Mechanisms of glucose regulation during aerobic exercise
To understand the possible metabolic responses to exercise in diabetes, it is useful to first describe the mechanisms of glucose regulation in people without diabetes.
People without diabetes
The increased metabolic demand of exercise requires a dramatic increase in fuel mobilization from sites of storage and an increase in fuel oxidation within the working muscle. Normally, the increase in fuel mobilization for oxidation is under neuroendocrine control. During the transition from rest to exercise, the working muscles shift from using predominantly free fatty acids released from adipose tissue to a complex mixture of circulating fats, muscle triglycerides (TG), muscle glycogen and BG derived from liver glycogen.
During the initial stages of exercise, muscle glycogen is the main source of energy, but the reliance on this limited fuel source decreases as the duration of exercise increases. As a result, contributions from circulating free fatty acids and glucose in the blood stream increase to replace diminishing muscle glycogen stores. This greater reliance on liver glycogen can have dramatic effects on BG levels.
The mixture of fuel utilization differs markedly depending on the intensity of exercise. During low to moderate intensities, plasma-derived free fatty acids make up the majority of oxidized substrate. As the intensity increases, there is a greater reliance on carbohydrates. During heavy exercise, BG utilization may be as great as 1 to 1.5 g/min and this fuel source must be continuously replaced at an equal rate or hypoglycemia will ensue.The mix of fuel utilization during exercise in people with type 1 diabetes appears to be similar to that of people without diabetes, except that individuals with diabetes may have a slightly greater reliance on fat as an energy source and a slightly lower rate of carbohydrate oxidation.
To facilitate the changes in glucose delivery during exercise, pancreatic insulin secretion decreases and circulating levels of glucagon, growth hormone, cortisol and catecholamines increase. The primary role of these hormonal changes is to ensure an adequate supply of glucose for the exercising muscles. Usually, the magnitude of change in these hormones is greater with increasing exercise duration and intensity. That is, during prolonged heavy aerobic exercise (i.e. exercising for over 30 min at 60 to 80% of VO2max), the reduction in insulin secretion is more pronounced, while the release of the other glucose counterregulatory hormones is increased to a greater extent.
