Without enough exercise, even those impossible bodies on display at the Olympics are in danger of rapidly morphing into the shape of a couch potato, a study of endurance athletes shows.

Researchers in France found that among 20 highly trained rowers, those who stopped training saw their weight, fat mass and cholesterol levels reach that of the average sedentary person within a year.

Given the athletes’ very low body fat during training, it’s not surprising that these changes happened quickly, the study’s lead author, Dr. Cyril Petibois, told Reuters Health.

However, increases in fat mass, cholesterol and triglycerides - another blood-fat-carrying molecule - should stabilize after a few months, according to Petibois, a researcher at the University of Bordeaux. Athletes in this study did not show such a stabilization, but instead had continuing alterations in body fat and blood fats during their year of “detraining.”

The concern is that, without enough exercise, these athletes will face an increased risk of cardiovascular disease in the future, according to Petibois.

He said the results highlight the importance of slowly cutting back on training at the end of a sports career.

“This information is very important in the post-Olympic period since it signifies the end of (sports careers) for a generation of athletes,” the researcher noted. He said retiring Olympians should continue training for the next year at levels of at least 25 percent of previous training levels.

For their study, reported in the Journal of Clinical Endocrinology & Metabolism, Petibois and his colleagues followed 20 rowers who had been training for more than 10 years.

During the study’s first year, all of the athletes performed endurance and weight training for about 22 hours a week, most weeks of the year. During year two, 10 athletes who wanted to retire from the sport were instructed to exercise no more than four hours each week, while the rest returned to their training regimen.

At the end of the second year, the researchers found, athletes who were not training gained enough weight and fat mass to qualify as officially out of shape. On average, their body fat increased from 12 percent of total body mass to 20 percent, while their body mass index, or BMI, reached 25-the threshold used to define “overweight.”

The rowers who stopped training rapidly lost the cholesterol benefits that their athleticism had given them. Within several weeks, their levels of “bad” LDL cholesterol went up, while concentrations of heart-healthy HDL cholesterol declined. The athletes’ triglycerides, another type of blood-fat-carrying molecule, rose by an average of about 40 percent.

Together, these changes gave the athletes the cholesterol “profile” of a sedentary person - one that could put them at risk of future cardiovascular disease, the researchers report.

According to Petibois, the effects seen in detrained athletes are related to liver metabolism. During hard endurance training, the liver appears to step up its production of much-needed fatty acids, and over a year of detraining, it continues to churn out high levels of fats.

But non-Olympians shouldn’t worry that one day all of their exercise will be for naught if they have to slow down. Petibois said that normal activity levels do not appear to trigger the metabolic changes that high endurance training can.

SOURCE: Journal of Clinical Endocrinology and Metabolism, July 2004.