In general, when people with diabetes exercise, they require less insulin for good control. This is due both to a more rapid absorption of insulin from exercised sites and to an increase in the effectiveness of the insulin. You must, therefore, reduce your insulin when you exercise, but there are few guidelines to help you. Some people only need to reduce their insulin a little, some need to reduce it by as much as half.
Exercise should generally be mild to moderate, and aerobic. Resistance exercise like moderate weightlifting is fine, but you should avoid power exercises like competitive weight lifting. In addition, you should try to exercise at the same time each day.
For mild exercise like walking or bike riding most people will only need to reduce their insulin by 1-2 units, although some people with diabetes will need to reduce their dose significantly more. For more strenuous exercise the insulin dose must be reduced even more. If in doubt, decrease the dose by a larger amount. If your next blood glucose is too high, you can always decrease the dose less next time.
If you do prolonged, strenuous exercise, the increased effectiveness of insulin may last longer than the exercise. This can be a serious problem and one that you should discuss with your doctor.
With intensive insulin therapy, since each insulin dose lasts only a short period, it is fairly easy to make alterations in the dose for exercise. This change, however, is very specific for each patient, and the guidelines that we use generally need significant individual alteration based upon the experience of the patient.
As a first guess, I generally lower the insulin by 2-4 units for mild to moderate exercise and by 4-8 units for moderate to heavy exercise. You need to check you blood glucose before and after exercise, and, working with your physician or diabetes educator, may need to make frequent changes to this scheme.