Doctors have long known that obesity increases a person's risk of heart disease, but in recent years the picture has grown more complicated.
Several studies have found that a high body mass index is associated with a lower risk of dying from heart disease and other chronic illnesses -- a mysterious phenomenon that has come to be known as the "obesity paradox." (Body mass index, or BMI, is a ratio of height to weight used to define obesity.)
According to a new analysis in the Journal of the American College of Cardiology, the paradox appears to be explained by the simple fact that BMI is a very flawed measure of heart risk. Waist size provides a far more accurate way to predict a heart patient's chances of dying at an early age from a heart attack or other causes, the study found.
As in previous studies, a high BMI was associated with a lower risk of death. But researchers found that heart patients with a high ratio of waist-to-hip circumference or a large waist size -- greater than 35 inches for women, or 40 inches for men -- were 70 percent more likely to die during the study period than those with smaller waists. The combination of a large waist and a high BMI upped the risk of death even more.
"What matters probably the most is the distribution of fat, more than anything else," says the lead researcher, Francisco Lopez-Jimenez, M.D., a cardiologist at the Mayo Clinic, in Rochester, Minn.
The new study provides more evidence of BMI's shortcomings in assessing heart risk, says Jean-Pierre Després, Ph.D., the director of research at the Quebec Heart and Lung Institute at Laval University, in Quebec City.
"If you measure body mass index, you don't assess body shape, you don't assess body fat distribution," says Després, who wrote an editorial accompanying the study. "I'm not saying BMI is useless. It's just that we need to go beyond that. BMI is the total cholesterol of lipids: We know that there is good and bad cholesterol, and there is good and bad fat."
More from Health.com:
Nor does BMI distinguish between fat and muscle, Després adds. Heart patients who lead a sedentary lifestyle may see a drop in BMI as they lose muscle mass, he explains, while heart-disease patients who become more active may actually put on weight and raise their BMI because they are adding lean muscle.
The findings also add fuel to the debate surrounding body type and the risk of developing heart disease. Several studies have suggested that people with an apple- shaped body who accumulate fat in their belly are more likely to develop heart disease than their pear-shaped counterparts, but that theory has been called into question by recent research.
Lopez-Jimenez and his colleagues analyzed data from nearly 16,000 heart patients who participated in one of four previously conducted studies or the Mayo Clinic's Cardiovascular Rehabilitation Program. More than one-third of the patients died during the studies, which ranged in length from six months to more than seven years.
A high BMI was associated with a 35 percent lower risk of death, but having a large waist in addition to a high BMI nearly doubled the risk of dying, the researchers found. (To zero in on waist size, they controlled for age, hypertension, diabetes, and other risk factors for heart disease.)
Even heart patients with apple-shaped bodies and BMIs in the normal range were at increased risk of dying sooner, which drives home the fact that normal-weight heart patients may need to lose some weight in their bellies too, Després says. "That's why it's so important for clinical cardiologists to measure waist circumference."
Why is belly fat so bad? It tends to be a sign of visceral fat, or fat that gathers around the organs in the abdomen, the study notes. This fat seems to promote insulin resistance and unhealthy cholesterol numbers, and may also boost inflammation.
Genetics plays a "very strong" role in whether a person gains weight around the waist, Després says. He estimates that about 30 percent of the population has this tendency to put on fat in these "undesirable sites."
The Morning Email helps you start your workday with everything you need to know: breaking news, entertainment and a dash of fun. Learn more