Some overweight people joke about being fat and happy, but what about being very fat and healthy? It seemed for a time that people could indeed be both. But new research calls the idea into question.

For years scientists wondered why some obese people didn't seem to have the health problems that often accompany being overweight, such as high blood pressure and out-of-whack blood sugar, insulin or cholesterol levels. To investigate, University College London researchers followed more than 2,500 men and women who were 39 to 62 when the study started. At the beginning, 181 were obese, with 66 of them classified as "healthy obese." 

Obesity is defined as having a body mass index (BMI) of 30 or higher or having a waist circumference greater than 35 inches for men or 40 inches for men. One in three U.S. adults are now considered obese, according to the Centers for Disease Control and Prevention.

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Every five years, the researchers did exams, looking for the development of high blood pressure, high cholesterol, high blood sugar or insulin problems that would re-classify the subjects as unhealthy.

Those who were classified as healthy obese were already in the minority when the study started, and their ranks dwindled as time went on. At the 20-year mark, 34 of the 66 originally described as healthy obese, or more than half, had become unhealthy obese, the researchers report in the Journal of the American College of Cardiology.

And what about the 32 who remained obese but healthy? The researchers speculate that this group may have a more favorable body fat distribution, specifically, a smaller waist. A bigger waist circumference has long been linked with a higher risk for heart disease and other health issues.

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"The main message is, even though people are healthy obese at baseline, about half of them came up unhealthy obese" by the study's end, says D.C. (Duck-chul) Lee, PhD, an assistant professor of kinesiology at Iowa State University, Ames. (He was not involved in the study.) And what happens to the "healthy obese" people after the 20-year mark is anyone's guess. 

Lee’s own studies have focused on a related question: Can fitness cancel out the negative effects of fatness? His team followed more than 3,100 men and women over time, tracking their BMI and body fat and evaluating their fitness with a treadmill test. At the six-year mark, many had developed high blood pressure, high cholesterol or other heart risk factors — especially those who had gained weight. Fitness reduced the effects of weight gain to some extent, but it didn't eliminate them. 

Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.