In general, exercise lowers blood glucose levels. This can be good or bad, depending on your blood glucose level before you start to work out. If you have type 1 diabetes, and your pre-exercise glucose result is under 100 mg/dl, exercise could lower glucose too much. Have a carbohydrate-containing snack before beginning to work out. Keep another snack handy to avoid low blood glucose levels during to exercise.
On the other hand, people with type 2 diabetes should limit snacking to assist in weight loss. Snacking before exercise can undermine your efforts. It can also work against the power of exercise to lower blood glucose levels. If you are using insulin or glucose-lowering drugs, talk to your doctor about what levels are safe before exercise.