-scores are a poor indicator of fatness among the 2 to 19 year olds with very high BMIs for whom it is used. (2)

BMI is just one of many inputs into the definition of obesity and not a very good one. It is time for doctors to stop hanging their hat on it.


John DiTraglia M.D.

John DiTraglia M.D. is a Pediatrician in Portsmouth. He can be reached by e-mail- jditrag@zoomnet.net or phone-354-6605.

John DiTraglia M.D. is a Pediatrician in Portsmouth. He can be reached by e-mail- jditrag@zoomnet.net or phone-354-6605.


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