Running long with Type 1: fuelling marathons and endurance training without the crash
Training for a marathon / long endurance runs with Type 1: how do people fuel and avoid crashing?
Long runs and marathons with Type 1 are absolutely possible, and the whole game is fuelling steadily while stopping your glucose crashing, both on the run and for hours afterwards. Endurance running burns through glucose, so most people take carbohydrate on the move and lower their insulin around the session, though how much insulin changes is always set with your diabetes team. The start line can throw a curveball, because nerves and adrenaline sometimes push you up before the run pulls you down. And the low you most need to plan for often lands late, that evening or overnight, as your muscles refill their fuel stores.
- Steady endurance running is aerobic and tends to lower glucose, so hypos during and after a long run are the main risk to plan around.
- People fuel on the move with fast carbs (gels, sports drinks, jelly babies), often taking something every 20 to 45 minutes on a long run.
- Many runners reduce their insulin around long sessions, but the amount is worked out with your diabetes team, never copied from someone else.
- Adrenaline at the start line can push glucose up first, so a pre-race high does not always mean you are safe from a later drop.
- The delayed low can come many hours later, including overnight, as muscles restock their glycogen (their stored glucose).
Why long running pulls your glucose down
When you run, your muscles pull glucose out of your blood to burn for fuel, and they keep doing it as long as you are moving. On top of that, exercise makes your body more sensitive to insulin, so whatever insulin is on board works harder than usual. Put those together and a long, steady run is a powerful glucose-lowering machine. That is great for general health, but it means a dose that is fine on a rest day can be far too much on a long-run day, which is why runners plan ahead rather than just set off. The longer and steadier the effort, the bigger the pull, and the more it keeps working after you stop. Sprints and hard intervals can behave differently, sometimes lifting glucose briefly, but the marathon-style steady miles are the ones that reliably bring you down.
Fuelling through the run without the crash
The core habit of endurance running with Type 1 is taking regular carbohydrate before you drop, not after. Most runners use fast, easy carbs they can stomach mid-stride: gels, sports drinks, jelly babies, dried fruit or flat cola. A common rhythm is something every 20 to 45 minutes on a long run, starting early rather than waiting to feel low, because once you are dropping fast the fix is always behind you. How much you personally need varies a lot, so the useful move is to learn your own pattern on training runs, not to save the experiment for race day. Carry more fuel than you think you need, split across pockets or a belt so a dropped gel is not a disaster, and keep some fast sugar that is purely for treating a hypo, separate from your planned fuel.
Adjusting insulin, and the start-line surprise
Reducing insulin around a long run is normal and often necessary, but it is firmly team territory: the mealtime dose beforehand, and any background or basal reduction, are worked out with your diabetes team, never read off a formula. If you use a pump, ask about a temporary basal reduction or an activity mode for long sessions, and when to start it. The start line adds a twist worth knowing: race-day nerves and adrenaline can push your glucose up before you have run a step, so a high reading at the start does not always call for a correction, and over-correcting there can set up a nasty low a few miles in. Many runners like to begin a long effort on the higher side of their usual range rather than already low or dropping, and to watch the trend arrow, not just the single number.
The delayed low, that evening and overnight
The run is not over when you stop. For hours afterwards your muscles pull glucose from your blood to rebuild their glycogen, their fuel store, and your insulin sensitivity stays raised, so a low can land that afternoon, that evening, or in the small hours while you sleep. After a marathon or a long training run, that overnight drop is the thing to respect most. People manage it with a check before bed, sometimes a snack, CGM alarms turned up loud enough to wake them, and settings adjusted with their team for the night after a big effort. Fast sugar by the bed is not optional on those nights. A low at 2am after a hard session is common enough that it is worth planning for every time, rather than treating as a surprise.
What changes how a run hits you
No two long runs land the same way, and race day is rarely a copy of training. Heat tends to lower you faster, insulin still working from a recent meal deepens the drop, and an evening run carries more of its effect into the night. Fitness changes it too: as your body adapts, the same run can move you differently. This is a pattern you build with your team from your own runs, not a fixed rule.
How long runs tend to go
Examples, not instructions or doses.
You start the race at 11 mmol/L on pure nerves and feel fine, so you carry on. By mile six you are dropping hard as the running takes over and the adrenaline fades. Starting a run high from nerves does not mean you are safe from a low.
You wait until you feel low to take a gel, and by then you are already dropping fast and it cannot catch up. Taking carbs early and on a regular clock, before the drop, is what keeps a long run steady.
You nailed the run and went to bed at a good number. At 2am your CGM alarms as your muscles refill their stores overnight. A snack before bed, a loud alarm and sugar within reach turned it from a scare into a quick fix.
Worth paying attention to
Questions that make an appointment useful
When it's urgent
Most exercise lows are routine to treat, but two things are emergencies. If a low ever becomes a severe hypo, where you are confused, cannot swallow safely, are fitting or pass out, you need someone else to help: they should call 999, put you on your side, not force anything into your mouth, and use a glucagon kit if one is to hand and they know how. This is why running with ID and telling someone your route matters. Much more rarely, if you feel unwell with high glucose, are being sick, have tummy pain, or are breathing hard and fast, that can be DKA (a dangerous shortage of insulin): check ketones if you can, contact your diabetes team or NHS 111, and call 999 if you are very unwell. Never let a test delay the call.