The amount of insulin circulating in the bloodstream during exercise is critical in determining performance and preventing fatigue from setting in early from hypoglycemia.
In people with diabetes and in most people with type 2 diabetes, insulin levels in the blood fall during exercise, and the rise in glucagon released from the pancreas stimulates the liver to produce more glucose. If insulin is injected, however, the body can't lower the circulating levels when starting exercise. Having too much insulin under those circumstances is bad news because it stimulates muscles to take up glucose from your bloodstream. Muscle contractions do the same thing, meaning higher insulin levels can result in double the glucose-lowering effect and rapid-onset lows.
But, some insulin needs to be in the body. If you have too little, the body will be missing the normal counterbalance to the rise in glucose-raising hormones, and you could end up hyperglycemic instead. To perform optimally, you need some insulin in the body to counterbalance the release of glucose-raising hormones, but not so much insulin that blood sugar drops excessively.
Timing of Exercise and Insulin Levels
The timing of exercise may also play a big role in the body's responses. For instance, a person is less likely to experience low blood sugar if he exercises before breakfast, especially before taking any insulin. At that time of day, only basal insulin (the insulin that covers the body's need for insulin at rest separate from food intake) is on board, so the circulating levels will generally be low, but there are usually higher levels of cortisol, a hormone that increases insulin resistance, to compensate.
If exercising after breakfast and a quick-acting insulin injection, the insulin dose may affect whether a person gets low because the dose will affect the levels of circulating insulin. In one study, exercisers with type 1 diabetes did 60 minutes of moderate cycling starting 90 minutes after taking their regular dose with an insulin pump and eating breakfast. To prevent low blood sugar, they reduced their rapid-acting insulin boluses by 50 percent and took no basal insulin. Their morning insulin reductions, however, turned out to be less than afternoon ones made for a similar workout. Thus, if hypoglycemia often develops during exercise, it would be best to exercise before taking any insulin to cover breakfast instead of afterward or later in the day.
Anyone with type 2 diabetes who still makes insulin is also more likely to have glucose levels drop if they are exercising after breakfast or another meal (as opposed to before) because of the insulin that is released in response to eating. Keep in mind, though, that if exercising long enough without eating, whether a person has diabetes or not, hypoglycemia can develop as the result of low fuel stores and liver glycogen after not eating overnight, so running a marathon without eating anything beforehand isn't a good idea.
This is adapted from Diabetic Athlete's Handbook.