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BMI: One size does not fit all



It’s an acronym many know. “What’s the BMI?” is a question many can quickly understand and relate to one’s being overweight or not. For the past decade, the body mass index has become a go-to for doctors and patients as a primary, or at least baseline, indication for whether a person should take a hard look at lifestyle changes or even surgery to lose weight.

The calculation can simply be broken down to multiplying ones weight in pounds by 703, then dividing that total by height in inches, squared. Sound like too much math for you? No problem. There are numerous BMI calculators on the Internet today that allow a person to plug in weight and height and voila! A BMI score pops up. The number is supposed to gauge a person’s approximate body fat percentage, experts say.

Internet BMI calculators have become increasingly popular among Americans. The National Heart Lung and Blood Institute’s Web site sees 25 million unique visitors to its BMI calculator annually, according to Karen Donato, NHLBI’s coordinator for overweight and obesity research applications. The National Institutes of Health division’s Web site also offers an iPhone app download for those who want to calculate BMI on the go.

BMI score ranges in the U.S. run as follows: below 16 is considered severely underweight; 16 to 18.5, underweight; 18.5 to 25, normal; 25 to 30, overweight; 30 to 35, obese; above 35, severely obese. A person with a BMI topping 40 is often recommended for bariatric surgery.


The BMI’s roots can be traced back to Belgium in the mid-1800’s. But its formal naming, the Body Mass Index, didn’t come until 1972, when American scientist Ancel Keys published a paper on it in the Journal of Chronic Diseases. In his research, Keys found it to be most appropriate for population studies, not necessarily individual diagnosis.

In 1998, after years of its own research, the NHLBI issued clinical guidelines for evaluation and treatment of overweight and obese people by primary care physicians. In it, the BMI became a baseline for doctors to gauge a person’s appropriate weight for their height. The final score could also be interpreted as an estimate of body fat percentage for an individual. Convenience and simplicity also played a role in recommending the BMI to primary care doctors.

“We found that it was a very good estimate of body fat … and that it was very easy to use,” Donato adds.

One of the factors that give the BMI credibility is the fact that it uses fairly wide weight ranges for height measurements. For example, on the U.S. Centers for Disease Control and Prevention Web site, a person with a height of 5 feet, 9 inches, can still be considered to be in the normal BMI range as weight can vary between 125 and 168 pounds. This helps to adjust for a person’s gender, frame or build.

The new guidelines also came with additional criteria. Physicians are encouraged not to simply look at the BMI number but to also measure a person’s waist circumference. Abdominal fat has been tied to more serious health consequences than fat on other areas of the body. The guidelines indicate that men with waists topping 45 inches and women topping 35 inches should consider a weight loss program.

In addition, primary care doctors should also look at additional risk factors, like high blood pressure, Donato says.

With added guidelines, the agency seemed to be pre-empting what would soon come, presumed “healthy” people with larger frames and high muscle content wondering why the BMI is categorizing them as “overweight” and even “obese.”

“We understood there were limits. The BMI can certainly over-estimate body fat in athletes and underestimate in an older person,” Donato admits.

Prior to the BMI recommendation in the late 90’s, many doctors used the Metropolitan height and weight table, a chart estimating ideal weight based on a person’s frame, height and weight. The table came about in the 1940’s when a statistician, Louis Dublin, grouped some 4 million people ensured by his employer, Metropolitan Life Insurance Company, in an attempt to categorize ideal weight ranges. The chart was revised in 1959 and 1983, many claiming the earlier version to be a better one. Still, by the late 1990’s a quest for a simpler gauge like the BMI was sought.

“We looked at various measures of weight to use in the primary care setting, BMI is still a really good gauge of fat. We knew by looking at data that as body fat went up, so did the onset of disease,” Donato adds. “Above (BMI) 30 the mortality rate increased a lot.”

BMI and bariatric surgery

Dr. James Atkinson, a valley bariatric surgeon, says the BMI, while used in his field a lot, only takes into consideration height and weight. The formula is a bit too simplistic for a fat percentage, according to the surgeon. Ideally, a submersion test will yield the best results for a fat percentage. But the tests are usually expensive and time consuming, he notes. But for the general population, the surgeon says BMI is a good place to start.

“When you’re talking about the general population that’s relatively sedentary, it can be a pretty accurate gauge,” he says.

Where Atkinson is concerned is that the FDA (Food and Drug Administration) recently lowered its recommendation for surgical band operations to BMI calculations between 30 and 35. Before it was only recommended for 35 and above.

“To me, a BMI in the range of 26 to 35, you’re still in the realm of good behavior where you can control things without surgery,” he says.

Atkinson understands that research attached to BMI notes health risks increase when someone tops a score of 27, theoretically, but he doesn’t want to be sending the wrong message about jumping to surgery too quickly.

“At 27 things are not as clear cut as they are for someone that’s say 35, 40 or 45,” he adds.

Allen Rader, an Idaho-based bariatrician who has trained numerous physicians around the country on his weight-loss programs, says BMI figures can “skew appropriate treatment.”

“Arnold Schwarzenegger’s BMI would probably qualify him for bariatric surgery,” the physician notes. “But his weight is all muscle.”

Rader says, while the NHLBI’s waist size considerations are in place, too many physicians still don’t address the location of fat on a person’s body.

“Somebody may be carrying around 40 pounds of fat, but a lady with a pear shape carrying it in her hips and legs will see totally different health consequences from somebody who may have it in the intra-abdominal cavity,” he adds.

In addition to calculating BMI, Rader uses a complex bioelectrical impedance analysis device to measure a person’s true body fat composition. These devices came on the market in the mid 1980’s and have become more sophisticated and easy to use through the years. Once he gets a sense of a person’s true fat content and evaluates further by studying the location of the fat, Rader then sets out to address losing fat specifically, instead of just weight.

“Many severely overweight patients can also have extra muscle mass,” he says.


For children, BMI calculations are a little more involved. Donato recommends visiting the CDC Web site for guidelines. There, a child’s BMI calculation is also measured against CDC growth charts that have been in place since the 1970’s. Where a simple calculation for an adult requires inputting only height and weight, a child’s BMI calculator on the site requires a birth-date, calculation date and gender specification as well. Final calculations not only give a BMI figure, but also rank a child on the growth chart by percentile.

Donato says regardless of BMI calculations for children, many parents have a hard time seeing their children as being overweight. She argues that many children who fall into the 85th or 90th percentile for height and weight may be overweight or obese, and, unfortunately, too many parents refuse to see the figures as an appropriate warning sign. Childhood obesity rates from the CDC hit 15.5 percent recently. Others have estimated upwards of 20 percent in the past.

“I think the perception of what normal weight is for people is out of whack,” she says. “Parents don’t see their kids as overweight.”

In an effort to address the issue of childhood obesity, the NHLBI recently launched its We Can! (Ways to Enhance Children’s Activity & Nutrition) initiative. On the We Can! Web site, the NHLBI and other NIH partners, like the National Cancer Institute and the Eunice Kennedy Shriver National Institute of Child Health and Human Development, offer suggestions for parents to engage in active lives and sound nutritional practices with children. Topics like the “Energy Balance” equation, which looks at appropriate calorie intake with regards to activity level; making activity plans for families, and sample eating plans are just a few covered by the program.

“This is all about educating parents and caregivers. We need to instill healthy lifestyle early in the curriculum,” Donato said.


The BMI has undergone its share of tinkering through the years. In 1998, when the NHLBI made its recommendation for primary care physicians, the previous cut-off score for normal/overweight categories was 27.8. That year it was reduced to 25, which ultimately meant 25 million Americans were suddenly redefined.

Since that time BMI and the topic of ethnicity has created more changes in the formula and weight ranges. In Japan, for instance, the BMI cut-off for being overweight is reduced to 23. Others, like Singapore, have revised language to indicate health risk factors instead of emphasizing being overweight or obese. Those in the normal range are considered “low risk” for health problems such as diabetes. Higher BMI figures bring “moderate risk” and “high risk” labels. Other race and ethnic considerations are still being taken into account.

“Right now we’re really looking at this in terms of what are the relevant points for all types of different ethnic population groups,” Donato adds. “Asians tend to be smaller and we’re asking ‘should it (a score) be lower?’ … We’re always in the process of updating clinical guidelines.”

While many will likely always question its merits, BMI calculations are still popular, given the amount of traffic the calculators see on the Internet. Even with so much caution about how much stock to put into the figures, the calculation can still help nudge and overweight person into a better lifestyle direction.

“I think it’s an interesting gauge for people to be aware of and it can be quite useful,” Donato adds.

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