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Is BMI a Trustworthy Tool?

Posted Jan 14 2010 12:00am

There continues to be much dialog about the usefulness and merit of the Body Mass Index (BMI) as a tool for screening and diagnosis of eating disorders. For example, there is current discussion underway about routinely monitoring BMI among patients diagnosed with a psychiatric illness. 

Today, my colleague and friend, Margarita Tartakovsky, MS, offers a very informative guest blog piece about BMI for our learning. Margarita is a writer/blogger for  Psych Central and authors very helpful articles about body image and eating disorder issues. Thanks to Margarita for sharing her guest post with us today.

Is BMI a Trustworthy Tool?

It’s used in doctor’s offices all over the country to ascertain fatness and health. It’s used in research studies to establish eligibility, significant change and even policy. Schools have sent report cards with it, using the number as the sole measure of a child’s health. City councils have used it to verify a model’s qualifications for the catwalk . Insurance companies use it to charge higher premiums. Health professionals use it as one indicator of recovery for patients with eating disorders.

Body Mass Index (see Dr. Shepphird’s post  on calculating BMI) has many uses, all of which are important and influential. Countless studies, from public health to psychology, use BMI regularly. Recently, however, experts have not only questioned the use of BMI but have called for its replacement with other more reliable measurements. 

The biggest issue with using BMI is that it isn’t a valid gauge of health. It’s common for people who are fit, healthy and active to have high BMIs. Using only BMI tells us that these individuals are overweight or even obese. According to BMI standards , Kobe Bryant and Will Smith are overweight. Tom Cruise is obese, and so was Arnold Schwarzenegger in his body-building days. Kate Harding of Shapely Prose has a slide show   of photos that illustrates how unreliable BMI can be.

In an article   on NPR , Keith Devlin writes:

“[BMI] makes no allowance for the relative proportions of bone, muscle and fat in the body. But bone is denser than muscle and twice as dense as fat, so a person with strong bones, good muscle tone and low fat will have a high BMI. Thus, athletes and fit, health-conscious movie stars who work out a lot tend to find themselves classified as overweight or even obese.”

Interestingly, the two individuals who contributed to creating BMI never intended for it to be used as an individual indicator of fatness or health. In 1832, Belgian mathematician Adolphe Quetelet developed the equation with the aim of identifying the normal man. As Jeremy Singer-Vine writes in Slate:

“This project had nothing to do with obesity-related diseases, nor even with obesity itself. Rather, Quetelet used the equation to describe the standard proportions of the human build—the ratio of weight to height in the average adult. Using data collected from several hundred countrymen , he found that weight varied not in direct proportion to height (such that, say, people 10 percent taller than average were 10 percent heavier, too) but in proportion to the square of height. (People 10 percent taller than average tended to be about 21 percent heavier.)”

Quetelet’s equation was essentially neglected until physiology professor and obesity researcher Ancel Keyes coined it the Body Mass Index after finding it to be the best measure of body-fat percentage among 7,400 men in five countries. After Keyes’ landmark study, BMI was quickly picked up by physicians who wanted a simpler and cheaper way of assessing body fat. Singer-Vine writes:

“His original paper warned against using the body mass index for individual diagnoses, since the equation ignores variables like a patient's gender or age, which affect how BMI relates to health. It's one thing to estimate the average percent body fat for large groups with diverse builds, Keys argued, but quite another to slap a number and label on someone without regard for these factors.”

(For a thorough history, see Singer-Vine’s article here or herewhich provides a great summary and where I initially found the Slate article.)

“It's now widely accepted that the BMI is uselessfor assessing the healthy weight of individuals,” Dr. David Haslam, the clinical director of Britain's National Obesity Forum, tells The UK Telegraph .

So if BMI is a lousy measurement, what are the alternatives? Tim Cole, a professor of medical statistics at the Institute of Child Health at Great Ormond Street Hospital, suggests using waist circumference , because “it is a direct measure of the part of the body that tends to accumulate fat. Having a waistband of more than 88cm (35in) in women and 102cm (40in) in men indicates the highest risk of cardiovascular and metabolic disease. There is an increased risk of the diseases for women with measurements of more than 80cm (32in) and men whose measurement is over 94cm (37in).” The American Society for Nutrition, the American Diabetes Association and other medical groups have encouraged using waist circumference to either supplement or replace BMI, writes Singer-Vine.

In the same article, Dr. Margaret Ashwell, a visiting research fellow in nutrition at Oxford Brookes University and a former member of the government's Food Advisory Committee, suggests using the waist:height ratio. She explains that a bigger middle indicates a big problem. It’s “where the excess fat gathers that causes problems,” she says.

BMI has been in our vernacular for years, and as Singer-Vine points out, regardless of where an individual may fall on the BMI spectrum, this number can be harmful. It can damage a person’s self-image and deceive another into thinking he or she is healthy. He writes:

“Faulty readings could promote a negative self-image among healthy people and lead them to pursue unnecessary diets. Or the opposite problem: People with a little too much body fat might be lulled into a false sense of complacency by a misleading BMI.”

So should BMI be banished from use altogether? Obesity expert Cathy Nonas, MS, RD, a spokeswoman for the American Dietetic Association, tells Medicinenet.com that BMI “is a very good and easy screening tool.” But BMI is not a diagnostic tool, experts agree. As the WebMD article summarizes, “BMI does not take into account age, gender, or muscle mass. Nor does it distinguish between lean body mass and fat mass.” But I wonder how often BMI is used as a diagnostic tool. It seems that in research, school report cards, doctor’s offices and insurance companies, BMI carries a lot of weight. And for a number that has so many limitations, one has to wonder it gets used at all. 

Do you think BMI has value? Do you use BMI in your practice? If so, why?

_______

Margarita Tartakovsky, MS, blogs about body image at Psych Central’s Weightless ( http://blogs.psychcentral.com/weightless/ ). Follow her on Twitter @Weightless blog ( http://twitter.com/Weightlessblog ).

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