Abstract

Objective- Reductions in insulin sensitivity in conjunction with muscle mitochondrial dysfunction have been reported to occur in many conditions including aging. The objective was to determine whether insulin-resistance and mitochondrial dysfunction are directly related to chronological age, or are related to age-related changes in body composition.

Research design and methods- Twelve young lean, 12 young obese, 12 elderly lean and 12 elderly obese sedentary adults were studied. Insulin sensitivity was measured by a hyperinsulinemic-euglycemic clamp and skeletal muscle mitochondrial ATP production rates (MAPR) were measured in freshly isolated mitochondria obtained from vastus lateralis biopsy samples using the luciferase reaction.

Results- Obese participants, independent of age, had reduced insulin sensitivity based on lower rates of glucose infusion during hyperinsulinemic-euglycemic clamp. In contrast, age had no independent effect on insulin sensitivity. However, the elderly participants had lower muscle MAPR than the young participants, independent of obesity. Elderly participants also had higher levels inflammatory cytokines and total adiponectin. In addition, higher muscle MAPR were also noted in men than in women whereas glucose infusion rates were higher in women.

Conclusions- The results demonstrate that age-related reductions in insulin sensitivity are likely due to an age-related increase in adiposity rather than a consequence of advanced chronological age. The results also indicate that age-related decrease in muscle mitochondrial function is neither related to adiposity or insulin sensitivity. Of interest, a higher mitochondrial ATP production capacity was noted in the men, whereas the women were more insulin sensitive demonstrating further dissociation between insulin sensitivity and muscle mitochondrial function.