How Long Does Exercise Affect Blood Sugar Levels?
Exercising in the morning before breakfast helps to prevent a high rise in blood sugar in diabetics only after breakfast and lunch. It does not prevent the high rise in blood sugar that follows the evening dinner meal (Medicine and Science in Sports and Exercise, Feb, 2014;46(2):232-238). Nine diabetics exercised before breakfast for three consecutive bouts of 20 minutes at approximately 60 percent of their maximum heart rate. The exercise lowered their rise in blood sugar after breakfast and lunch, but not after supper. The authors concluded that diabetics need to exercise every day to help prevent high rises in blood sugar after meals. They should also advise limiting foods that cause a high rise in blood sugar late in the day, when people tend to be inactive.
High Rises in Blood Sugar Cause the Side Effects of Diabetes
Preventing a high rise in blood sugar after meals helps to prevent heart attacks and all of the other complications in diabetics and in healthy people also. Blood sugar always rises after you eat. If blood sugar rises too high (greater than 160 mg/dL ), sugar sticks irreversibly to the outer membranes of cells. The sugar is then converted eventually to sorbitol that destroys the cells. This is what causes all of the side effects of diabetes: blindness, deafness, dementia, heart attacks, kidney failure, nerve damage, premature death and so forth.
How Exercise Controls Blood Sugar
Resting muscles draw no sugar from the bloodstream. However, contracting muscles can draw sugar rapidly from the bloodstream without even needing insulin (J Appl Physiol, 2005;99: 338-343 & 2005;8750-7587). This effect lasts maximally during exercise and up to an hour after you finish intense exercise, and tapers off completely around 17 hours after you have finished exercising (Am J Clin Nutr, 2008(July);88(1):51-57. Thus the most healthful eating pattern is to have your main meal mid-day, then have a light supper that is low in foods that cause a high rise in blood sugar.