The rate of muscle loss in 12 adults with an average age of 67 over 10 days was more remarkable and in a shorter period than a previous study of younger adults, said the research team led by William J. Evans, Ph.D., director of the Nutrition, Metabolism and Exercise Laboratory in the Donald W. Reynolds Institute on Aging at UAMS. The researchers attributed the increased muscle tissue loss to inactivity that results in a large decrease in the ability of muscle cells to make new protein, the primary constituent of muscle.
Evans said the study, “The Effect of 10 Days of Bed Rest on Skeletal Muscle in Healthy Older Adults,” demonstrated the “best case scenario,” as the subjects were healthy with no functional limitations. Older patients hospitalized with disease or trauma could also face other factors such as inflammation and decreased food intake that could contribute to further loss of muscle tissue and function, he said.
“We found a dramatic reduction in muscle protein synthesis brought on by inactivity that caused the muscle loss,” said Evans, the Jane and Ed Warmack Chair in Nutritional Longevity Nutrition, Metabolism, a professor in the UAMS College of Medicine and a research scientist for the Central Arkansas Veterans Healthcare System.
“This is a striking loss of muscle in healthy individuals. When you consider the chronically ill facing longer hospitalizations or bed rest, the magnitude of muscle loss is extraordinary and should be treated.”
Evans said continued research by the team will examine different strategies for preventing muscle loss. Those potential treatments included nutritional supplements, exercise and the use of insulin sensitizers to prevent the reduction in muscle protein synthesis.
The 12 study participants, described as moderately active prior to the study, remained in bed continuously for 10 days. During that time, they consumed a diet with the recommended daily allowance of protein (0.8 g/kg per day). Measurements taken before and after bed rest included the muscle protein synthesis rate over the course of 24 hours, lean body mass, urinalysis and a leg-strength test.
Evans said the protein synthesis rate drives the increase or reduction in muscle mass, as protein in muscle cells is always being created or broken down. When the rate decreases, more protein is being broken down than created, causing muscle loss. According to the study, there was a 30 percent decrease in the rate of protein synthesis in muscle cells between the measurements taken before and after the 10 days of bed rest. Evans said the researchers also found the bed rest likely caused an increase in insulin resistance, which he said further ratcheted down the protein synthesis rate. Muscle mass was measured as a change in lean body mass.
The study reported an average 1.5 kg reduction in whole body lean mass and 0.95 kg loss in leg lean mass after bed rest. Evans noted that the older adult participants in the UAMS study experienced more muscle loss in 10 days than did younger participants after 28 days as reported in a 2004 article in the Journal of Clinical Endocrinology Metabolism. In that study, an average loss of less than 0.4 kg in leg lean mass was reported after 28 days bed rest in subjects with an average age of 38.