Metabolic Inflexibility during Submaximal Aerobic Exercise Is Associated with Glucose Intolerance in Obese Older Adults

Objective: People with type 2 diabetes have reduced cardiorespiratory fitness and metabolic impairments that are linked to obesity and often occur prior to the development of type 2 diabetes. We hypothesized that obese, older adults with impaired glucose tolerance (IGT) have lower ability to shift from fat to carbohydrate oxidation when transitioning from rest to submaximal exercise than normal glucose tolerant (NGT) controls. Design and Methods: Glucose tolerance, body composition, and substrate oxidation (measured by RER:respiratory exchange ratio) during submaximal exercise (50% and 60% VO2max) and insulin infusion (3-hour hyperinsulinemic-euglycemic clamp) were assessed in 23 sedentary, overweight-obese, older men and women. Results: Obese subjects with NGT (n = 13) and IGT (n = 10) had similar resting RER, but during submaximal exercise those with IGT had a lower RER and less transition to carbohydrate oxidation than the NGT group (P < 0.05). The IGT group also oxidized less carbohydrate during insulin infusion than NGT (P < 0.05). RER at each exercise intensity independently correlated with120-minute postprandial glucose (r = -0.54 to -0.58, P < 0.05), but not with body composition, VO2max, or RER during insulin infusion. Conclusions: Obese, older adults have metabolic inflexibility during exercise that is associated with the degree of glucose intolerance independent of age and body composition.