New research using an updated definition of obesity suggests that many more people may have the disease than previous studies have suggested.
It’s estimated that 40 percent of adults in the United States have obesity. But much of the research used to produce those estimates has relied on body mass index, a measure of weight relative to height. Doctors have increasingly come to see B.M.I. as an imperfect tool for defining obesity, because it doesn’t distinguish excess weight from body fat or from bone and muscle.
Last year, after years of debate about whether obesity is a disease and how to measure it, an international commission defined “clinical obesity” as a chronic, systemic illness characterized by tissue and organ dysfunction resulting from excess body fat.
Dr. Brian P. Lee, a hepatologist at Keck Medicine of the University of Southern California and an author of the new study, set out to understand how many people might be reclassified under that newer definition. Using a nationally representative sample of 5,600 adults, he and his colleagues calculated the prevalence of excess body fat by waist and other body size measurements in addition to B.M.I., as the commission recommended. They also looked at whether patients had organ dysfunction or limitations to daily physical activities.
By their crude estimates, half of U.S. adults who are considered overweight but not obese according to their B.M.I. could be classified as having “clinical obesity.” The study, published in the Annals of Internal Medicine, suggested that even some people with normal body weight by B.M.I. standards could be considered to have obesity.
Dr. Francesco Rubino, the chair of the international Lancet Diabetes & Endocrinology Commission on obesity, commended the effort to understand the true prevalence of the disease, but said he was concerned the study had overestimated how many people had it. That’s because the study counted anyone who had excess body fat in addition to certain clinical characteristics — such as liver fibrosis, heart failure or difficulty walking — as having obesity.
While those clinical characteristics are linked to obesity on a population level, they can also arise from other causes — viral hepatitis, for example, or lupus. The study didn’t properly account for such alternative causes, Dr. Rubino said.
The findings may renew debate about the use of B.M.I., which many doctors still rely on as an efficient and affordable way to screen for obesity. The commission recommended using B.M.I. as a screening tool to help flag patients with elevated weight who need further testing, such as imaging or waist circumference measurement that can confirm the presence of excess body fat, Dr. Rubino said.
But even if B.M.I. does miss some cases, as the new study suggests, some experts argue it’s still the best practical measure. Confirming obesity with additional measures can be expensive, time-consuming and unnecessary, said Elizabeth Selvin, a professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health. There’s “a very strong correlation” between B.M.I. and other measures of excess fat, she said, and B.M.I. is a strong predictor of certain health outcomes. Moreover, Dr. Selvin said, “physicians are not using B.M.I. alone.”
B.M.I. is “a starting point — not a destination,” said Dr. Catherine Varney, a family physician and director of obesity medicine at the University of Virginia Health System. It doesn’t typically account for variations by sex, age and ethnicity, she said, nor does it indicate how and where the fat is distributed.
Waist circumference is also an imperfect metric, noted Mercedes Carnethon, the chair of preventive medicine at Northwestern University Feinberg School of Medicine. “It’s not a very reproducible measure — you could put that tape measure anywhere,” she said.
Regardless of how they calculate it, what doctors are most concerned about is excess body fat.
Excess fat — particularly abdominal, or visceral, body fat — releases inflammatory molecules and hormones that can affect important processes in the body, said Dr. Fatima Rodriguez, a professor of cardiovascular medicine at Stanford University. These include blood sugar regulation and heart function.
Excess body fat is also a precursor to a number of other metabolic conditions, including elevated blood sugar, cholesterol and blood pressure levels.
According to the commission’s definition, people who have excess fat but don’t yet have signs of organ damage would be classified as having “preclinical obesity.” Doctors expressed concern that the “preclinical” label might make it harder for patients to take the condition seriously — and to convince insurance companies to pay for treatment.
The people who tend to have preclinical obesity are younger adults, Dr. Carnethon said. But if they stay overweight or obese, subjecting their organs to the higher levels of inflammation and chemicals secreted from fat tissue, they will often go on to develop high blood pressure, diabetes or kidney disease. These are risk factors for heart attack, heart failure and stroke.
From the perspective of preventing chronic diseases, that’s an argument for acting sooner, Dr. Carnethon said.
“We have more options than ever to treat obesity,” she noted. “Why would we wait to use our most effective therapies?”
Nina Agrawal is a Times health reporter.
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