While the developed world gets all of the attention for obesity, the developing world is only different in one sense; they retain more severely undernourished people even while obese and overweight people in those countries are gaining weight.

Unless people get obese equally, that growing divide may force governments in the developing world to simultaneously care for starving people while treating health problems associated with obesity, including diabetes and heart disease.

The results were drawn from information collected in Demographic and Health Surveys (DHS), a project that tracks health and population trends in developing countries. Researchers analyzed the Body Mass Index (BMI) of 730,000 women living in 37 countries between 1991 and 2008 and found that as the average BMI in a population increases, the numbers of overweight and obese women are increasing at a much faster rate than the decline in the number of underweight women.

BMI is an indicator of body fat calculated by dividing a person's weight in kilograms by their height in meters squared. Obesity is defined as having a BMI of more than 30.0 kg/m2. Compared to people with a healthy weight (a BMI between 18.5 and 24.9 kg/m2), obese individuals and overweight individuals (who have a BMI between 25.0 and 29.9 kg/m2) have an increased risk of diabetes, heart disease and stroke, and tend to die younger.

At the same time, people who are underweight (BMI less than 18.5) also have an increased risk of death, perhaps from complications related to being malnourished.

"One might think that as a country grows economically, the majority of the underweight population would move into the average BMI range, but our study shows the opposite: people of average weight are disappearing," says Fahad Razak, the study's lead author and a U of T clinical fellow working at St. Michael's Hospital's internal medicine unit. "This growing trend of body weight extremes is going to pose a major challenge for health care and policy leaders. They will need to balance their priorities between addressing health issues afflicting the underweight who happen to be poor, and health issues afflicting the obese and overweight – the upper middle-class and rich." 

"The study is novel because for the first time we are showing that increases in BMI are not happening equally across the board; rather increases in average BMI are largely driven by populations that are already overweight or obese, with little to no change among underweight individuals," says S V Subramanian, professor of Population Health and Geography at Harvard School of Public Health, the senior author of the study. "This divergence in the population with fat getting fatter and lean remaining lean is aligned with general patterns of divergence on other domains such as income, and wealth, which of course, are primary drivers of weight status in these countries."

The researchers' future work will test whether these patterns are also observed in more developed countries.