Gut training
Evidence: moderate
Repeatedly feeding carbohydrate during training adapts the gut: it absorbs more, empties faster and protests less. A controlled trial found about two weeks of this roughly halved malabsorption and gastrointestinal symptoms. The principle is well supported mechanistically and in early trials, but the number of human studies is still small.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
The limit on race fuelling is rarely the muscle. It is the gut. Many runners cannot take in the carbohydrate they need because high intake during running causes nausea, bloating and worse. Gut training is the practice of deliberately rehearsing high-carbohydrate feeding in training so the digestive system tolerates and absorbs more on race day. It is a trainable performance factor, not a fixed constraint.
The gut adapts
Carbohydrate absorption across the intestinal wall is rate-limited by a transporter called SGLT1. A diet high in carbohydrate increases both the number and the activity of these transporters, raising the ceiling on how fast carbohydrate can be absorbed (Jeukendrup 2017). Repeated exposure also speeds gastric emptying and improves stomach comfort, so the same carbohydrate load sits more easily (Jeukendrup 2017). These adaptations begin within days and are substantial within about two weeks (Jeukendrup 2017). The gut, like muscle, responds to a training stimulus.
The best controlled test fed runners a high carbohydrate intake repeatedly over about two weeks. Malabsorption fell by roughly half, gastrointestinal symptoms roughly halved, and running capacity improved (Miall et al. 2018). The sample was small, which is the main limit on the evidence, but the direction and size of the effect match the mechanism. This matters more for runners than for cyclists, because the up-and-down jostling of running provokes more gut distress, so rehearsing race fuelling is part of a runner’s training rather than an optional extra.
Why it lets you reach 90 g per hour
A single carbohydrate such as glucose or maltodextrin saturates SGLT1 at about 60 g per hour (Jeukendrup 2017). Adding fructose, absorbed by a separate transporter, lifts the usable rate above 90 g per hour at a glucose-to-fructose ratio near 2:1 (Jeukendrup 2014). Gut training and multiple transportable carbohydrates work together: the transporter mix raises the theoretical ceiling, and gut training raises your personal tolerance toward it. Intakes around 90 g per hour are the practical target for long events, with little evidence that going higher adds to exogenous oxidation (King et al. 2018), though very high intakes near 120 g per hour can be tolerated by trained guts and may lessen muscle damage in mountain ultras (Viribay et al. 2020). Only runners whose guts can handle the load can use any of it, which is the point of training them. See in-race carbohydrate for the dosing by duration and glycogen for why the fuel matters.
A practical protocol
Building toward 90 g per hour
- Start where you are comfortable. Begin at the intake you already tolerate, often 30 to 40 g per hour, taken during longer runs.
- Rehearse with race products. Train the gut with the exact gels, drinks or mixes you will race on, at race-like intensity and timing. The adaptation is specific to the practice.
- Use glucose plus fructose. A roughly 2:1 glucose-to-fructose mix lets total intake exceed 90 g per hour without overloading a single transporter (Jeukendrup 2014).
- Progress gradually. Add about 10 to 15 g per hour every week or two as tolerance improves, building over several weeks toward the target for your event.
- Concentrate it on long runs. The long run and marathon-pace sessions are the natural place to practise high intake, because they rehearse race conditions and durability at the same time.
The honest framing. The mechanism is solid and the early trials are encouraging, but the human evidence base is still thin, so treat the roughly halved-distress figure as a promising single-trial result rather than a settled number. The cost of trying is low and the upside for long events is real, which makes gut training a sensible default for any runner racing beyond about two and a half hours.