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Why I Don’t Like BMI



While I am on a brief holiday in Berlin, I thought I’d rerun a few earlier posts that discuss the issue of measuring obesity and how such measures may or may not be helpful in obesity management – as many readers may not have seen these posts before, comments are very much appreciated.

The following was first posted on October 15, 2007

I often get asked to explain or define the term “obesity”. This is when, as a clinician, I am reminded that the conventional BMI-based definition of obesity is problematic.

To be fair, the concept of BMI has been most useful for population studies and there is no doubt that it reasonably reflects average body fat in a given group of people.

Yes, on average someone with a BMI of 30 will probably have more body fat than someone with a BMI of 25, but does this mean that everyone with a BMI of 30 needs obesity treatment and everyone with a BMI of 25 is safe? The diplomatic answer of course is “it depends”!

“Depends on what?” you may ask. Well, it depends on whether or not a) the higher BMI actually reflects more body fat in that individual and b) the person with the BMI of 30 actually needs treatment.

So the question really comes down to – does a given BMI level help me decide who needs obesity treatment? Well, most clinicians will probably agree that taken alone it doesn’t. You probably also need to know the age, gender, ethnic background, waist circumference, family history, current complaints (if any) and risk factor profile to decide who needs obesity treatment.

For example, a young pre-menopausal Caucasian woman, physically active, healthy diet, no risk factors with a BMI of 30 may be safe, whereas a 50 year old South Asian male with elevated triglycerides, hypertension, waist circumference of 95 cms, family history of premature heart disease and BMI of 25 may in fact significantly benefit from losing a few pounds (and keeping them off!).

Well, that is not what the current guidelines or regulators tell me – according to them, our BMI 30 lady has “obesity” and would thus qualify for obesity treatment; our BMI 25 male is not obese and would not qualify – nonsense!

So what is obesity? My rather simple clinical definition is the following:

Obesity is that level of excess fat that threatens or affects someones socioeconomic, mental or physical health – obviously, the level of excess fat that does that will vary from individual to individual depending on their “global risk”.

In fact, even with other risk factors such as dyslipidemia, diabetes or hypertension, we have now moved towards “global risk” where we factor in age, gender, co-existing disease, past history, etc. If this makes sense for dysplipidemia, diabetes or hypertension, why not adopt the same strategy for excess fat? – too complex for the busy practitioner?

Well, who said medical decision making has to be easy?

AMS

3 Comments

  1. Hi Dr. Sharma,
    I’ve been busy but still enjoying the conclusions from our conference in Montreal. I think we should do them once a year.

    As a fitness professional I have known for many years that BMI was not an accurate assessment. As a matter of fact, I have removed it from my assessment protocol as it offers nothing significant for either the fitness staff or patient as far as comparative analysis, program progress or goal setting for clients.

    I believe it will take mountains to remove it from Medicine as it is so easy.

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  2. Hi, Dr. Sharma 🙂

    I agree completely about the BMI. I have had all different levels of body fat at different weights. At a height of 6′ 2″ and a weight of 211 , back in 2007 I was ripped.

    Back in 1997 at age 21 , I was 211 and not that toned, and had a modest amount of fat to lose.

    I think a DEXA scan should be required for all patients ( except those obviously sinewy and low body fat ) to determine their true level of adiposity, but have no idea what this would cost.

    I also would like to see media hyped “successful” contestants The Biggest Loser get a detailed DEXA scan done to determine exactly how much body fat they carry. They lose weight, but that does not necessarily mean it came largely from fat tissue mostly.I have seen this in myself. I have had good results and also terrible. I lost 25 pounds several years in a row- some I looked fantastic – some I just looked like “smaller” version of my former self.

    I bet a lot of them lose a ton of water and considerable muscle. Although none of this is criticism for those with body fat troubles( I realize obesity is complex) , rather a criticism of the current treatment and diagnostic methods.

    I certainly sympathize with morbidly obese poeple and hope they can recover.

    Take care,

    Raz

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  3. Is there actually any evidence that people with lower body fat percentages do not have the same risks as people with higher body fat percentages? People with more lean mass look better (to most) and are likely more physically capable than people with higher body fat percentages at the same weight. But does that translate into lower disease risk? Everyone assumes that it does, but I’d be interested in whether or not that assumption has been tested.

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