Exercise-induced fall of blood glucose in insulin-treated diabetics unrelated to alteration of insulin mobilization

To investigate whether leg exercise accelerates the absorption of exogenous insulin from Its subcutaneous depot in insulin-treated diabetics, and whether the glycemie response to exercise in these patients is related to alterations in the insulin absorption process, we studied nine insulin-dependent diabetics and four normal subjects during rest and exercise. The diabetics received the regular dose of intermediate-acting insulin, reduced by 20 U, and their morning breakfast. One hour after the meal, 20 U regular insulin (Actrapid, labeled with 125I or unlabeled) was injected into the arm or the leg. Exercise (90 ± 10 W) was initiated 35 min later for three consecutive 10-min periods. In controls, no insulin was injected. Plasma glucagon and C-peptide levels were not changed during exercise, but blood ketone bodies and lactate levels increased significantly. Whether the insulin was injected into the moving or resting limb, external counting over the injection site demonstrated no exercise-induced alteration of the disappearance of 125I-radiation during the exercise period, but revealed a small postexercise enhancement of 125I disappearance only after leg injection. Determination of plasma insulin levels in four diabetics with newly insulin-treated diabetes revealed that circulating insulin had increased by 70% 30 min after the subcutaneous administration of regular insulin and had then plateaued for 45 min. Leg exercise had no additional effect on plasma insulin levels during the physical activity. In normals, exercise lowered circulating plasma insulin to a level approximately one-fifth that in diabetics. Regardless of the insulin injection site, there was an identical glycemie response to exercise, i.e., a pronounced fall of glycemia by 15% immediately and by 50% 30 min after the onset of exercise. In normals glucose homeostasis was not disturbed by exercise. This study demonstrates that moderate leg exercise does not necessarily accelerate the absorption of exogenous insulin; however, it can induce an acute and profound fall in glycemia, which presumably is due to the presence of a fixed hyperinsulinemic state in these patients rather than an alteration of the mobilization of insulin from its subcutaneous depot. Although this study investigated only a specific situation, the results indicate the possibility that exercise-induced hypoglycemia in insulin-treated diabetics is not necessarily prevented by changing the insulin injection site to nonmoving parts Of the body.