Many automobiles don't come with spare tires anymore, but many of us already have them. We couch potatoes often loll in front of the television instead of taking our extra pounds for a jog around the block. But new research shows that inactivity is not the whole problem. When an aging couch potato waddles into the gym to exercise, those extra pounds are harder to lose, partly because muscles lose the ability to burn fat as people get older.
Researchers at Washington University School of Medicine in St. Louis and the University of Texas Medical Branch in Galveston reached this dismal conclusion. They report their results in the December 1996 issue of the American Journal of Physiology--Endocrinology and Metabolism. In the study, they found that older people burn less fat than younger people when the two groups do similar excercise.
Principal investigator Samuel Klein, M.D., associate professor of medicine and director of the Center for Human Nutrition at Washington University, compared six adults whose average age was 73 with six adults whose average age was 26. Both the young and the old subjects were sedentary and not exercising regularly at the time of the study, which was supported by grants from the National Institute of Diabetes and Digestive and Kidney Diseases.
The researchers monitored fat and carbohydrate breakdown products in blood samples from the two groups while the subjects peddled stationary bikes for 60 minutes. Measuring oxygen consumption, the investigators learned how hard the two groups were able to exercise. The older subjects were asked to peddle at a rate that made them consume oxygen at half their maximum rate. The younger subjects also peddled at half their maximum so they could exercise at the same relative intensity as the older adults.
Klein and fellow investigators also observed the study subjects when they peddled at speeds that made them consume identical amounts of oxygen--the same absolute exercise intensity. During the one hour of exercise at the same absolute intensity, the elderly subjects oxidized less than one-third as much fat as their younger counterparts.
"The older people had a decreased ability to oxidize fat during exercise, both at the same absolute exercise intensity --the same exact workload--and at the same relative intensity, which is a lower workload because older people tend to be less fit than younger people," Klein explains.
Substituting One Fuel for AnotherAverage fat oxidation was 25 to 30 percent lower in the older people than in younger people at either the same absolute or the same relative intensity. As a consequence, carbohydrate oxidation was 35 percent higher. "Carbohydrate and fat are the two major fuels used during exercise--glucose from carbohydrate and fatty acids from fat," Klein explains. "If you use less fat as a fuel, you automatically use more carbohydrate. So it makes sense that if elderly muscles have difficulty converting fat into energy, they have to use more carbohydrate to compensate."
In the young adults, about one-half of the fuel metabolized during exercise came from fat with the other half from carbohydrate. In the elderly subjects about two-thirds came from carbohydrate, and only about one-third came from fat. Klein says burning carbohydrate rather than fat is not unhealthy. It simply substitutes one fuel source for another. But increased use of carbohydrate makes it harder for people to continue their workout. Carbohydrate oxidation leads to quicker fatigue and depletes blood sugar levels more rapidly, he says. As a result, sedentary elderly people cannot exercise for as long as sedentary young people.
The physiologic reasons that elderly people burn less fat are not known. Klein's team did not find any defect in the ability to mobilize body fat. There was little resistance of body fat to catecholamines, the hormones used to mobilize fat tissue for conversion to energy, and fat breakdown produced plenty of fatty acids for muscle to oxidize.
Klein believes the answer lies in the muscles themselves. "It appears the muscle tissue of older people is not able to--or prefers not to--oxidize fat as a fuel," he says.
Changes that occur in aging muscle may help explain why our muscles no longer oxidize fat as they did when we were younger. Muscle cells lose some of their mitochondria, the cellular structures that produce energy. Loss of mitochondria may contribute to a loss in the ability to oxidize fat.
"The mitochondria are where the cell does the major oxidation of fuel," Klein explains. "So with a decrease in mitochondrial capacity, elderly muscles may simply be unable to oxidize as much fat as younger muscles."
The Good NewsBut even before the precise mechanisms are understood, there's good news for elderly people. The paper mentions that an intensive 16-week exercise program helped the elderly subjects oxidize fat more like younger participants. "If older people train rigorously for about four months, they have more normal patterns of fat oxidation," Klein says. "It comes back to about where it is in younger adults. Apparently, training either corrects the defect or compensates for it in some way."
Normal muscles use two sources of fat. There is the fat our bodies store in adipose (fat) tissue. It is broken down and released into the bloodstream, which delivers it to muscles. The second source of fat comes from muscles themselves. Muscle tissue contains its own triglyceride droplets, so it can oxidize that fat as a fuel directly during exercise.
This study did not determine whether aging muscles have more trouble oxidizing fat from the bloodstream than fat stored in muscle or vice versa, but Klein now is starting to evaluate the use of intramuscular triglyerides and plasma fatty acids during exercise in elderly subjects.