People with a "normal" weight but extra pounds around the middle may have lower long-term survival odds than individuals who are obese, a U.S. study suggests.
What's considered a normal weight for adults is often based on a measurement known as body mass index (BMI), which assesses weight relative to height. For the current study, researchers focused on people's waist-to-hip ratio, which measures whether they're storing excess fat around the middle.
The found that men with a normal BMI but central obesity, the clinical term for belly fat, had twice the mortality risk of men who were overweight or obese according to BMI.
Normal weight women with belly fat, meanwhile, had a 32 percent higher mortality risk than obese women without excess pounds around the middle.
"Waist size matters, particularly in people who are a normal weight," said senior study author Dr. Francisco Lopez-Jimenez of the Mayo Clinic in Rochester, Minnesota.
"The lack of recognition of this leads people with abnormal distribution of fat to have a false sense of safety or reassurance that they don't need to exercise or they can eat whatever they want because they are "skinny" when in reality, if a person has a normal BMI and an abnormal waist size the risk is worse than if they have a high BMI."
To understand the connection between waist size and mortality, researchers analyzed data on more than 15,000 adults surveyed from 1988 to 1994 and then followed through 2006.
Based on BMI, about 40 percent of participants were normal weight, while 35 percent were overweight and 25 percent were obese.
According to World Health Organization (WHO) criteria, about 70 percent of participants were centrally obese, meaning their waist-to-hip ratio was at least 0.85 (for women) or at least 0.90 (for men).
Along with the waist-to-hip ratio, participants' waist circumference was also more helpful than BMI in predicting risk of mortality, although only about 29 percent of people in the study were centrally obese using the WHO sex-specific criteria for waist circumference - more than 88 cm (34.6 inches) for women and more than 102 cm (40.2 inches) in men.
Over an average of about 14 years, there 3,222 deaths, including 1404 due to cardiovascular disease.
A man with normal-weight central obesity had a 78 percent higher risk of death from cardiovascular disease than a man with a similar BMI but no fat around the middle, the study found.
For women in this same scenario, normal-weight central obesity more than doubled the risk of death from cardiovascular disease.
Limitations of the study include a measurement technique for waist circumference that's different than the method recommended by WHO, the authors acknowledge. Researchers also relied on self-reported data for many health complications such as high blood pressure, elevated cholesterol or diabetes.
Even so, the findings provide ample evidence suggesting that doctors should look beyond just BMI to identify people who are at the greatest risk due to excess pounds, Dr. Paul Poirier, of Laval University and the Quebec Heart and Lung Institute, noted in an editorial.
Measuring BMI is a good start for identifying patients at greater cardiovascular risk, but it is not sufficient to identify every person at risk, Poirier wrote.
"We need to talk about waist loss and not weight loss," Poirier said by email. "When you lose weight through exercise and proper nutrition then the first fat to go is the fat at the waist line."
Some preliminary research has previously pointed to the potential for diets low in carbohydrates to help eliminate waist fat, though more research on this is still needed, Lopez-Jimenez noted.
Because people with normal weight and excess pounds around the middle may not have as much muscle mass as people without belly fat, these individuals may benefit from an exercise routine that includes strength and resistance training in addition to aerobic activity, Lopez-Jimenez added.