People are often surprised when they start exercising and find that their blood glucose levels rise instead of fall. This happens when the body perceives exercise as an added stress, so adrenaline and other hormones come out to raise blood glucose levels. As your body gets fitter and the exercise is less stressful, blood glucose levels may fall with the same exercise. Intensity makes a difference: walking on flat ground for 60 min almost always lowers blood glucose levels, but hiking up a mountain for an hour may lower or raise blood glucose levels. Your level of fitness can change over time.
The biggest mistake people tend to make is to over-treat the high blood glucose after exercise. After a workout, the body needs to restore glucose into muscle (the muscle stores glucose as glycogen), and it is like a sponge sucking up water, whether the blood glucose level is high, low, or normal. This is a time when you need to give your body fuel (carbohydrates) and perhaps insulin, but less insulin than usual (maybe half a dose, but discuss with your healthcare team), to avoid immediate post-exercise rebound lows. Regardless of whether your blood glucose level is high after exercise, you are still at risk of lows 12-24 hours later (often at night), so watch for this and adjust your overnight insulin dose as needed.
The body responds to anaerobic exercise (high-intensity exercises like sprinting and powerlifting) by pumping up levels of stress hormones, which trigger the liver to pump out extra glucose to keep the muscles from becoming depleted. If you’re in a competition, like a race, that stress can also trigger hyperglycemia. Going into exercise a little bit high is fine—exercise will likely take care of the situation.
Strategies such as doing a cool-down of light walking or cycling for 15–20 minutes after strenuous exercise can help lower blood glucose levels more gradually.
Stay well hydrated during exercise, as dehydration can exacerbate highs. If you become dehydrated, you become resistant to the action of insulin and blood glucose levels can increase.