Carb counting for endurance sport with type 1 diabetes comes down to three numbers: what you eat before you start, how many grams per hour you take in while moving, and what you do afterwards to refill glycogen. The amount shifts with intensity, duration and how much insulin is still active, so there's no single fixed figure. Below is how experienced T1D athletes actually plan it, with the current evidence behind each step.
How many carbs do I need per hour for endurance exercise?
This is the number everyone searches for, and the honest answer is "it depends on intensity, not just time on your feet." A moderate-intensity bout lasting up to 30 minutes in a stable glucose state may only need a small amount, around 10 to 20 grams per hour, while longer and harder sessions push that up to 75 to 90 grams per hour. The consensus table used by sports diabetes teams breaks it down further by heart rate zone: below 60% of max heart rate for over an hour, you're looking at roughly 15 to 35 g/h, in the 60 to 75% zone it rises to 20 to 100 g/h, and above 75% effort it can reach 30 to 150 g/h.
That's a wide range, which is exactly why the number on a pack of gels isn't the number for you. Someone training in zone 2 for a long steady ride needs far less than someone doing hill reps at threshold. Start at the low end of the bracket that matches your effort and adjust session by session based on what your glucose actually does, not what the table predicts.
There's also a physical ceiling. When larger amounts of carbohydrate are needed to fuel prolonged endurance exercise, the maximum rate your gut can absorb glucose, around 1.2 to 1.7 grams per minute, becomes the limiting factor. Past that point, more carbs in your mouth doesn't mean more carbs available to working muscle. It just means a sore stomach.
What should my glucose be before I start training?
The general target is to begin exercise with a stable glucose between 7 and 14 mmol/L (126 to 252 mg/dL), with low ketone levels. If you're below that range, the advice is practical: ingest 10 to 20 grams of glucose if you're under 5.0 mmol/L (90 mg/dL), or 10 grams of carbohydrate if you're sitting between 5.0 and 6.9 mmol/L (90 to 124 mg/dL).
In real training life this looks like checking your CGM in the car park, not mid-warm-up. Check blood glucose 15 to 30 minutes before exercise, and again every 30 minutes to an hour during the session. Trend arrows matter as much as the number. A 7.0 mmol/L (126 mg/dL) that's falling fast behaves very differently to a 7.0 mmol/L that's flat or rising.
Do I need to eat the same for running, cycling and swimming?
No. Impact sport, load-bearing sport and non-weight-bearing sport all pull glucose differently. Running tends to drop glucose faster than cycling at a similar perceived effort, partly because of the extra muscle recruitment. Swimming and open water sessions are notoriously unpredictable because you can't check your sensor mid-lap, so many swimmers start slightly higher than the usual pre-exercise range and carry a tested gel at the wall.
Strength and interval sessions behave differently again. Short, hard efforts can push glucose up temporarily through adrenaline before it falls later, so a carb plan built for a steady long run will overfuel a track session and underfuel the recovery window after it. The lesson from working athletes is to build a separate mental model, and ideally a written one, for each sport you do regularly.
This is where
Use our free Insulin-to-Carb Ratio calculator to get a starting point. Takes 2 minutes.
Try Free ICR CalculatorAlways verify with your care team.Does automated insulin delivery change my carb plan?
Partly, but it doesn't remove the need to count. Regular physical activity is a cornerstone of care for people with type 1 diabetes, and the availability of automated insulin delivery systems has improved the ability to stay in target range around activity. Most pumps now have an exercise-specific setting, whether that's a temp target, an activity feature or a separate profile, and position statements from EASD and ISPAD now give detailed clinical practice points for managing physical activity with this technology.
A 2025 randomised trial on the Omnipod 5 system is a useful reality check on what these features actually do. Turning on the activity feature 30 or 60 minutes before exercise reduced insulin delivery, limited the glucose decline, and lowered hypoglycaemia risk compared with no activity feature at all. That's a genuine win. But it's not a free pass: even with the activity feature on, hypoglycaemia still occurred in roughly a quarter of participants, which suggests it should be set earlier before exercise or used as just one part of a broader exercise strategy rather than the whole plan. In plain terms, let your pump help, but still carry carbs and still check your Suggested IOB before you head out the door.
What about carbs after the session is done?
The work isn't over when you stop moving. Muscle and liver glycogen take time to refill, and getting this wrong is a common reason riders and runners wake up low overnight. Recovery depends on replenishing glycogen stores with carbohydrate soon after exercise, and current guidance suggests consuming around 1 to 1.2 grams per kilogram of bodyweight per hour in the first four hours afterwards. That's a nutrition target, not an insulin instruction, so talk to your care team about how any post-session correction or meal bolus should be shaped around it, especially if you're prone to delayed hypoglycaemia overnight.
How do I stop over or underestimating carbs mid-session?
Precision drops the moment you're tired, moving and trying to read a nutrition label through sweat in your eyes. This is the point where a lot of athletes either overcorrect with sugar out of fear, or underfuel because counting feels like too much effort when heart rate is already at 160. In practice, most people land closer to accurate when they build habits in training, not during a race.
Photographing your pre-session meal or your gels and bars while you're still at the kitchen table, rather than trying to do carb maths from memory on the trail, is one of the simpler fixes. This is one of the places T1Fit's photo-to-carb estimate genuinely helps, giving you a fast carb figure from a photo so you can log it and move on, rather than guessing under pressure. Whatever tool you use, the goal is the same: fewer decisions made on the fly, more decisions made in advance and rehearsed.
Endurance sport with type 1 diabetes rewards preparation more than perfection. Know your rough grams-per-hour for your actual sessions, start in range, let your pump's exercise mode do what it's built for, and treat recovery carbs as seriously as the ones you eat mid-ride. The rest is pattern recognition, built one long session at a time.
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