This is an excerpt from Clinical Exercise Physiology 4th Edition With Web Resource by Jonathan Ehrman,Paul Gordon,Paul Visich & Steven Keteyian.
Blood glucose levels should be monitored before an exercise session to determine whether the person can safely begin exercising, especially someone using insulin or selected glucose-lowering oral agents. Consideration must be given to how long and intense the exercise session will be. The following general guidelines can be used to determine whether additional carbohydrate intake is necessary.
If diabetes is managed by diet or oral glucose-lowering medications with little to no risk of hypoglycemia, most patients will not need to consume supplemental carbohydrate for exercise lasting less than 60 min. If blood glucose is less than 100 mg · dL-1 and the exercise will be of low intensity and short duration (e.g., bike riding or walking for -1, no extra carbohydrate is likely needed. If blood glucose is less than 100 mg · dL-1 and exercise is of moderate intensity and moderate duration (e.g., jogging for 30-60 min), 25 to 45 g of carbohydrate should be consumed. If blood glucose is 100 to 180 mg · dL-1, then 15 to 30 g of carbohydrate is needed. If blood glucose is less than 100 mg · dL-1 and exercise is of moderate intensity and long duration (e.g., 1 h of bicycling), then 45 g of carbohydrate should be consumed. If blood glucose is 100 to 180 mg · dL-1, 30 to 45 g of carbohydrate is needed. Remember that these guidelines will need to be used on a trial-and-error basis and individualized for each patient. In addition, someone trying to lose weight might benefit from a medication adjustment rather than increased food intake.
If the preexercise blood glucose is greater than 300 mg · dL-1, urine or blood can be checked for ketones. If ketones are present (moderate to high), exercise should be postponed until glucose control is improved. If a patient with a blood glucose greater than 300 without ketones is safe to exercise, be sure the patient is hydrated. These blood glucose values are guidelines, and actions should be verified with the patient's physician. Patients who use medication as part of diabetes treatment should be assessed to determine whether the timing and dosage of medication will allow exercise to have a positive effect on blood glucose. For example, a patient who uses insulin and had blood glucose of 270 mg · dL-1, had no ketones, and took regular insulin within 30 min will see a reduction in blood glucose from both the insulin and exercise. If this patient has not just administered fast-acting insulin and the previous insulin injection has run its duration, the patient has too little circulating insulin - more will be needed to help reduce the blood glucose before exercise. In this case, most physical activity would likely increase the blood glucose level. In all cases, adding medication must be cleared by a clinician with prescriptive authority. Those with type 2 diabetes who are appropriately managed by diet and exercise alone usually experience a reduction in blood glucose with low to moderate exercise. Timing of exercise after meals can help many patients with type 2 diabetes reduce postprandial hyperglycemia. Blood glucose should be monitored after an exercise session to determine the patient's response to exercise.
Patients are more likely to experience hypoglycemia (usually -1) after exercise than during due to postexercise replacement of muscle glycogen, which uses blood glucose. Periodic monitoring of blood glucose is necessary in the hours following exercise to determine whether blood glucose is dropping. More frequent monitoring is especially important when initiating exercise. If the patient is hypoglycemic, he needs to take appropriate steps to treat this medical emergency as previously discussed.
In poorly controlled diabetes, insulin levels are often too low, resulting in an increase in counterregulatory hormones with exercise. This circumstance causes glucose production by the liver, enhanced free fatty acid release by adipose tissue, and reduced muscle uptake of glucose. The result, more likely in type 1 than type 2 diabetes, can be an increased blood glucose level during and after exercise. High-intensity exercise can also result in hyperglycemia. In this case, the intensity and duration of exercise should be reduced as needed.