Hypo while running with type 1: why it happens

Running lowers glucose. Exercise helps insulin work better, and the insulin already in your body keeps working while you move. If you start near the bottom of your range, or with a lot of insulin still active from your last meal, a hypo can arrive fast.

The fix is not a trick. It is a plan you build with your care team, based on your own runs. This post explains what is going on, so you know which questions to ask. For the bigger picture across all types of training, start with our type 1 diabetes and exercise guide.

Why does my blood sugar drop when I run?

Steady running is aerobic exercise, and aerobic exercise tends to lower glucose. Diabetes UK lists jogging alongside walking, cycling and swimming as activities that usually bring levels down. The 2017 international consensus on exercise in type 1 diabetes says the same about walking, jogging and light cycling (ScienceDaily summary).

Two things stack up during a run. Being active helps insulin work better in your body (Diabetes UK), an effect that can last for up to 24 hours. And any insulin you have already taken is still working. If you have not eaten extra carbs or changed your insulin, glucose can keep falling. Diabetes UK names "doing a lot of exercise without having extra carbohydrate or without reducing your insulin dose" as a common cause of hypos (source).

Running is not always a one-way street, though. Hard efforts, race nerves and early-morning runs can push glucose up instead. More on that below.

Why does insulin on board make it worse?

Insulin on board, or IOB, is the rapid-acting insulin from your last bolus that is still working. Take a meal bolus and head out soon after, and you are running while that insulin is still working. Both the insulin and the running are lowering your glucose at the same time.

Eleanor, who has run parkrun most weeks for ten years with type 1, describes this in her Diabetes UK story. When she took insulin before her run, it worked for a while. Then, "within minutes of stopping running, my blood glucose is plummeting." In her words, "having insulin on board during cardio feels like the definition of an extreme sport."

Her story also shows the other side. Running first thing, before breakfast and insulin, sent her glucose high. She puts that down to the morning rise plus a surge of adrenaline. She settled on what works for her, and is clear that it is her experience, not advice.

That is the point. There is no single right answer for everyone. Your answer depends on your insulin, your timing and your running, which is why it belongs with your care team. Knowing your IOB before you lace up gives that conversation something concrete to work with. T1Fit shows your IOB on the dashboard for exactly this reason.

What should my glucose be before a run?

Diabetes UK's general guidance for people who check their own glucose (source):

• Below 4 mmol/L (72 mg/dL): do not run. Treat the low, have some starchy carbs, wait and check again.
• 4-7 mmol/L (72-126 mg/dL): you may need a carb snack first. If not, check regularly during the run.
• 7-13 mmol/L (126-234 mg/dL): you can start. Keep checking as you go.
• Above 13 mmol/L (234 mg/dL): speak to your care team before running. It could push you higher.

If you use a pump, your thresholds may differ. Diabetes UK also suggests that people on a hybrid closed loop system consider the activity or temporary target setting for around 90 minutes before and during exercise. Check how that applies to your system with your team.

If you have had a severe hypo in the last 24 hours, do not run. If you have had any hypo in that time, your risk is higher, so do not run alone.

As for food, Diabetes UK says your care team should tell you how many extra carbs you may need during exercise, depending on your levels. Carry hypo treatment and a snack with carbs in it on every run. Getting the carbs in your pre-run meal right matters too, because a meal you misjudged is hard to plan a run around.

Can I trust my CGM mid-run?

Not completely. A CGM reads glucose in the fluid between your cells, which lags behind your blood by up to 15 minutes. Diabetes UK notes the gap is more likely to be bigger during exercise. When you are dropping quickly, the number on your watch can be higher than your real level.

So if you feel low, believe your body. Stop running and check with a finger prick if you can. If you are low, treat it straight away, as your plan sets out. Diabetes UK's advice is to stop what you are doing, treat immediately, rest and then check that you are back above 4 mmol/L (72 mg/dL) before carrying on (source).

Why do I go low hours after a run?

Because the run has not finished with you. The extra insulin sensitivity can last up to 24 hours, which is why Diabetes UK suggests checking your glucose for up to a day afterwards. The consensus notes that exercise can cause hypos late in recovery, often while people are asleep.

Night-time lows are the ones to watch. You might not wake up, and you might only notice clues the next morning: feeling very tired, a headache, or a reading higher than expected (Diabetes UK). Keep hypo treatment by your bed. After a long or hard run, check before sleep. If you see a pattern of lows after running days, take it to your care team, because your insulin may need to change.

What should I ask my care team?

Bring a few weeks of runs to your next appointment: time of day, distance, effort, when you last ate and bolused, and your glucose before, during and after. Then ask:

1. How long is my rapid-acting insulin active, and how should I time runs around meal boluses?
2. Should I change anything before or after a run, and by how much?
3. What should I eat before longer runs, and how much should I carry?
4. What target should my CGM low alert be set to while I run?
5. What should I do differently on days after a hypo?
6. If I use a pump or closed loop system, how should I use its exercise settings?

Your answers will be different from other runners'. That is expected. Build them from your own data, check them with your team, and adjust as your running changes.