CHAPEL HILL -- The recommended upper limit of a healthy body mass index (BMI) -- the term doctors and others use to indicate how much people weigh for how tall they are -- might need to be revised downward to protect people from becoming glucose intolerant or developing type 2 diabetes, a new University of North Carolina at Chapel Hill study suggests.
Conducted with information gathered from 2,626 aborigines in remote parts of Australia, the study also suggests doctors should recommend lower BMIs for some groups than for others. Whites, for example, appear to be somewhat less sensitive to the unhealthy effects of excess weight than blacks and various indigenous populations around the world.
A report on the findings appears in the June issue of the journal Diabetes Research and Clinical Practice, which has just been published. Lead author is Dr. Mark Daniel, assistant professor of health behavior and health education at the UNC School of Public Health.
"We calculate people's body mass index by dividing their weight in kilograms by their height in meters squared," said Daniel, also an assistant professor of epidemiology. "The resulting number, which is considered to range from 20 to 25 in healthy people, is a better indicator of how much extra weight a person carries around as fat than their weight alone because, obviously, people of the same ages and weights vary considerably in height."
In the new study, Daniel worked with Drs. Kevin Rowley of the University of Melbourne, Robyn McDermott of the Tropical Public Health Unit of Queensland Health in Cairns and Kerin O'Dea of the Menzies School of Health Research in Darwin. They examined Australian aborigines in 15 remote settlements since diabetes and the impaired glucose tolerance (IGT) that precedes it have grown as health threats almost worldwide. Areas studied ranged from central desert regions of the Outback to lands subject to tropical monsoon rains in the north.
Besides calculating subjects' body mass indexes, researchers gathered data on how well or poorly their bodies processed glucose while fasting and two hours after eating. They also divided subjects into three age groups -- between 15 and 34.9, between 35 and 44.9 and over 45 years -- as well as five BMI groups – below 22, between 22 and 24, 25 to 29.9, 30 to 34.9 and 35 or more.
BMI scores from 25 to 30 are considered overweight, and scores above 30 indicate obesity.
"What we found was that the age- and sex-adjusted prevalence of impaired glucose tolerance was 14.9 percent and of diabetes was 14.8 percent," Daniel said. "For both conditions, the prevalence of disease increased with increasing BMI."
The age- and sex-adjusted risk of developing impaired glucose tolerance was three times greater for people with BMIs of 22 or more compared to people with lower BMIs, he said. The risk of diabetes was four times greater for subjects with BMIs of 22 or more. Even in just the 22 to 25 BMI range, which is supposedly healthy, the risk of having diabetes was close to three times greater.
"We also calculated that if we could prevent gains in BMI beyond 22, we could prevent an estimated 46 percent of diabetes cases and 34 percent of impaired glucose tolerance cases, which is just astounding," Daniel said.
The UNC report is among the first of several studies showing that an increased risk of diabetes exists in the 22 to 25 BMI range, which was established based on earlier research on whites and, until recently, has rarely been questioned, he said.
"The world health community might want to revise the recommended upper healthful limit downward, especially among such groups as American and Canadian Indians, Pacific Islanders, Africans and U.S. blacks and many indigenous people, who all appear to be at increased risk," Daniel said. "We believe programs to reduce the risks of diabetes are urgently needed in many parts of the world, including Australia and the United States."