August 13 (HealthDay News) -- Moderate exercise, such as walking 30 minutes a day, may offer better protection against diabetes and heart disease than a more rigorous workout regimen, concludes a U.S. study that included 240 middle-age, sedentary people.

"On the surface, it seems to make sense that the harder we exercise, the better off we'll be, and by some measures that's true," lead author and exercise physiologist Cris Slentz, of Duke University Medical Center, said in a prepared statement. "But our studies show that a modest amount of moderately intense exercise is the best way to significantly lower the level of a key blood marker linked to higher risk of heart disease and diabetes. More intense exercise doesn't seem to do that."

Perhaps even more surprising is that some of the benefits achieved through moderate exercise seem to last much longer than the benefits gained through more intense training, Slentz said.

The study was published in the August issue of the Journal of Applied Physiology.

The participants in the study were divided into four exercise groups: high amount/high intensity; low amount/high intensity; low amount/moderate intensity; and a control group that did no exercise. The volunteers started with a two- to three-month "ramp-up" period and then continued their exercise programs for six months.

The Duke team found that no amount of exercise significantly changed levels of low- density lipoprotein (LDL -- "bad" cholesterol). However, length and intensity of exercise did improve levels of high-density lipoprotein (HDL -- "good" cholesterol), and that benefit was sustained over time.

The study also found that low amount/moderate intensity exercise significantly lowered levels of triglycerides, which are particles that carry fat around the body and are also a good indicator of insulin resistance, a marker for diabetes. Reducing triglyceride levels lowers a person's risk of diabetes and heart disease.

"A proper exercise program appears to be able to lower a person's insulin resistance in just a matter of days. We were also amazed to see that the lower triglyceride levels stayed low even two weeks after the workouts ended," senior author and cardiologist Dr. William Kraus said in a prepared statement.

SOURCE: Duke University, news release, July 30,2007