During physical activity, how much energy your muscles expend increases substantially, supported by fuels like glucose and fats in your bloodstream, as well as glycogen and triglycerides stored within your muscles. The intensity and duration of any exercise are the primary determinants of the relative use of these metabolic fuels. For low-intensity exercise like slow walking, particularly if sustained for long intervals, your muscles will use more fats circulating in your bloodstream, with relatively slow depletion of muscle glycogen. During prolonged lower-intensity activities, though, your blood glucose use can still become quite significant as your muscle glycogen stores become depleted over time, increasing the risk of hypoglycemia in diabetic exercisers.
During moderate intensity exercise, your body’s blood glucose use is usually increased three- to four-fold above rest, and you’ll mobilize and use more fats as well. Both how long you exercise and how trained you are affect your relative use of fats. Doing longer duration exercise and having a higher fitness level favor a greater use of fat, although the greatest utilization of triglyceride (fat) stores in your muscles occurs during recovery from such activities rather than during, unless your glycogen levels get really low. During short-duration activity, taking in some extra carbs can effectively control your blood glucose levels, but for more prolonged exercise sessions, most athletes with diabetes will need to reduce their insulin doses as well (if taken). In some cases, type 2 diabetic athletes not taking insulin may alternately need to reduce doses of certain sulfonylureas (e.g., Diabinese, DiaBeta, Micronase, and Glynase, in particular) or Byetta to avoid hypoglycemia or sluggishness.
Although aerobic activities use a fuel mix of muscle glycogen, blood glucose, and lipids, during higher-intensity, prolonged activities like running, carbohydrate is your body’s fuel of choice, and near depletion of both muscle glycogen and blood glucose is inevitable if you exercise for long enough. If you do higher-intensity, repeated interval training, you may also significantly deplete your muscle glycogen and a higher risk of hypoglycemia.
In the next blog, I’ll continue to talk more about how your body responds, physiology-wise, to different types of training and activities.