Sports and exercise can affect the blood sugar in various ways. The use of energy by the body during the exercise will have the tendency to lower the blood sugar, as might be expected. If the exercise is vigorous, stressful to the body, or competitive, the release of stress hormones may occur, which will actually serve to raise the blood sugar.
Also, so-called “isometric”exercise (meaning that tensing or rigidity of the muscle against forceful resistance is involved, such as in weight-lifting) tends to raise the blood sugar more than “isotonic” exercise (such as repeated movement against minimal resistance, such as jogging, swimming,or dancing), which will generally tend to lower it.
Finally, the management of the blood sugar in the aftermath of exercise, meaning from hours to as much as half a day later, may be as important and challenging as during the period of activity itself.
This is because the replenishment of depleted glycogen (starch) into the muscle requires drainage of glucose from the bloodstream into the muscle as the building blocks for the starch.
Exactly how best to manage your blood sugar during and after exercise is somewhat unique to each individual and becomes clearer after you have performed the same activity several times. It helps to plan ahead for the calorie consumption that may occur during any-thing other than brief exercise.
Taking additional calories prior to, and sometimes during, exercise is preferable to cutting or stopping your diabetes medication during or prior to exercise, although sometimes it can be beneficial to reduce the insulin dosage modestly when exercise is planned ahead.
This planning is helped by knowledge of your blood sugar levels leading up to the exercise and sometimes during it and certainly after it.
Finally, account for the replenishment of glycogen stores over the hours following significant exercise by eating a sustaining meal later in the day afterward.
It is not unusual for the blood sugar to be significantly lower, occasionally seriously so, the morning after exercise the previous evening if this is not factored into meal consumption.