Students Weighed and Measured

Does BMI = TMI?

Body Mass Index (BMI) and Childhood Obesity. Tetra Images - Rob Lewine/Brand X Pictures/Getty Images

There are, admittedly, many limitations and some potential liabilities attached to the BMI, or body mass index.  The BMI measures weight adjusted for height, and is typically expressed as weight in kilograms divided by the height in meters squared.  For children, the desired values are adjusted for age and sex.  For adults, a BMI above 25 is considered overweight; above 30 is stage I obesity; above 35 is stage II; and above 40 is stage III, or severe.

The principal limitation of the BMI is that it measures heaviness, not fatness, when fatness is what we really need to know about from a public health perspective.  The measure does not account for frame, and those who are wider or “heavier boned” will tend toward higher BMIs even without surplus body fat.  That is even truer of athletes with lots of muscle mass, the obvious extreme being body builders who have enormously high BMIs, but at times almost unbelievably low body fat levels.  Not that it makes them healthy, but the perils of competitive bodybuilding are a topic for another day.

Despite these and other limitations, the BMI has proven enormously useful in public health, and is, in fact, the principal measure used to gauge weight trends in the general population. It is courtesy of the BMI that we are able to say what percentage of Americans (or others) are overweight or obese, and whether those numbers are rising, falling, or steady.

  The BMI is used for this purpose because it is good enough, and the measures that would be more perfect tend to be the enemies of getting good epidemiologic monitoring done.  Body composition testing, for instance, at the level of the whole population or in a representative sample, would be terribly expensive and time consuming.

The BMI works, because it does reflect what is going on in the world pretty reliably.  We don’t have an epidemic of athleticism or muscularity- we don’t need any measures at all to know that.  We do have epidemic obesity, in adults and children alike.  While the BMI might ignore the distinction between muscle and fat in any given individual, at the population level, trends in BMI indicate trends in fatness with considerable fidelity.

Other limitations of the BMI pertain to specific cut-points.  The cut-point of 25 is used to define overweight in adults, and an age- and sex- adjusted analogue to that cut-point is used in children, because the risk of weight-related metabolic harms rises at the level.  But studies have questioned whether that is reliably so, and in particular, whether that is true in older people.  Cut-points on any continuous scale are somewhat arbitrary, as are one-size-fits-all categories; this is no exception. 

There is also the fact that some people can have BMIs below 25 and still have an excess of body fat.

  Some entire population groups in Asia are prone to this so-called “lean obesity,” with slight frames but an excess of abdominal fat just the same.  And some people, including many women before menopause, can accumulate excess body fat rather safely in the lower part of the body- hips, buttocks, thighs- and have no obvious harms from a BMI above 25.

All such limitations are surmountable.  The BMI can be used for public health tracking, with more customized assessments appended to guide the care and counseling of individuals.  But there is another concern about a different kind of liability: what if reporting the BMI of children in the first place does harm?

This is the very issue being raised by the Academy of Eating Disorders, among others, which recently issued a press release opposing the reporting of student BMIs by schools.  The so-called “fitnessgrams” report student BMI home to families, and indicate where they fall relative to national standards.

The reason for the Academy’s opposition is self-evident: they are concerned that this focus on weight will propel children toward eating disorders.  This worry is far from new, and arises with regularity and in diverse contexts among all of us who are working to treat, manage, and prevent obesity and its consequences.  As Editor-in-Chief of the peer-reviewed journal Childhood Obesity, I have waded into these very waters before.

My opinion now is much as it was then.  What happens when students, or students’ bodies, are weighed and measured is all a matter of context. 

We of course “weigh and measure” students in schools all the time; that’s what grades are.  We don’t tend to think that grades are a bad idea because they are a source of blame, or shame, or stigma.  But of course, poor grades can be exactly that.  Despite those perils, we seem to believe in the need to know.  And we rely on sensible priorities and prudent behaviors to put grades to good use, rather than bad.  Grades are not intended to shame children, but empower them and their families to know where their studies need additional work.  We have resources in place, if not always adequate ones, to help address difficulties revealed on report cards because addressing them early has the potential to improve performance, and over time, opportunity.  Better opportunities in turn can mean a better job, and a better life.

The intent of measuring BMI in children is the same.  Substantial evidence indicates that parents often overlook incipient obesity in their children.  If this happens, kids are increasingly prone at ever younger ages to complications, from type 2 diabetes to fatty liver disease.  Obesity and its consequences translate into poorer performance in school, and frequent absences.  A rising BMI that is overlooked until obesity develops is thus apt to do direct harm to the body, and indirect harm to the learning that is the reason for being in school in the first place.

The critical questions about the practice, then, are these: (1) how is this information reported home? And (2) what actually happens when families get the information?

The answers to the first are a work in progress, and have been ever since the practice was first adopted state wide in Arkansas when Mike Huckabee was governor.  The reporting needs to be compassionate and informative, not judgmental.  It needs to indicate that obesity is a public health problem, not an indicator of personal deficiencies in adult or child.  The reporting needs to indicate clearly that the focus is on health, not weight per se- but that weight is often the first indicator of looming health problems.  We might, for instance, measure and report the blood pressure of children- because high blood pressure is an increasing problem among our obese children and grandchildren.  But weight changes first.  And finally, the reporting needs to indicate the constructive steps that can be taken to nip an inchoate problem in the bud, just as problematic grades need to lead to a guidance counselor.

The answer to the second is, to the best of my knowledge, quite reassuring.  The reporting of student BMI by New York City schools is just one of many practices aimed at controlling rates of childhood obesity.  The evidence has long been available that obesity rates in NYC schools were in fact declining slightly while rising elsewhere, with a recent report indicating that the same is true of severe obesity rates.  This leaves us a long way from confetti and champagne, but is clearly good and promising news.  I have not found any evidence of a concurrent rise in the rates of eating disorders among these same children.

Eating disorders are a serious public health concern in their own right, and a societal fixation on elusive thinness, propagated in particular in the media and fashion industries, has been implicated in their spread.  The reporting of BMI by schools could figure in this, but there is no evidence that it does, and no reason why it should.  The reporting should be about informing and empowering, not judging.  It should be standard and universal, not selective and stigmatizing.  The focus should be on health, not weight per se- but the importance of weight to health should not be ignored, any more than that of blood pressure or other important indicators.

Does the BMI equal TMI (too much information)?  It should not.  Our children are learning the lessons of our attitudes all the time.  If we consider weight one useful predictor of health outcomes and react accordingly to information about it, our children will treat it as such.  If we are in fact inclined to blame obesity on its victims and indulge in biases against them, our kids will indeed be victims.  But in that case, the fault lies not with the informative fitnessgrams, but with ourselves, and our faulty reactions to them.

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