I have a paunch, but I hide it pretty well. Like lots of men, I collect calories in my belly—what scientists call the visceral region, my gut. It’s easy to conceal: many ordinary men’s shirts are pyramid-shaped, so they fit neatly at the shoulders and then billow a bit as they go south. But they can also hide a health problem: even if you’re not overweight, having excess belly fat can lead to serious cardiovascular disease and other illnesses. In fact, as new research shows, those who have normal weight but concentrated “central” fat are more than 50% more likely to die earlier from all causes than those who are obese.
A team of eight scientists presented the research in Munich on Aug. 27 at a European Society of Cardiology meeting. Led by Dr. Francisco Lopez-Jimenez, a professor of medicine at the Mayo Clinic, the scientists examined data for 12,785 Americans who had been tracked for approximately 14 years for a major CDC study. Lopez-Jimenez and his team reviewed information on both body-mass index (BMI)—a measure of how fat you are in proportion to your height—and waist-to-hip ratio (WHR), the circumference of your belly in relation to the circumference of your hips. (You can calculate your BMI here and your WHR here.)
The researchers divided the survey participants into three categories of BMI—normal, overweight, and obese. They also divided them into two categories of waist-to-hip ratio—normal and high. (They defined normal as less than .85 for women and less than .90 for men and high as .85 or higher for women and .90 or higher for men.) That left them with six subgroups: normal BMI/normal central fat; normal BMI/high central fat; and so on. This all gets confusing, but the chart here shows all six groups.
At the end of the 14-year follow-up period in the CDC survey, 2,562 people in the sample had died. When Lopez-Jimenez and his colleagues analyzed the deaths, they found that those with a normal BMI but excessive belly fat had the highest risk of mortality of all six groups, even after accounting for early-death risk factors such as age, sex, race, smoking, hypertension, and diabetes. In fact the normal BMI/high central fat group had a higher death rate than all of the participants who were obese, irrespective of waist-to-hip ratios.
The risk of dying during the CDC survey was 2.1 times higher for those with normal BMI and high waist-to-hip ratio than for people of normal weight and waist ratio. For obese people, the risk of mortality was still high—about 1.4 times higher than normal—but, says Lopez-Jimenez, “The level of risk attributed to normal weight [but] central obesity appears to be similar to that of smoking a pack a day.”
That’s a substantial risk, and the reasons central fat is so hazardous to our health aren’t entirely clear yet. But Lopez-Jimenez says there are three main theories to explain the connection:
1. Previous research shows that belly fat is metabolically different from other types of fat. Visceral fat cells are more likely to release certain hormones that promote insulin-resistance, a condition that can lead to diabetes. “The fact that visceral fat is next to the gut makes it highly active from a metabolic standpoint,” says Lopez-Jimenez. “So that, by itself, would cause inflammation [and] insulin resistance.”
2. As some recent data is showing, other kinds of fat—particularly fat in the legs and buttocks—is associated with a healthier metabolic profile. “It seems to be exactly the opposite of visceral fat,” says Lopez-Jimenez. “The more fat in the legs, the better the profile: better cholesterol, lower glucose, lower triglycerides, and lower insulin.” He admits that this research is “very new and very strange,” but at least two studies (one here and one here) have found that lower-body fat may offer some metabolic benefits, especially in comparison with belly fat.
3. Lopez-Jimenez speculates that people with lots of abdominal fat have less muscle mass, possibly because they exercise less. Muscles are fed by glucose, so they tend to soak up the fat we eat far more efficiently than fat cells. That’s one reason exercise can help people from gaining weight (although exercise alone contributes little to weight loss).
The new study is fascinating but controversial. As my colleague Alice Park has written, previous research suggests that BMI and waist-to-hip ratio have about the same predictive value in assessing risk for heart disease. The data presented in Munich does go a step further—evaluating not only risk for heart ailments but the likelihood of death from all causes—but the analysis will have to be replicated before the debate can be settled.
In the meantime, what this means for your diet isn’t so different from any other nutrition advice. As writer Michael Pollan has said, “Eat food. Not too much. Mostly plants.” As for my own paunch, I’m trying not to kid myself. Even though I still fit into my low-slung jeans, my belly now hangs over. I’m not obese, but I may be worse off.