The finding came from a look at 5,115 men and women in four regions of the United States who were enrolled in a study between the ages of 18 and 30, tested for fitness on a treadmill and followed for 15 years.

"Participants with low fitness were three- to six-fold more likely to develop diabetes, hypertension, and the metabolic syndrome (excess abdominal fat, elevated blood pressure and triglycerides and low levels of so-called 'good' cholesterol) than participants with high fitness," the study concluded.

"Previous work has demonstrated that engaging in a regular exercise program can improve fitness. If the association between fitness and cardiovascular risk factor development is causal, and if all unfit young adults had been fit, there may have been 21 percent to 28 percent fewer cases of hypertension diabetes, and metabolic syndrome," the authors said.

"The key point from this study is that the development of risk factors for heart disease and stroke isn't just the natural result of aging," said Mercedes Carnethon of Northwestern University's Feinberg School of Medicine, chief author of the study.

"All Americans -- including women and minorities -- can protect themselves against those risks by maintaining their physical fitness," she said.

"Americans don't have to run marathons to improve their physical fitness," added Cheryl Nelson of the National Heart, Lung, and Blood Institute which partly sponsored the study.

"They should try to engage in at least 30 minutes of a moderate-intensity physical activity such as brisk walking on most and, preferably, all days of the week. Being physically active will not only improve their fitness but also help them maintain a healthy weight, which in turn will protect their heart health," she added.

The study was published in this week's Journal of the American Medical Association.