Is BMI a reliable indicator of obesity and overall health?

Ever been told your BMI (Body Mass Index) was unhealthy, or have you checked an online BMI calculator to see if you fall into the overweight or obese categories? Well, you may not have to be as concerned as you thought about the number, on its own.

The American Medical Association now says BMI is a flawed tool when used alone as a measurement, and doctors should consider other factors that affect it, including:

• Age
• Gender
• Ethnicity
• Variation in body types
• Genetics
• Metabolism
• Illness or medical conditions

To be sure, the AMA does recommend that people pay attention to their weight, but suggests BMI be used with other measurements, such as waist circumference, when assessing weight. The association says BMI’s usefulness is limited because it doesn’t directly measure body fat and the data it’s based upon came from non-Hispanic white people. In a policy statement adopted in June, the AMA says BMI lines up well with fat levels in the general population, but becomes less reliable at the individual level.

“BMI is a very simple method of calculation – very easy – but it has lots of limitations,” said Arash P. Tadbiri, MD, who practices family and geriatric medicine at Bourne Primary Care at the Bourne Health Center. “It should be used only for screening.”

What is BMI?

BMI, or Body Mass Index, is a measurement of weight relative to height. It’s calculated by dividing a person’s weight by the square of their height, according to the Centers for Disease Control and Prevention.

BMI’s weight/height ratio surfaced during the mid-1800s, according to a July article in the AMA Journal of Ethics, and its use surged in recent decades with the development of weight loss drugs and concern about soaring obesity rates. In 1997, the World Health Organization, relying upon a report prepared using drug company grants, approved categorizing BMI scores of 30 and above as obese. The U.S. federal government followed suit the same year. In 2004, Medicare allowed reimbursement for weight-loss drugs and in 2013, the AMA said obesity was a disease.

The 1998 guidelines of the National Heart, Lung and Blood Institute set forth these BMI score categories:

• Normal - 18.5-24.9
• Overweight - 25-29.9
• Obese - 30 and above

The AMA Journal of Ethics article concluded by stating BMI poorly measures body fat and that some research questions whether an individual’s body fat reliably correlates with the state of their health. The CDC says BMI doesn’t determine a person’s amount of fat or state of health, but works as a screening tool to be used with other information, such as skinfold measurement, as well as a person’s diet, exercise and family history.

Pluses and Minuses

While the AMA officially recognized BMI’s limitations this year, its shortcomings have been noted for years. For example, a 2006 article in the Archives of Disease of Childhood noted gender, age and ethnicity all affect BMI’s accuracy; a wide range of fat mass amounts exist among people with the same BMI; and that a person could exercise and thereby add muscle mass while losing fat, and still maintain the same BMI.

“BMI was originally created for white men,” Dr. Tadbiri said. “In women, their percentage of body fat is higher. And kids, most of their weight is muscle mass, so it’s misleading.”

BMI doesn’t account for trends of decreasing muscle mass and increasing fat mass in older adults, he said. Nor does it reflect differences in body structure for different ethnic groups.

“African Americans, they have bones that have more density – stronger and heavier,” Dr. Tadbiri said. “That increases bulk, but they may not be overweight.”

However, there are many reasons to pay attention to BMI, because it’s known that as BMI rises, so do health risks, he continued. Excess weight is associated with many health conditions, including heart disease, Type 2 diabetes, some types of cancer, high blood pressure, fatty liver disease, breathing difficulty, fertility, pregnancy and sexual problems, osteoarthritis and gout, and diseases of the kidney, pancreas and gallbladder, according to the National Institute of Diabetes and Digestive and Kidney Diseases. This makes BMI useful to decide if a patient’s health needs further assessment. It’s not necessary for people who are “obviously overweight,” he said.

“We should not judge obesity or overweight by just BMI,” Dr. Tadbiri said. “The most important thing is how fit is the patient?”

He said a patient with a BMI of 27 shouldn’t be called overweight if they run four to five miles a day. The same applies to someone with a BMI of 28-29 whose job consists of eight hours a day of physical exertion, he added.

“Age, sex, ethnicity, muscle mass – these are the things that affect BMI. Bone mass is another thing,” Dr. Tadbiri said. ”We have to consider all these factors.”