BGLs OK during exercise, high afterwards and then low. Why?

Matt from Sydney asked: Hi Allan, why is it that sometimes my BGL is OK during exercise but go high afterwards and when I correct with insulin I go really low?

Sometimes strategies used to avoid hypoglycemia during exercise like increasing carb intake and/or reducing insulin dose can set you up for going high afterwards. This is especially so with higher intensity exercise. The stress of exercise stimulates the release of glucose into blood. Given your BGL is not going high during exercise it is reasonable to assume you have a found a balance between insulin adjustment and carb intake that works for you.

Exercise stimulates production of counter-regulatory stress hormones that increase glucose production. If during exercise glucose production is matched by glucose utilisation (i.e., the amount of glucose used by muscle) BG will remain reasonable steady.However this equation changes when exercise stops and glucose use by muscle is reduced while the effects of counter-regulatory glucose raising hormones persist. Under such circumstances it is normal for BG levels to remain elevated for 1 to 2 hours even in people without diabetes. Of course for those with a functioning glucose auto-regulation system this is sorted with natural insulin production.  This is not the case for people with T1D and correcting an elevated BGL with extra insulin at this time is challenging as the effects of counter-regulatory hormones wear off and insulin sensitivity increases to encourage replenishment of the glucose stores (i.e. glycogen) used to fuel the exercise.  Here BGLs can change rapidly from high to low.

Take home: If you find your BGLs are elevated after exercise but return to ‘normal’ within 2 hours treat them as a transient deviation. But if you find exercise results in consistently prolonged periods of hyperglycaemia you should consult your diabetes health care professional about taking correction insulin immediately after exercise. Note – extra care should be taken here because the BG lowering effect of insulin is likely to be magnified significantly.

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