Some blog topics draw in a continuous trickle of commentary long after they've been published.  One such blog is entitled “Determining Ideal Body Weight”. And little wonder.

 

The other day I answered a comment by a young reader named Charlotte. It started me thinking how urgent her question might be for others who may not have read the original blog (or at least not for some time).  Charlotte wrote:

"I am an 18 year old female with a history of initially EDNOS and then AN B/P subtype, for over 4 years now. I have never lost my period for more than 3 months, however it has been irregular for basically the duration of my eating disorder.

I am currently in residential treatment. When I told my dietitian that I had my period this month and last after missing it completely for 2 months, she recalculated my goal weight to give me a BMI of 17.3, on the grounds that she didn't want to push me too far and cause relapse (as has happened before).

However, I am unsure whether I can actually be healthy with a BMI of 17.3. I have never been big; most of my childhood I was either underweight or just barely normal. My height is on the 90-95th percentile (accelerated from the 50-70th just before my ED started). My weight has historically followed around the 25th percentile, increasing to the 50th as my height increased.

I was wondering what your opinion on this new goal weight is? On the one hand, my eating disorder is very excited, but I'm not sure if it would be healthy or not?"

To which I replied:

"How sensible you are! It is very unlikely that a BMI of 17.3 is adequate for you or pretty much anyone. You said yourself that your weight tracked before the illness to between 25-50%tile, which -- if you're 18 years old -- is not consistent with a BMI of 17.3 percent.

But more than that, calculating a goal weight by BMI is not sensible on an individual (not population) basis. It is the opposite of "state not weight”.  As to your "state", irregular periods in an 18 year old who is likely to have had her periods for around 6 years, may mean that the cycles are anovulatory for too many reasons to go into here. You had regular periods premorbidly (before the illness) and should strive to achieve that again.”

To be afraid to "push someone too far and cause relapse" is, IMO, a misguided understanding of AN. You cannot appease this illness by allowing someone to remain underweight."

 

As I reflect on this exchange I feel it needs a little expansion. To the issue of determining BMI in an individual patient and sharing it with them (presumably as a proxy for weight):

 

Setting a goal weight for someone who has had an eating disorder since early-mid adolescence or even earlier is hard enough, complicated as it can be by stunted/arrested growth.  Adding an individual BMI calculation to the mix makes it less accurate, not more.  

 

And BMI cannot be a proxy for health either because, famously, it varies with age and Tanner stage and also because it can be terribly skewed by “fitness” and muscle mass.  A very muscular person may have a BMI technically in the “obese” range and yet have very little bulk other than lean body mass.

 

Ancel Keys (of Human Starvation fame) worked a great deal with the concept of BMI as it relates to “fatness”. According to Wikipedia’s entry for body mass index: “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” (emphasis mine).

 

So it may be simple, but it ain’t right.

 

Even more important than the controversy surrounding BMI Charlotte’s query on my blog brings up the issue of trying to “appease” anorexia nervosa by minimizing weight gain.  As I stated, there is no weight low enough to appease an eating disorder that has been present since this young woman was in middle school. That is a fool’s errand indeed.  Once you are at the “acceptably lower weight”, it will eventually also become unacceptable.

 

Do patients panic when they have to gain weight?  You bet.  And a typical time for this to happen is when patients cross the threshold of  “approximately 90% of their goal” and reach what we call their “phobic weight”.  But that does not mean that you recalculate the goal weight to avoid this.  There is no avoiding it. It must be worked through.

 

I do get a great deal of criticism for telling patients “I will not let you get fat”.  By this I mean that I will return them to a weight that reflects their own state of recovery, which is the real goal.  What is that state?  For women it is the weight at which they can have regular, ovulatory periods, participate meaningfully in their life and the life of their loved ones, earn a living (if old enough), and/or go to school and find a social equilibrium that is normal for them.  For males, and very young females, it's all of the above minus the periods.  Such as state is likely to involve temperature, heart rate, blood pressure, estradiol and other hormones, glucose and insulin metabolism, thyroid functioning, leptin and possibly a pelvic ultrasound.  Sound complex?  That’s because it is!

 

So what I really want to stress is: anorexia nervosa is a disease like any other and for which we have some biological guides for a return to functioning.  We can’t appease the brain disorder and the anosognosia by skimping on the weight restoration.  We can’t both - patient and provider - be afraid of complete weight restoration.

 

Be strong.  Act as a beacon to your patients, not one more voice singing the siren song of the eating disorder.