Training Your Gut For Race-Day Carb Intake

Let's face it — most marathon runners never train their gut at all. They decide what to eat on race morning and hope for the best. I've run every BRT marathon squad through this 8-week build-up, and the difference it makes is significant. You've had the mid-long-run stomach cramp, the sudden need for a portaloo at mile 14, or the gel that sat like a stone. None of that is bad luck — it's an untrained gut, and like any other system in your body, it adapts with repeated, deliberate exposure.

My advice to you is straightforward: start at 30g of carbs per hour in week one if you're currently fuelling ad hoc or not at all. Add roughly 5–15g/hr each week, tapering the increments as you near your ceiling, and build toward 60–90g/hr on mixed glucose-fructose sources by week 8. Progression only happens on marathon-pace or long-run efforts — never easy days, where your gut isn't under the same digestive load it'll face in the race. Below you'll find the week-by-week table, what to do when progress stalls or symptoms return, and what to avoid while you train.

The Week-by-Week Build

Your gut has two carbohydrate transporters — one handles glucose, the other fructose — and both adapt to repeated exposure the same way a muscle adapts to load. Weeks 1–3 use single-source carbs (glucose only) to establish a baseline. From week 4 you introduce glucose-fructose blends in roughly a 2:1 ratio. That blend is what allows you to push past the 60g/hr ceiling a single transporter can handle alone.

Here's my rule with every athlete I work with: if you experience GI symptoms in the week you increase the dose, stay at that week's number for an extra week rather than pushing forward on schedule. Adaptation needs repeated exposure at a tolerated dose. Forcing the escalation teaches your gut to associate carbs with distress — and that sets you back further than the extra week ever will.

When Progression Stalls or Symptoms Return

Three specific situations derail the standard protocol, and each has a different fix. If symptoms return after illness or a training break, drop back two rungs on the table and rebuild over two weeks rather than resuming where you left off — your gut de-trains faster than your legs do. If symptoms only show up in the heat, that's a hydration and dilution problem, not a carb-tolerance problem — reduce your gel concentration or space your fuel with more water rather than cutting carbs altogether. And if you plateau at a fixed g/hr despite consistent practise, switch your carb source — gel to chews, or chews to real food. Texture and osmolality, not total carb load, is often the actual limiter.

I worked with a sub-3:30 marathoner who was stuck at 45g/hr on gels for three straight weeks. We switched him to a chews-and-real-food mix in weeks 6 and 7. He reached 75g/hr comfortably by race day. Same gut, different product — that's usually all it takes when a plateau won't budge.

What to Avoid While Gut Training

I'd like you to keep this list short and non-negotiable. Avoid high-fibre and high-fat foods within three hours of a practice run — both slow gastric emptying and work against everything you're trying to train. Never test a new product for the first time within two weeks of race day. And dial in the caffeine content of your gels during training, not on race morning — caffeine sensitivity varies enough between runners that race day is the wrong place to find out where you sit.

This protocol reflects the current sports-science consensus on multiple-transportable-carbohydrate absorption, and it's what I run every BRT marathon squad through before race week. If under-fuelling generally — not just gut tolerance — is your real problem, start with why you keep running out of energy in training instead. If gels themselves are the issue rather than the dose, try the real-food and low-FODMAP alternatives before assuming you need more gut training. And if you want to see what happens when none of this gets rehearsed, read what happens when race-day fuelling goes wrong.

Head back to the full marathon fuelling strategy hub for the rest of your race-day nutrition plan.

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