Training Your Gut to Tolerate Fuel

Does practice work?

Enough to plan around. In a 2023 review of 8 trials of 4 to 28 days of repeated feeding during running or cycling, gut discomfort fell by 47% on average in the two trials that used two weeks of carbohydrate feeding². A 2025 position statement gives repeated feeding its best evidence grade, with the benefit mainly for carbohydrate the gut fails to absorb and the symptoms that come with it, and calls the field understudied³. A 2025 review of 29 trials calls gut training protocols promising and adds that symptoms have many causes, so each runner has to work it out individually⁴.

You are training three things: how much volume the stomach holds without discomfort, how fast it empties, and how well the small intestine absorbs carbohydrate, because carbohydrate left unabsorbed draws in water, ferments and slows the whole gut. Stomach and absorption are the two most trainable.

When to start

Give yourself at least 12 weeks before a race you want to fuel hard in, so there is time to lay out the plan, climb the ladder below and rehearse it. If you have frequent stomach trouble, give it three to four months and keep working it on long runs.

How long the climb takes depends on where you are going. To fuel a road marathon at 60 to 90 grams an hour, rehearse near that rate for four to six weeks. To go from a normal 30 to 40 grams an hour up to 90 to 120, expect about six to ten weeks if you are well trained, and two to three months or more if you are not.

The order of the work

Build volume first and the carbohydrate rate second. Fluid and the amount in the stomach are the base, and the rate sits on top of it.

1. Volume, for two to three weeks. On easy runs (conversational pace) and moderate runs, take more fluid at once than usual and eat higher-volume food, even fruit. Bloating from drinking can be trained away in about this long.
2. Carbohydrate rate. Start at about 60 grams an hour for a couple of weeks if that is comfortable, then go to 80 to 90, with enough fluid to move the food along. Use the products and the clock you plan to race with.
3. Harder sessions last, because the gut copes least with fuel at high effort. The long run takes the top of your reasonable range, quality sessions take a rate inside your range, and easy runs need little.

Match the work to your symptoms

Upper-gut trouble (belching, regurgitation, nausea, vomiting) is a volume problem. Add carbohydrate and fluid volume across repeated long runs. Lower-gut trouble (bloating, gas, the urgent search for a toilet) is an absorption problem and needs a stronger stimulus: easy runs on consecutive days for around ten days, taking close to your race-rate carbohydrate each time. In the first trials, runners practiced five days a week for two weeks. Because the runs are easy, this fits the last weeks before a race, and the carbohydrate helps load the muscle too.

If you have no stomach trouble, you do not need a block. Fuel your long runs the way you will race and add rate in steps. The runner who gains most is the one who has struggled, or who is aiming at 90 to 120 grams an hour.

Keep a log. For every long run, write down the grams an hour, the form, how your stomach felt and how you felt the next day. Hold the last rate that felt comfortable when symptoms show up, and fit the work around your training so it does not cost the runs it is there to support.

It fades in weeks, so time it

Two to three weeks without practice and the top-end rates, 90 to 120 grams an hour, get hard again, while 60 to 80 stay comfortable. After that gap, four to six sessions bring you back to 120. Do the hard work close to the race, not months before it.

You do not need to fuel at the maximum on every session all year. Save the high rates for the long runs and hard sessions that need them, and once your gut is training well, ease off.

Your daily eating counts

What you eat all week shapes what your gut absorbs. Months on a fat-heavy diet lower carbohydrate absorption, and a runner who never eats fructose will struggle to absorb it in a gel. A low-carbohydrate, high-fat diet does not prevent or treat these symptoms³. If you still have trouble after practice, a low-FODMAP diet for the day or two before a race reduced symptoms in trials, but it is restrictive, one trial found more intestinal injury with it before running in heat, and it should come with carbohydrate loading and fueling during the run³⁴.

What has not helped

Do not spend money on these. There is no evidence that a gluten-free diet helps runners without celiac disease. Probiotics gave mixed results: in a systematic review, four of five studies found no effect on symptoms. Neither has earned a place in your plan³⁴.

For how much to take and in which form, see the guide on carbohydrate during exercise. For the race-day plan, see race-day fueling, and for drinking, see hydration and electrolytes.