By DRS. SHARI BARNETT and SWATI SHROFF

Whether you are spry in your 80s might depend on how active you are in your 40s, a new study shows more clearly than ever before.

Specifically, the study, published today in the Archives of Internal Medicine, shows that exercising in midlife staves off a range of dangerous diseases, even some cancers and cognitive conditions.

"It has been known for decades that people who are more fit live longer, but what has been unclear is that people who are fit live better," says Dr. Jarett Berry, lead investigator and assistant professor of medicine at University of Texas Southwestern in Dallas.

Studies in the past have found a clear relationship between fitness and mortality. This new study observed men and women older than 65 and enrolled in Medicare, and found that those with higher midlife fitness levels had fewer chronic diseases decades later in life.

The researchers measured the fitness levels of those in the study with exercise treadmill testing. They then separated patients into groups depending on their fitness level. For the next 26 years, the researchers looked at whether the patients developed certain kinds of chronic disease. The diseases monitored were: heart failure, heart disease, stroke, diabetes, pulmonary disease, chronic kidney disease, Alzheimer's disease, and colon and lung cancer.

"In general, we saw if you increased your fitness by 20 percent, you would decrease your disease burden by 20 percent," Berry said.

Higher midlife fitness also appeared to delay the development of the chronic diseases; those with a higher level of midlife fitness spent a greater proportion of their final five years of life with a lower burden of chronic disease, suggesting an improvement in not only quantity, but quality of life.

"The benefit of fitness persists to the end of life," Berry says.

Health experts not involved with the research say it is further proof that a little exercise in midlife can have big benefits later.

"The best time to take off extra weight is before chronic disease develops because many of these conditions can be prevented," says Dr. Walter Willett, chairman of the department of nutrition at Harvard School of Public Health in Boston. "Losing weight is often more difficult after diagnosis of these conditions because many of them limit activity."

Connie Diekman, director of university nutrition at Washington University in St Louis, said, "Developing a healthy lifestyle, which include making the right food choices and getting regular activity, is essential to quality of life now and in the future."

And Dr. Gerard E. Mullin, associate professor of medicine at Johns Hopkins University School of Medicine in Baltimore, believes that physicians should start writing for exercise just like they would for a new prescription drug.

"As physicians," he said, "we all should carry pre-printed exercise prescription pads to promote disease-fighting physical activity."