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SAN FRANCISCO -- Duke University Medical Center researchers have found that the positive effects of exercise in reducing levels of potentially harmful cholesterol in the blood can persist weeks after exercise cessation, suggesting a long-term bodily adaptation to exercise.
However, the researchers also found that the extent of the body's positive adaptations to exercise appear to depend on the amount and intensity of exercise, as well as the gender of the exerciser.
The researchers divided 182 sedentary overweight men and women at risk for developing diabetes or heart disease through eight months of exercise training into three groups -- low, moderate or high. They then measured the changes in the levels of lipids and other particles in the blood at three points -- one day, five days, 14 days -- after stopping the exercise program.
"In general, while all groups saw short-term benefits, we found that those participants in the high amount/vigorous intensity exercise group saw a great number of benefits over the 14-day detraining period," said Johanna Johnson, Duke exercise physiologist who presented the results of the Duke study today (May 28, 2003) during the 50th annual scientific sessions of the American College of Sports Medicine. "Also, these sustained benefits in the high-intensity group were seen by both men and women alike.
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"Interestingly, a low amount of exercise at moderate intensity resulted in lower levels of triglycerides in both men and women one day after exercise cessation; however, only the men sustained this benefit over 14 days," Johnson continued. "More research is needed to better understand the mechanisms behind the body's adaptations to exercise, as well as why the long-term triglyceride benefits only occurred in men."
Cholesterol and triglycerides are energy-rich fats, or lipids, that must "attach" to protein particles in order to circulate throughout the bloodstream and nourish tissues. Abnormal levels of these lipoprotein units have been linked to the progression of atherosclerosis and heart disease.
While it has been known that exercise has a beneficial effect on lipid levels, these are little data correlating the amount and intensity of exercise with these benefits, the researchers said.
To answer this question, the Duke team, led by cardiologist William Kraus, M.D., received a $4.3 million grant from the National Heart, Lung and Blood Institute in 1998. The results of that five-year trial, dubbed STRRIDE (Studies of Targeted Risk Reduction Interventions through Defined Exercise), are now being published and presented.
For the current analysis, the researchers randomized the participants into one of three exercise groups: high-amount/vigorous intensity (the caloric equivalent of 20 miles of jogging per week); low-amount/vigorous intensity (equivalent of jogging 12 miles per week); low-amount/moderate intensity (equivalent of walking briskly for 12 miles per week).
The exercise was carried out on cycle ergometers, treadmills or elliptical trainers in a supervised setting. In order to determine the role of exercise alone, participants were not allowed to change their diet during the course of the study.
To measure how blood lipid levels changed, the researchers took blood samples before the beginning of the exercise program, and then at three different times after exercise cessation -- one day, five days and 14 days.
"While there was significant improvement in the lipid profiles among all three groups one day after exercise cessation compared to pre-training levels, only a few of those benefits were maintained throughout the 14-day detraining period," Johnson said.
Specifically, both men and women in the high amount/vigorous intensity group experienced improvements in HDL cholesterol (the so-called "good" form of cholesterol), HDL size and large HDL for the entire 14 days. A previous study by the Duke group in the Nov. 7, 2002, issue of the New England Journal of Medicine demonstrated that exercise can increase the size of HDL particles. Other research has shown that small HDL particles are associated with atherosclerosis, and that large HDL particles can be protective against the disease.
Johnson said that the moderate exercise group did not sustain any of the benefits over 14 days.
She pointed out that compared to most exercise studies, the STRRIDE regimen is longer and more intense. Participants were given individualized exercise programs based on their weight and level of fitness, and were required to exercise a specified number of minutes per week at a designated intensity.
"When we asked the participants to stop their exercise program, there were a lot of grumpy people," Johnson said. "After eight to nine months of exercise, it had become a part of their routine."
Other studies are underway among this group of participants measuring the relationship between the intensity and adherence to an exercise regimen.
Joining Johnson were Duke colleagues Cris Slentz, Ph.D., Brian Duscha, Kevin Ketchum and Kraus, as well as Joseph Houmard, Ph.D., and J.S. McCartney from East Carolina State University.