Aerobic Exercise Beneficial in McArdle's Disease

Thirty to 40 minutes of moderate exercise on a stationary bicycle four days a week markedly improved the exercise capacity of eight people with McArdle’s disease, researchers at the University of Texas Southwestern Medical Center in Dallas and the University of Copenhagen (Denmark) have found.

MDA grantee Ronald Haller at UT Southwestern and colleagues reported in the June issue of Annals of Neurology that regular aerobic exercise is safe and beneficial in this condition.

Aerobic exercise relies on “oxidative” energy production, which takes place inside the muscle cells’ mitochondria (metabolic centers). In McArdle’s disease, which is also known as muscle phosphorylase deficiency, patients cannot utilize the stored form of sugar, known as glycogen, for energy, so they have to rely on alternative fuels from the bloodstream, such as the sugar glucose.

Dietary interventions to improve exercise capacity in McArdle’s, except for a high-sugar meal 40 minutes before exercise, have been ineffective, the investigators say. (The benefit of a high-sugar meal is attributed to the increased blood glucose levels; the down side is a relatively short time of benefit and possible unwanted weight gain due to extra calories.)

A concern about exercise in McArdle’s disease is that pushing exercise beyond one’s fuel supply limits can cause potentially severe muscle damage and even kidney damage from a protein (myoglobin) that leaks out of disintegrating muscle cells.

The Texas and Copenhagen investigators found, however, that when patients exercised with a heart rate of 60 percent to 70 percent of maximum for their age, they were able to perform 30 to 40 minutes of exercise, four times a week, without evidence of muscle damage.

Study participants were monitored for 14 weeks, during which they were asked to pedal a stationary bicycle at an intensity that brought their heart rate to between 95 and 115 beats per minute for the oldest participant and 110 to 130 beats for the younger subjects.

“Since maximal heart rate declines with age and can be estimated as 220 beats per minute minus age in years, we recommend a target heart rate range of 120-140 for patients 20 years old, 110-130 for those 40 years old, and so on,” Haller said. “Ideally, training should be discussed with and monitored by a physician familiar with McArdle’s disease.”