Losing muscle and strength late in life may not be the unavoidable bane of aging that scientists have thought, according to a study by researchers at the Keck School of Medicine of USC and the University of Texas Medical Branch.
Many believe that the decline of muscle as people grow older-called sarcopenia-and the accompanying loss of strength and function occurs because muscle proteins begin to break down faster than they can be created and restored. But a three-year-long study casts that into doubt, the researchers reported in the Sept. 12 issue of the Journal of the American Medical Associa-tion.
The team studied 48 healthy men-the largest group to date on the topic-during non-eating hours and tracked the rate at which their bodies created muscle proteins and the rate of muscle protein breakdown. They found that turnover rates were similar in younger and older men. That means that other factors must account for the muscle loss and weakening that come with age.
“From a medical standpoint, it’s very good news,” said Elena Volpi, assistant professor of medicine in the division of endocrinology and diabetes at the Keck School and the paper’s lead author. “It means sarcopenia is not an inevitable effect of age.”
Volpi conducted the study with colleagues at the University of Texas Medical Branch before moving to the Keck School last year.
Muscle loss in aging might be linked to eating habits or the body’s ability to use protein from food, as well as hormonal changes, she suggested. Reductions in physical activity and the effects of disease also are potential targets for investigation.
Further research may eventually lead to interventions to keep elderly people stronger further into old age. These might include physical activity programs, improved diets, medicine to help the body better use nutrients and hormone therapy. “It is a positive finding, because such interventions would be easier to create than one that targets a reduction of basal muscle protein synthesis,” Volpi said.
As seniors lose muscle mass and strength, they may eventually lose their independence. In addition, reduction in lean body mass may contribute to metabolic problems including type 2 diabetes and osteoporosis. Such health challenges will mount in the coming years, as demographers expect the number of Americans over age 65 to double-swelling to 20 percent of the population-by 2030.
The body uses amino acids to create muscle proteins, which routinely break down and are replaced by new proteins in a natural recycling process, Volpi explained. Researchers introduced a labeled amino acid intravenously into the volunteers during their basal state (when they were not eating). A label in the amino acid allowed them to recognize it and measure it in muscle biopsies and blood samples to see how much was taken up by muscle to make proteins. They also measured how much the labeled amino acid was diluted by unlabeled amino acid-already present in the body-due to muscle protein breakdown. They calculated muscle volume, as well.
Researchers used three different methods to examine the rates of protein synthesis and protein breakdown.
Between 1997 and 2000, study researchers recruited 22 healthy seniors with a mean age of 70. They compared them with 26 younger healthy men with a mean age of 28. In both old and young participants, the balance between synthesis and breakdown was similar-although they found that muscle protein both was created and broken down a little bit faster in older people than in the young group.
The findings may differ from earlier studies because those studies did not directly measure muscle protein breakdown and net muscle balance in older men, she said. In addition, Volpi and colleagues did not change or control participants’ diet or activities before the study because these changes could have affected basal muscle protein turnover. The health of participants in other studies may have differed from those in the current investigation, as well.
Researchers hope to conduct more detailed investigations into muscle breakdown as tests are refined to detect the turnover rates of individual proteins within muscle. Volpi also is pursuing research funded by the National Institute of Aging into insulin’s effects on muscle protein turnover in older people.
Blake Rasmussen, assistant professor of kinesiology, also participated in the study before joining USC.
The study in JAMA was funded through the National Institute of Aging, the University of Texas Medical Branch’s Claude D. Pepper Older Americans Independence Center and the National Center for Research Resources.
Elena Volpi, Melinda Sheffield-Moore, Blake B. Rasmussen, Robert R. Wolfe, “Basal Muscle Amino Acid Kinetics and Protein Synthesis in Healthy Young and Older Men.” Journal of the American Medical Association, Vol. 286, No. 10.