Silvia Jansen

Does being physically fit counteract some of the undesirable health consequences of being overweight? That question, of pressing interest to those of us who exercise while carrying a few extra pounds, prompted an important new study that focused on aerobic fitness and weight swings.

The study, which was published last month in The Journal of the American College of Cardiology, examined health information about more than 3,100 adults who’d visited the Cooper Clinic in Dallas for medical checkups. During the exams, physicians gathered information about each person’s cardiovascular health, including blood pressure, cholesterol profile, abdominal girth and body fat percentage. They also measured the patients’ aerobic fitness using treadmill tests.

For years, scientists and the news media have been debating the relative risks of being fat but fit. While it might seem as if aerobic fitness could ameliorate many of the health problems associated with extra body fat, studies on the topic have produced mixed results. Some have suggested that being in shape virtually eliminates the health risks of extra pounds, especially in terms of heart health. But others have come to the opposite conclusion, finding that excess fat contributes to heart disease and premature death, even if someone is physically active.

Many of these studies, though, have compared people’s fitness and fatness at only one point in time, which is an artificial measure because, as we all know from experience, bodies change.

So, for the new study, scientists from the University of South Carolina and other institutions turned to Cooper Clinic data that covered the same patients over the course of at least six years and three checkups.

They chose 3,148 adult men and women, most in their 40s at the start of the study and all normally active but not athletes. None at first had any indications of heart disease or other risk factors, like high blood pressure or cholesterol.

The researchers then compared the patients’ body fat and aerobic fitness during their second checkup, usually two or three years after the first. A majority of the people had, by that time, gained body fat. Paradoxically, many also had become more fit, a surprising statistic, unless you consider that these were men and women who were dutifully showing up for medical checkups and receiving repeated admonitions to exercise.

None during that second visit yet showed discernible risk factors for heart disease.

But by the time they showed up for their third checkup several years later, almost a quarter had developed high blood pressure, unhealthy cholesterol levels or a combination of risk factors called metabolic syndrome.

Those at greatest risk for these health problems were, unsurprisingly, those who’d both lost fitness and gained fat. If someone had grown less fit over the years while adding fat, he now had a 71 percent greater chance of suffering from metabolic syndrome than those who’d lost fat.

But fitness offered some protection to those who gained fat. A person who had improved fitness but added girth had a 22 percent lower risk than someone who was both fat and unfit.

Not surprisingly, dropping fat mass also reduced people’s risks of suffering from metabolic syndrome and the other health problems, but very few people in the study lost fat.

The bottom line: Exercise by itself won’t erase the heart risks of extra body fat, but it may blunt them.

“What this tells us is that both fitness and fatness matter, separately and together, for heart health,” says Duck-Chul Lee, a research fellow at the University of South Carolina who led the study.

More encouragingly, simply maintaining fitness may be all that is required to protect your heart. “We did not see a great deal of added benefit from improving fitness,” Dr. Lee says, “compared to maintaining it.”

People who had bettered their aerobic fitness had 28 percent less risk of developing high blood pressure than people who’d let their fitness slide. But those who’d merely kept up their baseline fitness throughout the study, adjusted for age (aerobic capacity declines with age, even among people who exercise regularly) had 26 percent less risk of the condition, a barely discernible difference.

“The message is simple,” Dr. Lee concludes. “So much attention gets focused on weight reduction, but reducing body fat is very difficult for most people. Our study suggests that,” in terms of heart health, “maintaining your fitness over your lifetime is just as important, and for most people is probably more achievable.”