BMI often not an accurate indicator of body fat

Body mass index, or BMI, long considered the standard for measuring the amount of fat in a person’s body, may not be as accurate as originally thought, according to new research.

A research team from Michigan State University and Saginaw Valley State University measured the BMI of more than 400 college students – some of whom were athletes and some not – and found that in most cases the student’s BMI did not accurately reflect his or her percentage of body fat.

The research is published in the March issue of Medicine and Science in Sports and Exercise, the official journal of the American College of Sports Medicine.

BMI is determined by this equation: A person’s weight divided by his or her height squared. Generally a BMI of 25 or above indicates a person is overweight; 30 or above indicates obesity. A person with a higher BMI is thought to be at a greater risk of heart disease, diabetes and other weight-related problems.

“The overlying issue is the same criteria for BMI are used across the board,” said Joshua Ode, a Ph.D. student in the MSU Department of Kinesiology and an assistant professor of kinesiology at Saginaw Valley. “Whether you’re an athlete or a 75-year-old man, all the same cut points are used.”

“BMI should be used cautiously when classifying fatness, especially among college-age people,” said Jim Pivarnik, an MSU professor of kinesiology and epidemiology. “It really doesn’t do a good job of saying how fat a person really is.”

The problem, especially among younger people and athletes, is that BMI does not distinguish between body fat and muscle mass, said Ode.

“A previous study of NFL football players found that a large percentage of them – around 60 percent – were considered obese,” he said. “But when you look at an athlete like that, you see that in many cases he is not obese. Many athletes have huge BMIs because of muscle mass, but in many cases are not fat.”

The answer, said Pivarnik, is instead of having one cutoff point for everyone, perhaps have different classifications for different people.

“Is a BMI of 25 for overweight and 30 for obese the right ones to use with 20- to 29-year-olds in terms of disease risk?” he asked. “Maybe it could be as simple as this: If you’re a regular exerciser or an athlete, maybe 28 is overweight for you and 33 is considered obese.”

BMI, said Pivarnik, is used as a “surrogate” for percent fat, even though body fat doesn’t really fit into the equation.

“What if you take fat out of the equation?” he said. “Is there something unique about BMI that may increase a person’s risk of chronic disease, unrelated to fatness? Are people with high BMIs but low fatness as much at risk of heart disease as other people? The answer is no one knows.”

In their research, Ode and Pivarnik used carefully measured height and weight to calculate BMI in more than 400 college-age people. They then used a standard method of measuring body fatness and found that BMI was not providing an accurate portrayal of the amount of fat a student had.