For the new study, published this month in Medicine & Science in Sports & Exercise, researchers at the University of Alabama at Birmingham gathered 72 older, sedentary women, ages 60 to 74, and randomly assigned them to one of three exercise groups.
One group began lifting weights once a week and performing an endurance-style workout, like jogging or bike riding, on another day. Another group lifted weights twice a week and jogged or rode an exercise bike twice a week. The final group, as you may have guessed, completed three weight-lifting and three endurance sessions, or six weekly workouts.
The exercise, which was supervised by researchers, was easy at first and meant to elicit changes in both muscles and endurance. Over the course of four months, the intensity and duration gradually increased, until the women were jogging moderately for 40 minutes and lifting weights for about the same amount of time.
The researchers were hoping to find out which number of weekly workouts would be, Goldilocks-like, just right for increasing the women’s fitness and overall weekly energy expenditure.
Some previous studies had suggested that working out only once or twice a week produced few gains in fitness, while exercising vigorously almost every day sometimes led people to become less physically active, over all, than those formally exercising less. Researchers theorized that the more grueling workout schedule caused the central nervous system to respond as if people were overdoing things, sending out physiological signals that, in an unconscious internal reaction, prompted them to feel tired or lethargic and stop moving so much.
To determine if either of these possibilities held true among their volunteers, the researchers in the current study tracked the women’s blood levels of cytokines, a substance related to stress that is thought to be one of the signals the nervous system uses to determine if someone is overdoing things physically. They also measured the women’s changing aerobic capacities, muscle strength, body fat, moods and, using sophisticated calorimetry techniques, energy expenditure over the course of each week.
By the end of the four-month experiment, all of the women had gained endurance and strength and shed body fat, although weight loss was not the point of the study. The scientists had not asked the women to change their eating habits.
There were, remarkably, almost no differences in fitness gains among the groups. The women working out twice a week had become as powerful and aerobically fit as those who had worked out six times a week. There were no discernible differences in cytokine levels among the groups, either.
However, the women exercising four times per week were now expending far more energy, over all, than the women in either of the other two groups. They were burning about 225 additional calories each day, beyond what they expended while exercising, compared to their calorie burning at the start of the experiment.
The twice-a-week exercisers also were using more energy each day than they had been at first, burning almost 100 calories more daily, in addition to the calories used during workouts.
But the women who had been assigned to exercise six times per week were now expending considerably less daily energy than they had been at the experiment’s start, the equivalent of almost 200 fewer calories each day, even though they were exercising so assiduously.
“We think that the women in the twice-a-week and four-times-a-week groups felt more energized and physically capable” after several months of training than they had at the start of the study, says Gary Hunter, a U.A.B. professor who led the experiment. Based on conversations with the women, he says he thinks they began opting for stairs over escalators and walking for pleasure.
The women working out six times a week, though, reacted very differently. “They complained to us that working out six times a week took too much time,” Dr. Hunter says. They did not report feeling fatigued or physically droopy. Their bodies were not producing excessive levels of cytokines, sending invisible messages to the body to slow down.
Rather, they felt pressed for time and reacted, it seems, by making choices like driving instead of walking and impatiently avoiding the stairs.
Despite the cautionary note, those who insist on working out six times per week need not feel discouraged. As long as you consciously monitor your activity level, the findings suggest, you won’t necessarily and unconsciously wind up moving less over all.
But the more fundamental finding of this study, Dr. Hunter says, is that “less may be more,” a message that most likely resonates with far more of us. The women exercising four times a week “had the greatest overall increase in energy expenditure,” he says. But those working out only twice a week “weren’t far behind.”
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