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12Apr

This is a really complex subject so I will try and break it down:

BMI and the DSM

The DSM threshold for anorexia nervosa is a BMI of 17.5.  Two points to notice.  The first is that a person with a BMI of 17.6 is still “underweight”, albeit “moderate”.   What is that about?  I note that you are not “mildly” or “moderately” obese.  This leads back to my “Fat is Bad and other urban myths” blog.  The second is someone can be suffering from an eating disorder at a BMI of 20 +.  You don’t have to be thin to be anorexic, or bulimic, or have an eating disorder.

BMI is one of my pet hates.  I can do no better than quote from Wiki on this to explain my chuntering.

The body mass index (BMI), or Quetelet index, is a heuristic proxy for human body fat based on an individual’s weight and height. BMI does not actually measure the percentage of body fat.

While the formula previously called the Quetelet Index for BMI dates to the 19th century, the new term “body mass index” for the ratio and its popularity date to a paper published in the July edition of 1972 in the Journal of Chronic Diseases by Ancel Keys, which found the BMI to be the best proxy for body fat percentage among ratios of weight and height;[3][4] the interest in measuring body fat being due to obesity becoming a discernible issue in prosperous Western societies. BMI was explicitly cited by Keys as being appropriate for population studies, and inappropriate for individual diagnosis. Nevertheless, due to its simplicity, it came to be widely used for individual diagnosis, despite its inappropriateness.

However, BMI has become controversial because many people, including physicians, have come to rely on its apparent numerical authority for medical diagnosis, but that was never the BMI’s purpose; it is meant to be used as a simple means of classifying sedentary (physically inactive) individuals with an average body composition (Highlighting mine)

I have said it so many times before, but just in case someone hasn’t heard it before:

No one patient is the same.  Therefore, no one treatment will work for everyone.”  This applies as much to the diagnostic criteria, as anything else.  Once it is understood that Eating Disorders are brain disorders and that the physical symptoms are the end stages of the disease, rather than early warning signs, perhaps we will have a better chance of early intervention.

Weight

Everyone has an “optimum function” weight range.  This is a spread of weight where someone is at their best physically and mentally.  The “normal” weight range is a BMI of 20 – 25 (well 24.9 to be precise but what’s a couple of pounds between friends?).  There is a tiny (and I mean tiny) percentage of the population that is healthy at a BMI of 18.5.  Why do some clinicians insist that every single eating disorder sufferer falls into this category?

I refuse to believe that all eating disorders patients are of the lean, slim, exceptionally fit marathon runner genotype.  If they were, wouldn’t someone have noticed this?  Some may be but I would argue that they are the exception to the rule and that the majority of eating disorder patients fit into the “normal” weight range (BMI of 20 – 25) for their personal optimum function.

Whether it is the insurance companies in the US or the National Health systems of the UK, Australia and New Zealand, discharging a patient at a “healthy” BMI of 18.5 is saving money, in the short term.  There is a very strong argument that discharging a patient at a low weight  is more likely to result in a relapse and readmission.

Saying “haven’t we done well” because a patient has reached a number on a scale and is now “recovered” does not mean a patient is mentally and physically well.  Apart from the fact that BMI is a population screen and not a diagnostic tool, expectations that a patient is now “cured” can be confusing, disheartening and disempowering for the patient and the carer.

So, a “recovered” BMI of 18.5 is a nonsense really and should be ignored.  It is a just a number.  It is NOT a measure of mental or, I would argue, physical health

Brain Disorder

“Once it is understood that Eating Disorders are brain disorders and that the physical symptoms are the end stages of the disease, rather than early warning signs, perhaps we will have a better chance of early intervention.”

Now I am not going to make a habit of quoting myself but I need to just get a point across here.  I listened to Dr Thomas Insel’s speech at the F.E.A.S.T. conference in Alexandria and he contended that the physical symptoms of eating disorders are the “end” stage of the disease, in the same way that psychosis is the “end” stage of Schizophrenia.  If that is true, we should be offering intensive curative care to patients.  Discharging them before they have reached their “optimum function” weight range seems to me to be offering palliative care.  Does this mean that clinicians who discharge patients before they are recovered believe that nobody can recover from an eating disorder?  Or do they believe it is an eating disorder is a “choice” by the patient?

For further information on Optimum Function, please watch State not Weight.

Courtesy of Charlotte Chattering and Chuntering

  

5 Responses to An Explanation of BMI and its shortcomings

  1. This is really interesting as I do think people get hung up on BMI – I know I do as my BMI is showing me as overweight!

    And I do agree that a lot of it – whether that’s under or over eating is a mind thing.

    Really interesting blogs – thank you.

  2. Absolutely. The ranges are derived from the Met Life Insurance actuary tables. Wtf, right?! I wrote an article on this last year, which echoes everything you wrote here but without the ED focus here.

  3. Pingback: What every doctor should know | talkhealth Blog

  4. Brava Charlotte! Thank you for so clearly laying out this information. Wonderful. I’ll share with as many docs as I can…tough to change ideas that have been so long embraced but we must keep trying.

    Becky Henry
    Hope Network, LLC

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