For many people, being afraid of having your blood glucose go too low when you’re active has long been a barrier to getting and staying regularly active. It’s a valid fear since many different factors can increase your risk for hypoglycemia (low blood glucose) associated with physical activity, including what activity you do, how long you do it, and how hard you are working out.
What usually happens when you exercise is that your blood glucose goes down, although there may be times and situations when it goes up instead, at least temporarily. What you may not know is that your blood glucose responses can vary based on when you last ate (1), the time of day you’re active, your biological sex, and how much total activity (intensity and duration) you do (2). Those are a lot of factors to figure out and balance appropriately when you have to take insulin.
The amount of insulin you have “on board” (injected, pumped, or inhaled) during activities impacts how likely you are to go low. When your insulin levels are higher, you use more blood glucose during the activity. This effect is compounded by how doses of insulin are absorbed—the insulin peaks later and lasts longer when you take more rapid-acting insulin (3). What’s more, the blood flow to your skin increases when you’re working out, which can speed up insulin absorption (from where it was injected or pumped), which makes you more likely to get low (4). You want to reduce the doses of insulin you take within two to three hours of exercising or lower your basal rates on your pump around exercise; it also may help to exercise more than three hours after you took any mealtime or correction insulin.
To avoid going low, you may need to eat more, monitor your glucose more frequently, and be more careful when you’re exercising in the heat, as that can speed up glucose use and cause you to go low and get dehydrated more easily. When you try a new or unusual activity, you’re more likely to get low until your body adapts to it (known as a “training effect”). Once you’re more trained, your body can switch to using slightly more fat and less carbohydrate (blood glucose included), which keeps your glucose levels more stable during that activity. Training is very specific, though, and this effect only works on the activity you have adapted to doing.
It is also possible to go low after you work out and not just during the activity. If you use up a lot of the glucose stored in your muscles (glycogen) while active, your body will remain more insulin sensitive for hours to days afterward, which can cause you to get low when you’re not expecting it—like during the middle of the night when it’s really inconvenient and harder to detect and treat quickly. Doing repeated interval workouts or intense resistance training can really use a lot of glycogen and raise your risk of later-onset lows (5). You will likely need to monitor more often after and possibly cut back on your insulin or eat more food (particularly carbohydrates) to prevent them.
Finally, you should know that most people with type 1 diabetes have impaired hormonal responses to going low, meaning that your body releases less of glucose-raising hormones like adrenaline and glucagon that helps your body recover from hypoglycemia. Having a prior low or exercising may make your responses to the next low (or exercise bout) even more blunted, so just be on the lookout for lows that can sneak up on you around times of activity. While it is possible to be active safely and effectively regardless, it may take more vigilance and a period of trial and error to get it right. You’ve got this!
1. Yardley JE. Reassessing the evidence: prandial state dictates glycaemic responses to exercise in individuals with type 1 diabetes to a greater extent than intensity. Diabetologia. Aug 18 2022. doi: 10.1007/s00125-022-05781-8.
2. Steineck IIK, Ranjan AG, Schmidt S, Norgaard K. Time spent in hypoglycemia is comparable when the same amount of exercise is performed 5 or 2 days weekly: a randomized crossover study in people with type 1 diabetes. BMJ Open Diabetes Res Care. 2021;9(1):e001919.
3. Nosek L, Roggen K, Heinemann L, Gottschalk C, Kaiser M, Arnolds S, et al. Insulin aspart has a shorter duration of action than human insulin over a wide dose-range. Diabetes Obes Metab. 2013;15(1):77-83.
4. Koivisto VA, Felig P. Effects of leg exercise on insulin absorption in diabetic patients. N Engl J Med. 1978;298(2):79-83.
5. Joy NG, Tate DB, Davis SN. Counterregulatory responses to hypoglycemia differ between glimepiride and glyburide in non diabetic individuals. Metabolism. 2015;64(6):729-37.