DALLAS (AHA) - A six-month endurance-training program reversed the decline in cardiovascular fitness that occurs during middle-age, according to a landmark study that began 30 years ago with five men in their 20s.

The follow-up report published in two parts in today's Circulation: Journal of the American Heart Association is one of the longest studies to evaluate the effects of age and physical activity on cardiovascular capacity.

"This pair of studies together underscore the relationship between physical activity and cardiovascular fitness, or aerobic power," says Darren McGuire, M.D., assistant professor of internal medicine at University of Texas Southwestern Medical Center at Dallas, and lead author of the studies. "An endurance exercise program using a relatively modest intensity of training was able to return the group to the levels of aerobic power they had 30 years before."

Researchers studied five healthy men in 1966 as part of a research project to determine the adverse effects of bed rest on physical fitness. Fitness data obtained at that time in what became widely known as the Dallas Bed Rest and Training Study were used for comparative purposes in the 1996 study.

In the latest report - a project developed by study co-author Jere Mitchell, M.D., clinical professor of internal medicine at UT Southwestern - researchers put the same men, now in their 50s, through a six-month endurance-training program. The tests in the follow-up compared age-related changes in cardiovascular performance over 30 years to cardiovascular performance levels achieved after the training program. The men in the study were not athletes and only one of the subjects engaged in regular physical activity.

The men, ages 50-51, participated for up to 24 weeks in an exercise training program that included walking, jogging or cycling. By the end of the study, the subjects were exercising weekly about 4 ½ hours divided into four or five sessions, says McGuire. A measurement of maximal oxygen uptake (Vo2max) during treadmill exercise was used to document changes in cardiovascular performance.

"The studies indicate that middle-aged men can actually reverse many of the negative results of non-exercise, even after being physically inactive for a long time," says Benjamin D. Levine, M.D., study co-author and medical director of the Institute for Exercise and Environmental Medicine at Presbyterian Hospital in Dallas. "We found that 100 percent of the age-related decline in aerobic power that occurred over 30 years in these men was reversed by six months of endurance training."

The participants' Vo2max was measured before and after the training program. On average, Vo2 max increased 14 percent (2.9 to 3.3 L/min) after the training period, achieving the level of aerobic power observed when the men were studied 30 years ago.

"Our findings should be a beacon of hope for all those people who wrongly assume that reduced physical fitness is an inescapable part of growing older," says Levine, who is also an associate professor of medicine at UT Southwestern. "This sends a strong message that regular exercise should be a part of everyone's lifestyle, and that it is never too late to start exercising."

Another part of the follow-up study examined age-associated changes in the subjects' cardiovascular capacity - the body's ability to take in and use oxygen. Researchers compared the cardiovascular capacity lost over 30 years of aging to that lost during three weeks of bed rest when the men were 20 years old. "It's remarkable to note that three weeks of bed rest - which is the ultimate 'sedentary' state - in these subjects when they were 20 years old had a more profound negative impact on their cardiovascular fitness than did 30 years of aging," says Levine.

Cardiovascular capacity declined over the 30-year interval primarily because the men's muscles had a decreased ability to extract oxygen from the blood. On average, Vo2 max decreased 11 percent - from 3.30 liters per minute (L/min) as young men to 2.90 L/min as middle-aged men. In addition, body weight increased 25 percent on average (from 77 to 100 kilograms), and percent body fat increased 100 percent (from 14 percent to 28 percent).

"Except for their ages and the fact that all were Caucasians, there were wide variations in the physical characteristics and lifestyles of these men," Levine notes. "In 1966, one was a semi-pro football player and one was a long-distance runner, while the other three were basically sedentary. Over the 30 years, some gained weight and got out of shape, while others stayed reasonably fit."

To determine their levels of fitness before they began the 1996 follow-up studies, each man was asked to recall and describe his physical activity during a typical seven-day period. The researchers found that the more active the men were, the less fitness capacity they had lost compared to 30 years earlier.

These studies re-emphasize the potentially devastating effects of physical inactivity, especially prolonged enforced bed rest for all patients, either young or old, McGuire points out.

Other researchers taking part in the bed rest follow-up study include Jon W. Williamson, Ph.D; Peter G. Snell, Ph.D.; C. Gunnar Blomqvist, M.D., Ph.D.; and Bengt Saltin, M.D.

Copyright 2001 The American Heart Association. All rights reserved.