"It was not a lifestyle link; it was an estrogen link," said Marybeth Brown, professor of physical therapy in MU's School of Health Professions. "The role of female sex hormones on skeletal muscle has been unclear in the past. In fact, some reports have suggested that the hormones have no effect."
Brown studied the effect of low hormone levels combined with physical inactivity in rats. The rats' ovaries were removed, and the rats were then put on "simulated bed rest" for four weeks. Surprisingly, the loss of muscle mass and strength that occurred with the simulated bed rest was similar in rats with and without normal estrogen levels. What was not the same was the recovery of muscle mass in rats that lacked estrogen. Brown found that the muscle atrophy that occurred during the bed rest was not regained in rats that lacked estrogen. Rats with normal estrogen levels - intact ovaries - had a robust recovery of muscle mass and strength after the simulated bed rest was complete.
"Women now spend a third of their lives in an estrogen-depleted state; years ago, women did not even survive long enough to have a menopause," Brown said. "Consequently, we need to know as much about the potential consequences of estrogen loss as possible. If estrogen-deficient women are failing to recover muscle mass from the bed rest associated with the flu or a hospitalization, the implications for old age are quite significant. We need all the muscle mass we can get to make it through old age without losing independence."
Women lose 10 to 15 percent of their muscle mass between the ages of 20 and 50, and then the decline accelerates. Loss of muscle mass and strength is a natural phenomenon, and once muscle is lost, it cannot be regained. According to Brown, women can increase their muscle mass through resistance exercise, which will help maintain independence for a longer period of time. If women are sedentary, they will lose muscle mass and strength faster than normal. Results from the study suggest that sedentary living combined with bouts of bed rest may put estrogen deficient women at a very high risk for losing independence.