It is advisable to achieve a blood glucose level that is at least 6.7 mmol/l (120 mg/dl), if not higher, before the start of exercise to avoid hypoglycaemia. You may need repeated blood glucose tests, spaced about 30 minutes apart, to determine if your blood glucose is falling or rising prior to beginning your exercise. If the blood glucose is 5 mmol/l (90 mg/dl), not rising, and the activity is primarily aerobic, there is a high risk of hypoglycaemia during the exercise. If you have had an episode of hypoglycaemia prior to exercising (even during the previous night) you may have an increased risk of hypoglycaemia during your physical activity. If you exercise within 2 hours before the meal, you usually need to lower the bolus dose by 30-50%. For prolonged exercise (90 minutes or more), a greater reduction may be needed.
In one study, cross-country skiers with Type 1 diabetes were able to carry on for several hours when reducing the premeal dose by 80%, compared with only 90 minutes if the dose was reduced by 50%. Some people find that lowering their premeal insulin dose may cause an initial rise in their blood glucose which impairs their performance. In such a case, it is probably better to rely on extra carbohydrate intake rather than dose reduction for best performance. If you have been exercising your leg muscles, insulin injected into the thigh will be absorbed rather more quickly from the subcutaneous tissue (more so with short-acting than with rapid-acting insulin). If you inject insulin deep enough to enter the muscle it will be absorbed much more quickly and you will risk having hypoglycaemia. It is important to remember that exercise alone will not lower the blood glucose level at all. Insulin has to be present to enable this to happen.
Glucose from the bloodstream needs insulin in order to enter the muscle cells. The rate of glucose uptake into the muscles of an adult is approximately 8-12 g per hour when exercising at an ordinary rate, and is more than doubled with heavy exercise. The levels of the hormones, adrenaline, glucagon and cortisol in the bloodstream increase during physical exercise. Glucose is released from the liver depot and new glucose is produced in the liver from proteins. If the liver were unable to increase its glucose production, the blood glucose level would drop by about 0.1 mmol/l (2 mg/dl) per minute during exercise, soon resulting in hypoglycaemia. A high level of insulin in the blood counteracts the production of glucose in the liver which, in turn, increases the risk of hypoglycaemia. In people without diabetes, the level of insulin in the blood drops during exercise.