Just as important, the trial participants who exhibited the greatest decline in physical status during inactivity benefited the most from exercise training, according to the researchers.
These findings linking the ability of exercise training to reverse the negative effects of inactivity can be attributed to the exercise alone, because the participants did not alter their diets during the trial, the researchers said.
"Continuing to lead an inactive lifestyle leads to a gradual decline in many important markers for cardiovascular health," said Jennifer Robbins, an exercise physiologist at Duke, who presented the results of the study June 2, 2006, at the annual meeting of the American College of Sports Medicine in Denver.
"The good news is that a small amount of physical activity can make a big difference in reducing the risks for developing such conditions as heart disease, stroke or diabetes," she said. "Our findings demonstrate that while the cost of choosing a sedentary lifestyle can be high, switching to an active way of life can be beneficial at any time."
The current study stemmed from a recently completed trial known as STRRIDE (Studies of a Targeted Risk Reduction Intervention through Defined Exercise). The trial, funded by a $4.3 million grant from the National Heart, Lung and Blood Institute, investigated the effects of exercise on sedentary overweight adults at risk for developing heart disease, diabetes, or both.
The STRRIDE trial, in which the intervention ran for six months, randomly assigned 334 participants into three different exercise groups and one control group.
"At the end of the trial, we were surprised to see that many markers of cardiovascular health declined in participants in the control group, who did not exercise," Robbins said. "Our Duke group decided to see if these negative effects could be reversed after the participants spent the same amount of time in an exercise program."
Of the 61 STRRIDE participants randomly assigned to the control group 53 agreed to the take part in the new study, which ran an additional six months. The researchers measured 17 biological factors known to increase cardiovascular risk, including waist size, physical fitness, visceral fat levels, body mass index, cholesterol levels, insulin sensitivity and indicators of metabolic syndrome, a precursor of diabetes.
"In the new analysis, we found that waist size, time to exhaustion, visceral fat and metabolic syndrome scores deteriorated significantly during the six-month period of inactivity during the original STRRIDE trial," Robbins said. "However, after six months of exercise training in the study, 13 of the 17 variables had either reverted to original baseline levels or even improved."
According to Robbins, only a moderate amount of exercise is needed to counteract the detrimental effects of inactivity in these individuals. The STRRIDE trial measured three levels of physical activity: the equivalents of 12 miles of walking per week, 12 miles of jogging per week or 20 miles of jogging per week. Participants worked out on treadmills, elliptical trainers or cycle ergometers in a supervised setting.
"When looking at the group as a whole, we found it wasn't the participants with the highest intensity of exercise who accounted for the combined beneficial effects," Robbins said. "That should be reassuring for people to know they don't have to do a high-intensity workout to get these benefits of exercise."
A previous analysis by the Duke group of the same STRRIDE participants, reported in 2005, found another unhealthy effect of physical inactivity: inactive participants gained an average of 2 pounds in six months.
"At that rate, it can be assumed that this group of inactive people would gain 20 pounds in five years," Robbins said. "This means this population of sedentary people needed to exercise just to maintain their current weight. However, our earlier studies have shown that people who exercise can derive many of the cardiovascular risk benefits even in the absence of weight loss."
The STRRIDE trial was led by Duke cardiologist William Kraus, M.D. The Duke team is currently enrolling patients in STRRIDE II, in which the team will study the effects of weight training, aerobic training, and aerobic and weight training combined on cardiovascular health.
Joining Robbins in the current study were Cris Slentz, Brian Duscha, Johanna Johnson and Lori Aiken of Duke, and Joseph Houmard and Jennifer McCartney of East Carolina University.