By Amy Norton
THURSDAY, Feb. 4, 2016 (HealthDay News) -- Many overweight and obese Americans might be perfectly healthy when it comes to blood pressure, cholesterol and blood sugar levels -- while many thin folks may not be the picture of good health, a new study contends.
Using a government health survey, researchers found that nearly half of overweight U.S. adults were "metabolically healthy."
That meant they had no more than one risk factor for type 2 diabetes and heart disease -- including high blood pressure, unhealthy cholesterol or triglyceride levels, elevated blood sugar, or high concentrations of C-reactive protein (a marker of inflammation in the blood vessels).
Among obese adults, 29 percent were deemed healthy -- as were 16 percent of those who were severely obese based on body mass index (BMI, a ratio of weight to height).
On the other hand, more than 30 percent of normal-weight Americans were metabolically unhealthy.
The researchers estimate that nearly 75 million Americans would be "misclassified" as heart-healthy if BMI is the only yardstick.
"The bigger picture we want to draw from our findings is that the dominant way of thinking about weight -- that higher-weight individuals will always be unhealthy -- is flawed," said Jeffrey Hunger, one of the researchers on the study and a doctoral candidate at the University of California, Santa Barbara.
The study, published Feb. 4 in the International Journal of Obesity, is far from the first to find that obese adults can be in good shape as far as heart health. Researchers have debated the "fat but fit" theory for years.
By the same token, studies have shown, being thin is no guarantee of good health.
But, Hunger said, the new findings also help "solidify" the number of Americans who could be mistakenly deemed unhealthy based solely on BMI.
That has potential "real-world consequences," Hunger said. Many larger U.S. businesses offer employee wellness programs, which can include discounts on health insurance premiums for meeting certain goals, such as weight loss. Some employers penalize employees for not participating.
Hunger's team says the U.S. Equal Employment Opportunity Commission has proposed rules that would allow employers to charge workers up to 30 percent of their health insurance costs if they fail to meet certain health criteria, including a specified BMI.
The new study's findings are based on more than 40,000 U.S. adults who took part in a nationally representative federal health study between 2005 and 2012.
Obese men and women were, in fact, the most likely to fall into the unhealthy category: Depending on the severity of their obesity, 71 percent to 84 percent had risk factors for heart disease and diabetes. That compared with 24 percent of underweight and 31 percent of normal-weight adults.
Still, Hunger said, weight is not the be-all and end-all.
"Right now, we have this laser focus on weight when we should be talking about health," he said. "The general public should try to focus on improving their health behaviors -- eating well, staying active and getting enough sleep -- and forget about the number on the scale."
But Dr. Gregg Fonarow, a professor of cardiovascular medicine at the University of California, Los Angeles, cautioned that weight does still matter.
He noted that some recent studies have been challenging that idea of "metabolically healthy obesity."
Last year, researchers reported on a long-term study of more than 1 million Swedish men showing that those who were obese but fit -- based on a cycling test -- were 30 percent more likely to die prematurely than men who were out of shape but thin.
But another study, published in the Journal of the American College of Cardiology, followed 2,500 British adults for 20 years. It found that among those who were obese but healthy at the outset, more than half eventually developed high blood pressure, diabetes and other risk factors for heart disease -- often within five years.
It's true, Fonarow said, that at any point in time, obese people may be metabolically healthy. But over the years, obesity takes its toll.
"So individuals who are classified as obese by BMI are at increased risk for a variety of obesity-related ills," he said.
Still, Hunger and his colleagues warn against "obsessing" over weight, which may only worsen heavier people's well-being. Instead, healthy eating and regular exercise should be the focus, rather than BMI, Hunger said.
"Practically speaking," he said, "the conversation needs to shift."