Activity Before and After Insulin

The same walk becomes a different proposition depending on what treatment you are on. This catches people out, because nothing about the walk has changed.

On metformin alone, exercise carries essentially no risk of low blood sugar. Metformin does not force insulin release. You can walk, cycle or swim without checking anything or eating beforehand.

On a sulfonylurea, that changes. These tablets push insulin out regardless of what your blood sugar is doing, so exercising while one is working can drop you low.

On insulin, planning becomes routine. Injected insulin cannot be recalled once given, and exercise makes your body more sensitive to it — so the same dose lowers your glucose further on an active day than on a quiet one.

• Check your glucose before you start, and again afterwards.
• Carry fast-acting glucose with you, every time, without exception.
• Be aware of when your insulin is at its strongest, and prefer to exercise away from that window.
• For longer or harder sessions, your doctor may advise reducing a dose or eating extra carbohydrate beforehand.

There is a delayed effect worth knowing about. Your muscles refill their glucose stores for hours afterwards, so a low can arrive four to twelve hours after exercise — including in the middle of the night after evening activity.

None of this is a reason to exercise less. It is a reason to change how you prepare when your treatment changes.