Anaerobic Exercise

See why sprints and heavy lifts push blood sugar up, then down again hours later.

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Sprints, heavy lifts and intervals near the top of your heart rate range — vigorous to all-out effort1 — set off a stress response that raises blood sugar, even while you are burning fuel at a very high rate.2 The risk of a low comes later, often overnight.

The workout is fixed: 30 minutes of intervals at about 90% of your maximum heart rate. Choose Injections, Pump or AID (on a pump, a Temp Basal: a Start, Back to usual and a Rate; on AID, when Exercise Mode starts), and move the Food and the Bolus (each a Time and an Amount) around the workout. Watch blood sugar rise during and just after the effort, when the spike peaks and begins to fall, how a meal dose still working changes it, and how the risk of a low comes hours later despite the earlier high.

Educational tool, not medical advice. Talk to your care team before changing your management.

What’s happening here?

During anaerobic exercise, your body releases stress hormones — chiefly adrenaline and noradrenaline — that signal your liver to release stored glucose rapidly.2,3,4 This release outpaces how fast your muscles can use it, so blood sugar rises even though you are working hard.

Unlike aerobic exercise, which typically lowers blood sugar during the workout, anaerobic effort can push blood sugar up significantly, which makes it one of the most counterintuitive situations in diabetes management. The rise often peaks 5–15 minutes after the hardest effort ends, not during it.

Carbs eaten during the intervals work differently from carbs eaten at rest. Working muscle runs on its glycogen and on glucose from the blood, and once you eat more than it takes to hold blood sugar level, most of the rest goes to the muscles as fuel instead of raising blood sugar: about 80% of it.5

Boluses here model rapid-acting insulin — aspart, lispro, or glulisine.6 If you use Regular or another mealtime insulin, the concept holds but the curve shifts later and runs longer; the Bolus Insulin Types simulator compares them side by side.

The post-workout high can last for hours. Without diabetes, the body releases extra insulin in early recovery to clear it; in type 1 nothing does, so it stays up until a dose or refilling muscles bring it down.2 In one study, a correction at the usual dose brought it back down, and lows were rare;7 correcting again and again, on top of insulin still working, can bring a low later, often overnight.2
Phase 1
During Effort

Stress hormones make the liver release glucose faster than working muscle can use it. With little insulin working, the liver’s output outpaces it and blood sugar rises. With a meal dose still working, the liver keeps releasing it, but the working muscles take up more of it, so blood sugar rises less or falls instead, and drops further afterwards.8,9,10 A dose also absorbs faster while you work; long-acting insulin doesn’t.

Phase 2
Early Recovery

Stress hormones clear, but in type 1 the high does not: there is no burst of insulin to bring it down, so it can stay up for hours. A correction given now lowers it much as it would at rest.

Phase 3
Late Recovery

Muscles refill their glycogen, and that night insulin can work harder again. A dose — above all a correction stacked on the evening’s — can lower blood sugar more than usual, and a delayed low is likeliest here, often overnight.

The day after

  • Insulin can work harder overnight, and the risk of a low can carry into the next day
  • Meals and corrections during this window may need smaller doses than usual

Applying this to your own life

Anaerobic workouts create two separate management challenges, one after the other: a high during and after the effort that can last for hours, followed by a higher low risk later, often overnight.

When doing intense workouts, watch how your blood sugar responds during the workout itself and for several hours afterward. A continuous glucose monitor is especially useful here because the delayed low can happen well after the workout feels finished.

  • Expect the high, then the low. Note how far blood sugar rises, how long the high lasts after you finish, and when it starts to come down.
  • Correct with care. A usual correction brings the high down; correcting again and again, on top of insulin still working, can land as a low later.
  • Watch the night. The low risk returns in the small hours, so the low from a late workout can arrive during sleep.
  • On AID, learn how your own system works. Every AID system works its own way, from Omnipod 5, Control-IQ and MiniMed 780G to Loop, iAPS, twiist and CamAPS FX. They differ in the target they aim for, how fast they react, and what their exercise or activity setting does. Some offer an extended bolus or a temporary target, and some don’t. The AID on the simulator is one example of how such a system can act. It is built along the lines of the open-source Loop app, but it isn’t how any particular pump works. Learn how yours works from its maker’s guides and your care team.
  • Keep notes. Peak, drop, any delayed low, and how your next meal’s insulin compared with a rest day.
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Where the numbers come from

The chart is a model, set to match published studies of people with type 1 diabetes. Here is where each part comes from.

  • The rise. With little insulin working, intense intervals pushed blood sugar up by about 70 mg/dL (3.8 mmol/L), and with no extra insulin it stayed up for hours.7,11 Even a single 10-second sprint raises it a little, by slowing how fast muscle takes glucose up afterwards.12 More insulin working doesn’t stop the liver. In studies that traced glucose, its release during intense exercise rose as much with insulin held steady or doubled, while the insulin made working muscle take up more glucose,8,9 and in type 1 the amount of insulin changed how much glucose muscle took up, not how much the liver released.10 At about twice the basal insulin level, intense exercise needed as much glucose as moderate,13 and at home, with the day’s insulin on board, interval sessions lowered blood sugar by about 14 mg/dL on average.14 In the model the muscles take up more of the rise as the insulin working climbs above basal, and all of it at 2½ times basal. No study has yet traced the liver in type 1 during intervals with a meal dose working.
  • Why it stays up. Without diabetes, a burst of insulin after hard effort clears the rise; giving people with type 1 that insulin did the same.15
  • The correction. A correction at the usual dose brought the high back down, and lows were rare.7
  • The meal. Carbs only: juice, candy, a sports gel. A small amount is absorbed within about an hour.16,17 The stomach passes sugar on at about half a gram a minute, so 60 g takes about 3¼ hours.18,19 During exercise the gut takes it at about a gram a minute — 60 g an hour — so the same 60 g is through in about 2 hours.20 A meal with fat and protein digests more slowly; the Bolus Timing simulator shows how other meals digest.
  • Fuel stores. Intervals burn muscle glycogen fastest — about 40% of what the working legs hold in half an hour21,22 — and the adrenaline drives the liver to release glucose 7–8 times faster than at rest, from its own glycogen.3,4
  • Refilling, and the night after. Muscle refills its glycogen slowly with nothing eaten and about four times faster with carbs, in type 1 diabetes nearly as fast as without.23,24,25,26 The longer and harder a workout, the more insulin’s effect rises afterwards: in adolescents with type 1, each extra half hour of afternoon activity made a low that night or the next day about 30% more likely.27 Eating afterwards refills the muscle but does not take that rise away. A meal and snack after evening exercise protected the first hours, not the late night,28 and the morning after a marathon insulin worked no harder although the muscles were still low on glycogen.29

These curves are representative — every body is different. Use the simulators to understand how the factors work together, not to make management decisions.

Sources
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  2. Riddell MC, Gallen IW, Smart CE, et al. Exercise management in type 1 diabetes: a consensus statement. Lancet Diabetes Endocrinol. 2017;5(5):377–390. doi.org
  3. Marliss EB, Vranic M. Intense exercise has unique effects on both insulin release and its roles in glucoregulation: implications for diabetes. Diabetes. 2002;51(Suppl 1):S271–S283. doi.org
  4. Kreisman SH, Halter JB, Vranic M, Marliss EB. Combined infusion of epinephrine and norepinephrine during moderate exercise reproduces the glucoregulatory response of intense exercise. Diabetes. 2003;52(6):1347–1354. doi.org
  5. McConell G, Fabris S, Proietto J, Hargreaves M. Effect of carbohydrate ingestion on glucose kinetics during exercise. J Appl Physiol. 1994;77(3):1537–1541. doi.org
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  9. Sigal RJ, Fisher SJ, Halter JB, et al. Glucoregulation during and after intense exercise: effects of beta-adrenergic blockade in subjects with type 1 diabetes mellitus. J Clin Endocrinol Metab. 1999;84(11):3961–3971. doi.org
  10. Nguyen TP, Jacobs PG, Castle JR, et al. Separating insulin-mediated and non-insulin-mediated glucose uptake during and after aerobic exercise in type 1 diabetes. Am J Physiol Endocrinol Metab. 2021;320(3):E425–E437. doi.org
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  12. Fahey AJ, Paramalingam N, Davey RJ, et al. The effect of a short sprint on postexercise whole-body glucose production and utilization rates in individuals with type 1 diabetes mellitus. J Clin Endocrinol Metab. 2012;97(11):4193–4200. doi.org
  13. Shetty VB, Fournier PA, Paramalingam N, et al. Effect of exercise intensity on exogenous glucose requirements to maintain stable glycemia at high insulin levels in type 1 diabetes. J Clin Endocrinol Metab. 2021;106(1):e83–e93. doi.org
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  22. Vigh-Larsen JF, Ørtenblad N, Andersen OE, et al. Fibre type- and localisation-specific muscle glycogen utilisation during repeated high-intensity intermittent exercise. J Physiol. 2022;600(21):4713–4730. doi.org
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