DIABETES - Inactivity may be a greater factor in insulin resistance than aging

An article entitled, “Endurance exercise as a countermeasure for aging,” published online on August 20, 2008 in the journal Diabetes, concluded that the reduction in insulin sensitivity that often occurs in one’s later years may not be an inevitable consequence of aging.  Researchers at the Mayo Clinic College of Medicine in Rochester, Minnesota sought to determine whether long-term endurance training could improve lowered insulin sensitivity (a factor in the metabolic syndrome, which is more prevalent with aging) and mitochondrial dysfunction, a widespread condition of aging which has been associated with declines in insulin sensitivity. (Mitochondria are organelles within the cell that produce energy.) The team enrolled 22 adults aged 18 to 30 years, and 20 adults between the ages of 59 to 76 years for the current study. Participants were divided into those who reported less than 30 minutes of exercise per day two times weekly, and those who participated in at least one hour of running or cycling per day six days per week over the past four years. Blood samples were tested for lipids, glucose, and other factors, and dual x-ray absorptiometry was used to measure fat and fat free mass. Insulin sensitivity, whole-body peak oxygen uptake, muscle mass, mitochondrial function, and SIRT3 expression were also measured. SIRT3 is a mitochondrial gene of the sirtuin family that has been linked with longevity, whose expression has been found to increase with calorie restriction. Older participants had less muscle mass, greater adiposity and diminished whole-body oxygen peak, however, among those that were exercised trained, oxygen peak was higher and fat was lower than in the age-corresponding sedentary groups. For trained subjects, insulin sensitivity was significantly greater compared to the sedentary groups, with no significant difference between young and old groups noted. The age-related decrease in mitochondrial oxidative capacity observed in older individuals was not seen in exercised-trained participants. Although mitochondrial DNA was higher in trained compared with sedentary participants, it remained greater in younger than in older subjects. No decline of SIRT3 expression with age was observed among trained adults, although a significant decline was noted in older sedentary participants.  In their summary of the findings, the authors write that “endurance exercise-trained young and older people have substantially higher insulin sensitivity than the sedentary groups and no differences between young and older people were observed in either sedentary or exercise trained groups. Secondly, in contrast, we found age-related declines in various markers of mitochondrial function in the sedentary groups, but these age-related differences were partly, but not completely, abolished in people who practice regular endurance exercise. Finally, we show that endurance exercise may exert similar potentially lifespan-enhancing effects as caloric restriction through elevated SIRT3 expression in both young and older adults.” The authors conclude that exercise could have similar effects on life-span as those observed with calorie restriction in other organisms.