The Skinny On The BMI Doctors And Statistics Agree That The Body-mass Index Is A Reliable Weight Gauge

It’s a health trend that makes the news more often than just about any other: Americans are fatter than ever.


Many of the reasons have been well-hashed over, including sedentary lifestyles and an overreliance on fast food and other unhealthy eating habits.
On a regular basis, bad news about obesity is reported by the media, such as the recently unveiled Framingham Heart Study, which tracked almost 3,500 Massachusetts residents over more than four decades and determined that obesity, on average, cuts significant years from one’s life.

That study, and many others, use something known as the body-mass index (BMI) as a gauge.

“If you look at large studies involving tens of thousands of people, body-mass index correlates very well with the consequences of excess fat. Cases of diabetes and heart disease are more common when BMI is higher, so it becomes a good number to be aware of,” said Dr. Stuart Chipkin, chief of Endocrinology, Diabetes, and Metabolism at Baystate Medical Center.

But as the bad news continues to flow, some have asked whether the index is a realistic measure of physical health — and, for that matter, why the index was developed in the first place.

This month, The Healthcare News delves into those questions in an effort to deliver the skinny on the BMI.

Height and Weight

With so many different body types out there, a means of leveling the playing field when it comes to weight was inevitable, Chipkin said.

“It’s that old joke: ‘I’m not really overweight; I’m just too short,’” he said. “The body-mass index was developed to essentially compensate weight for height. The feeling was that this would be a way to put everyone on an even footing.”

Developed by the federal government in 1998, body-mass index basically assigns a number based on a formula that incorporates height and weight. Calculating it is simple: multiply one’s weight in pounds by 703, then divide by height in inches, and divide by height in inches again. A number over 25 indicates someone is overweight, while more than 30 signals obesity. A number between 18.5 and 24.9 falls within the “normal” range. By this scale, about half of all Americans are currently either overweight or obese.

Given those statistics, BMI may seem like a harsh scale. But the National Institutes of Health (NIH) calls it a reliable gauge, while research such as the Framingham Heart Study seems to back that up.

In that long-term study, 3,457 participants, all of them between the ages of 30 and 49 at the start, were tracked throughout their lives — over 42 years, in fact. The idea was to determine how overweight and obese conditions around age 40 affect life expectancy years down the road.

The study concluded that significant decreases in life expectancy were associated with being overweight or obese. Forty-year-old female nonsmokers lost 3.3 years of life expectancy, and 40-year-old male nonsmokers lost 3.1 years, due to being overweight, compared with their trimmer peers. Forty-year-old female nonsmokers lost 7.1 years, and 40-year-old male nonsmokers lost 5.8 years, because of obesity.

mong smokers, obese females lost 7.2 years of life expectancy, and obese male smokers lost 6.7 years, compared with normal-weight smokers. Obese female smokers lost 13.3 years, and obese male smokers lost 13.7 years, compared with normal-weight nonsmokers.

Idenfying who was overweight and obese at the start of the study was determined by applying the new body-mass index standards to the participants’ heights and weights as recorded at the start, and the drastic mortality results seem to suggest that the BMI is a fair measure of who is carrying too many pounds.

“When you statistially analyze thousands of people, there are always going to be exceptions,” Chipkin said. “It won’t hold 100{06cf2b9696b159f874511d23dbc893eb1ac83014175ed30550cfff22781411e5} of the time — but it holds pretty well most of the time.”

Theresa McAndrew, clinical dietician at Holyoke Hospital, noted two common exceptions in which case the BMI is not useful. “It can overestimate body fat in athletes who have a high muscle mass, and it can underestimate body fat in older adults who have lost muscle mass,” she said. “In those two areas, it might not be that accurate.”

“The BMI doesn’t work as well when somebody is very muscular,” Chipkin agreed. “The extreme example is a really strong athlete. A bodybuilder is going to have a very heavy weight and have a higher BMI, but he has so much muscle that he’s really not at risk. There’s always going to be that kind of exception.”

Because of such exceptions, the NIH characterizes the BMI as a starting point, stressing that other factors must be considered, including waist circumference and risk factors for diseases and conditions associated with obesity, before a complete picture of one’s health emerges.

“It’s just a guideline the general public can use to calculate where they stand,” McAndrew said. “There are differences in gender anyway: women have higher body fat mass than men. From there, people need to see what kinds of risk factors they may be up against.”

hose risk factors include high blood pressure, cholesterol, triglycerides, and blood sugar; family history of premature heart disease; physical inactivity; and, as the results of the Framingham study suggest, smoking.

For those looking for an objective scale who don’t want to depend completely on BMI, many doctors also consider waist circumference a risk factor all its own, as it’s a good indicator of abdominal fat, which can be a predictor of heart disease and other problems. This risk increases with a waist measurement of more than 40 inches in men and more than 35 in women, according to the NIH.

Stemming the Tide

Clearly, although it isn’t the end-all measure of health, the BMI is taken seriously by doctors. However, the Centers for Disease Control and other bodies have been examining whether exercise and healthy eating might reduce some of the dangers touted by the studies, such as diabetes, for overweight people.

“If you don’t think body-mass index is applicable to you, just check with your doctor,” Chipkin said. “There are exceptions, but most people do fit the rule.”
A key is staying active, doctors say. Inactivity is cited as a key reason why obesity has reached epidemic proportions among children and adolescents. The CDC reports that 15{06cf2b9696b159f874511d23dbc893eb1ac83014175ed30550cfff22781411e5} of all Americans age 6 to 19 are overweight and inactive, although some studies have put that figure even higher. Overweight young people, studies show, are much more likely to develop diabetes, hypertension, or heart disease down the road.

“It’s getting younger and younger, which is too bad,” McAndrew said. “It’s a combination of things, such as fast food and lack of physical activity. Nowadays, kids are playing at age 5 with Nintendo and Gameboy, where 20 or 30 years ago, they were outside, being more active.” The rise of the Internet culture hasn’t helped, she said.

The bottom line for those who don’t see the BMI as a realistic gauge, doctors say, is that the scale hasn’t changed, while the habits of Americans have, even in the five years since the inception of BMI. Blaming the messenger doesn’t help matters.

“If you look at the difference just in the last 10 years,” Chipkin said, speaking of not only overweight young people, but adults, “the rates have climbed exponentially, faster than anything else out there. It’s just scary.”

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