Link Found Between Aging and Muscle Loss
Study finds stem cells in older mice turn into fat instead of muscle
MONDAY, Feb. 11, 2002 (HealthDayNews) -- Scientists think they may have found a cellular mechanism that turns muscle to fat as you get older, and the findings might point toward ways to keep you strong as you age.
All human adults carry a reserve of stem cells in their muscles, and elsewhere in their body. These cells are "generic," meaning they don't have the characteristics of the cells of a particular part of the body but they could develop into such cells.
When activated, stem cells in muscles express genes characteristic of muscle cells, which lets them repair muscle fibers. This way, the body maintains a more or less constant number of muscle fibers.
In a new study, muscle stem cells from young (8-month-old) and old (23-month-old) mice were isolated and examined. The cells from the young mice expressed the genes of muscle cells, but the genes from the older cells behaved quite differently.
"We found that if you isolate the stem cells in increasingly older mice, they seem to express genes that are more characteristic of fat," says Charlotte Peterson, lead author of the study. "Our hypothesis is, as you get older, in addition to getting more fat outside and around the muscles, these stem cells may be directly contributing to increases in fat in your muscles."
The findings, which appear in the March issue of Mechanisms of Aging and Development, represent the first time this process has been observed in muscle. A similar phenomenon has been noted in bone stem cells, which normally devote themselves to forming new bone. In older people, however, they seem to express genes characteristic of fat cells. This apparently contributes to decreases in bone density.
Peterson and her colleagues now wonder if their findings and others might have wider implications.
"Our hypothesis is that, with age, this may be a default program," says Peterson, an associate professor of geriatrics at the University of Arkansas for Medical Sciences and a scientist at the Central Arkansas Veterans Healthcare System. "As you get older and older, no matter what they're supposed to do -- make nice bone or nice muscle or nice skin -- for some reason, there's this default program that gets activated."
Though intriguing on some levels, experts say the research also has some problems.
For one thing, the experiment was conducted in an artificial environment.
"The observation is very interesting, but it's all outside of the body and in an environment that is not very relevant to what happens in the body. It's very extreme," says Dr. Nir Barzilai, director of the Institute for Aging at Albert Einstein College of Medicine in New York City. "This is done outside of the metabolic environment, so you cannot quantify the real impact."
There also may be issues with whether age, rather than fat or body mass, were responsible for the differences in cell behavior, adds Barzilai.
"They're metabolically such different animals that it may not be a cause-and-effect relationship. It may be secondary to other differences. Being overweight has a huge effect on a variety of metabolic issues," says Barzilai.
By way of example, Barzilai tells of another study in which one animal was allowed to eat as much as he wanted while his brother was given only 60 percent of what he was consuming. The brother lived twice as long as the first animal.
"To see if aging does something independent of food and fat, many experiments do caloric restrictions," says Barzilai.
"I wouldn't say that this is not an important observation or that it shouldn't be followed, but it's a little bit premature to make anything more out of it than insinuation," says Barzilai. "It's kind of a descriptive that needs to be proven by 10 more experiments before you can call it a mechanism."
Peterson and her colleagues are already going ahead with new experiments to see if the process occurs in humans. This will involve taking needle biopsies from the thigh muscles of volunteers and, again, studying their reactions under a microscope.