Type 1 diabetes and exercise: a practical guide

Exercise is good for you with type 1 diabetes, but it moves your glucose in different directions depending on what you do. Steady efforts like running, cycling and swimming usually push it down. Short, hard efforts like sprints and heavy lifting often push it up. Both can bring a low hours later, sometimes while you sleep.

That is the whole picture in four sentences. The rest of this guide explains why it happens, what to check before you start, and how to build a plan with your diabetes care team.

Is exercise safe with type 1 diabetes?

Yes. Diabetes UK says people with type 1 can take part in almost every type of physical activity. A few extreme sports, such as scuba diving and mountaineering, need a medical assessment first. Some complications change the picture too. With certain eye, kidney, nerve or foot problems, high-intensity work or heavy lifting may not be right for you, so check with your team before starting something new.

The upside is real. Regular activity helps your body use insulin better, which makes it easier to keep glucose in your target range. The 2017 international consensus on exercise in type 1 diabetes, led by Professor Michael Riddell and published in The Lancet Diabetes & Endocrinology, notes that active adults with type 1 are less likely to have eye and kidney disease, and more likely to reach their HbA1c, blood pressure and body weight targets (ScienceDaily summary).

The biggest barrier is not safety. It is fear. The same consensus names fear of hypos and not knowing how to manage exercise as the main reasons people with type 1 hold back. Understanding what each kind of training does to your glucose is the first step past both.

How do different types of exercise affect glucose?

Think of it as a spectrum, not a rule.

• Steady aerobic work such as walking, jogging, cycling or swimming at a comfortable pace tends to lower glucose (Diabetes UK).
• Short, sharp efforts such as sprints, heavy weights, squash or HIIT tend to raise it, at least for a while. The consensus describes this rise as temporary.
• Gentle work such as stretching may leave it roughly where it was.

Other things move the needle as well: how hard you are working, how fit you are, the time of day and stress. Diabetes UK points out that nerves before a tournament can push glucose up on their own. So the same session can play out differently on different days. That is normal, and it is not a sign you got something wrong.

Two posts go deeper on each end of the spectrum. If steady cardio is your thing, read why hypos happen while running. If you lift, read why blood sugar goes up after lifting weights.

What should my glucose be before I start?

Diabetes UK publishes general guidance for people who check their own glucose (source). Ideally you start somewhere around 5.6-12 mmol/L (101-216 mg/dL). In more detail:

• Below 4 mmol/L (72 mg/dL): do not start. Treat the low, follow it with some starchy carbs, wait and check again.
• 4-7 mmol/L (72-126 mg/dL): you may need a carb snack first. If you skip it, check regularly as you go.
• 7-13 mmol/L (126-234 mg/dL): you can begin. Keep checking during the session.
• Above 13 mmol/L (234 mg/dL): speak to your care team. Exercise could push you higher, and you may need a different plan. If you do not know why you are high and you have been taught to test for ketones, test and follow your team's advice.

Pump users have different thresholds, so check your own plan. Two more rules from the same page are worth remembering. If you have had a severe hypo in the last 24 hours, do not exercise. If you have had any hypo in the last 24 hours, your risk is higher, so avoid training alone.

These are general guidelines, not a personal plan. Agree your own numbers with your care team.

How do food and insulin fit around a session?

There are two levers: the carbs you eat and the insulin already working in your body. Diabetes UK says people who use insulin may need to change their doses around exercise, depending on how long they move for and what they do (source). The consensus covers both nutritional and insulin adjustments. How much, and when, is individual. That is a conversation for your care team, not a formula from a blog.

What you can do yourself is make the conversation easier. Diabetes UK lists doing a lot of exercise without extra carbohydrate, or without adjusting insulin, as a common cause of hypos (source). So:

• Carry hypo treatment and a snack with some carbs in it, such as a sandwich, a piece of fruit or a cereal bar.
• Wear diabetes ID so people around you can help.
• Record what you ate, when you took insulin, what you did and your glucose before and after. Show it to your diabetes nurse or doctor.

Accurate carb counts make those records worth more. A pre-training meal you guessed at is hard to learn from. This is the problem T1Fit is being built around: estimate the carbs on your plate from a photo, log the session, and see its Exercise Impact build into a pattern over time.

Why can I go low hours after exercise?

Exercise keeps working after you stop. Diabetes UK explains that your body can respond better to insulin for up to 24 hours after activity, so it recommends checking glucose for up to 24 hours afterwards (source). The consensus notes that both aerobic and anaerobic exercise can cause hypos late in recovery, often while people are asleep.

Night-time lows are easy to miss. You may not wake up. Signs the next morning include feeling very tired, a headache, or a reading higher than you expected (Diabetes UK). Keep hypo treatment by your bed. If you suspect lows after training, check before sleep and during the night, then take the readings to your care team, because your insulin may need to change.

Can I trust my CGM during exercise?

Mostly, with one catch. A continuous glucose monitor reads glucose in the fluid around your cells, not your blood. Diabetes UK says it lags behind blood glucose by up to 15 minutes, and the gap is more likely to be bigger when you are eating or exercising.

When glucose is falling fast mid-session, your CGM may show a number higher than your real level. Two habits help:

• Trust how you feel. If you feel low but the reading looks fine, stop and do a finger-prick check.
• Confirm before you treat. Diabetes UK suggests a finger-prick check when treating a hypo for the most accurate result.

Trend arrows and low alerts are still your best early warning. Set alerts at a level your care team is happy with, so you get time to act.

Once you know what to watch for, build your plan one step at a time. Start with one activity you already do and change one thing at a time. Run the same session at the same time of day a few times, and log it. Patterns show up faster than you expect. Then take those patterns to your care team and agree what to adjust. Everyone's response is different, so your plan should be yours, not someone else's.

Exercise with type 1 takes more planning. It still pays off, for your glucose and for everything else.