How do you train your gut for race-day nutrition?

Why does the gut even need training?

During exercise, your blood gets diverted away from the digestive system and toward the working muscles and skin. Splanchnic blood flow can drop by 60 to 80 percent at high intensities, which means the gut is working on a fraction of its normal oxygen and nutrient supply. On top of that, the mechanical jostling of running shakes intestinal contents, and the sympathetic nervous system suppresses normal digestive function. Put these together and the intestine becomes a much less reliable organ than it is at rest.

In this compromised state, the intestine still has to absorb the carbohydrates you take in during the session. Absorption happens through specific transporter proteins on the gut wall — SGLT1 for glucose-type sugars and GLUT5 for fructose — and those transporters have a finite capacity. When you exceed their capacity, undigested sugars sit in the gut lumen, draw water in by osmosis, ferment, produce gas, and eventually trigger some combination of cramping, bloating, reflux, nausea, and diarrhea. This is the classic GI distress pattern that wrecks athletes in the second half of long races.

The critical insight from Jeukendrup's work is that this transporter capacity is trainable. Athletes who chronically take in more carbs during training upregulate their intestinal transporters over weeks and become able to absorb significantly more before hitting the distress threshold. The gut is not a fixed organ — it adapts to its workload the same way muscles and mitochondria do, just through a different mechanism.

What does the research actually say about carb intake rates?

The progression of recommended carb intake for endurance racing has moved steadily upward over the last two decades, driven by the transporter research and the clinical experience of elite endurance coaches.

• 30 g/h was the old recommendation — it roughly matches the capacity of the SGLT1 transporter alone when saturated by glucose. This is what most people without gut training can tolerate without meaningful GI issues, and it's broadly safe for untrained guts.
• 60 g/h became the recommendation as soon as it was understood that SGLT1 alone could absorb up to about 60 g/h of pure glucose at peak capacity. This is still a reasonable target for most amateurs, especially in races under 2 hours.
• 90 g/h became possible when Jeukendrup's group showed that mixing glucose and fructose (typically in a 2:1 ratio) uses two separate transporters — SGLT1 for glucose and GLUT5 for fructose — which stack rather than compete. The dual-transporter mechanism allows absorption up to about 90 g/h, assuming both transporters are upregulated by training.
• 120 g/h is now the upper range used by elite endurance athletes and Tour de France cyclists, again using glucose-fructose mixtures and carefully gut-trained protocols. Products like Maurten 320, Precision Hydration PF 90, and SIS Beta Fuel are designed around this upper range.
• Beyond 120 g/h, the evidence is thinner and more individual. Some elite endurance athletes take 130 to 150 g/h successfully; others cannot tolerate more than 90 to 100 g/h no matter how much they train. Individual variance is real and it does not disappear with training alone.

The jump from 60 to 90 to 120 g/h is not a marketing story — it is a specific biochemistry story about the upregulation of two specific intestinal transporters, and the research has been replicated across multiple labs and populations. It is one of the cleanest stories in sports nutrition.

How does the glucose-fructose mix actually work?

Glucose is absorbed through the SGLT1 transporter, which is saturated at roughly 60 g/h of intake for most trained athletes. If you take more than 60 g/h of pure glucose, the excess sits in the gut, causes osmotic stress, and triggers GI distress. This is why pure maltodextrin or pure glucose products cap out around 60 g/h in practice.

Fructose uses a completely different transporter — GLUT5 — which sits on a different part of the intestinal cell and operates independently of SGLT1. GLUT5 is saturated at roughly 30 to 45 g/h of pure fructose. When you add fructose on top of a fully saturated SGLT1, the fructose transport happens in parallel and the total absorption rises — you can now absorb up to about 90 g/h (60 from glucose + 30 from fructose) through both transporters combined.

This is why every modern high-carb race nutrition product uses a glucose-fructose mix, typically at a 2:1 ratio (two parts glucose for every one part fructose). That ratio matches the relative capacities of SGLT1 and GLUT5 and minimizes the risk that either transporter is overwhelmed. The products have slightly different labels — some call it maltodextrin:fructose, some use glucose:fructose directly — but the underlying principle is the same across Maurten, SIS, Precision, Tailwind's higher-carb options, and GU Roctane.

The one exception worth knowing is Maurten's hydrogel technology. Maurten suspends the carb solution in a sodium alginate matrix that supposedly minimizes gut stress by encapsulating the carbs until they reach the small intestine. The research on hydrogels is mixed — some studies show less GI distress, some show no difference versus conventional drinks at the same carb content. The 2:1 glucose-fructose rule still applies to Maurten products regardless.

What is the actual gut training protocol?

Gut training is a progressive exposure protocol, conceptually similar to how you build mileage in a running plan. You start below your current tolerance threshold and progress the intake over weeks until you reach the target rate for race day.

• Week 1: Start at 60 g/h during long sessions. Use a mixed glucose-fructose product (any modern sports nutrition brand). Practise this during your weekly long run or long ride. Most trained athletes will tolerate 60 g/h without problems after a week or two of exposure.
• Week 2–3: Progress to 75 g/h during the long session. This is the transition zone where some athletes start to notice mild stomach fullness or bloating. If you feel discomfort, stay at 60 g/h for another week before pushing up.
• Week 4–5: Progress to 90 g/h during the long session. This is the rate at which the dual-transporter mechanism is fully engaged. You should be taking in the equivalent of about 1.5 gels every 20 minutes or one serving of a high-carb drink every 20 minutes.
• Week 6–8: Push to your target race rate if it's above 90 g/h. For most marathoners and 70.3 athletes, 90 g/h is the ceiling they actually need. For full Ironman athletes and ultra-distance racers, the target can be 100 to 120 g/h, and the final weeks of gut training are spent rehearsing that rate.
• Throughout: Practise with your exact race-day nutrition products, not substitutes. The brand, flavor, and concentration all matter — the gut responds to specific products, and switching products on race day can reset the adaptation.

The simplest version of the protocol is: every long session in the 6 to 8 weeks before a race includes the rate of carbs you intend to use on race day, using the product you intend to use on race day. If your long run is 2.5 hours and you plan 90 g/h on race day, you take 225 g of carbs during that run. Not a substitute amount, not a lower test amount — the actual plan.

What does gut training look like in practice for different races?

The target rate and total intake shift with the race distance and duration. The gut training protocol should be calibrated to the race you're actually preparing for.

• Marathon (2.5 to 5 hours): Target rate is usually 60 to 90 g/h. For a 4-hour marathoner, that's 240 to 360 g of carbs over the race. Gut training focuses on the long runs, practising the exact gel-to-water ratio you'll use on race day. A typical plan might be 4 to 5 gels across the race, taken every 5 to 6 km, with water at every aid station.
• 70.3 Ironman (4 to 6 hours): Target rate is 75 to 100 g/h. Most athletes fuel primarily on the bike (where GI tolerance is best) and lightly on the run. Gut training focuses on long rides with full race-day nutrition intake, plus practice of a higher-carb drink during brick workouts.
• Full Ironman (8 to 15 hours): Target rate is 80 to 120 g/h on the bike, lower on the run. Gut training here becomes a months-long project because the absolute volume of carbs is enormous (640 to 1,400 g total), and the gut needs to be conditioned not just to a rate but to sustained high intake over many hours.
• Ultra-distance (6 to 30+ hours): Target rate is usually 60 to 90 g/h with flexibility for real food at checkpoints. Gut training for ultras is less about the peak rate and more about tolerating a variety of foods, flavors, and textures at fatigue. The intestine gets more sensitive as the hours pass, and the gut training protocol should include late-stage tolerance testing.
• 5K to 10K: Gut training is essentially irrelevant — the race is too short to require meaningful in-race fueling. Focus on pre-race nutrition instead.

What causes GI distress even in gut-trained athletes?

Even with a careful gut training protocol, some athletes still struggle with GI symptoms on race day. The causes are usually knowable and mostly preventable.

• Too-concentrated drinks. A drink with a carb concentration above 8 to 10 percent empties from the stomach more slowly and draws water into the gut lumen, which triggers cramping and fullness. The fix is to match the recommended mix ratio on the product label and not 'concentrate it for efficiency'.
• Dehydration. A dehydrated gut is much more prone to distress than a well-hydrated one. If fluid intake falls behind carb intake, GI symptoms rise quickly. The fix is to hydrate to target and not skip aid stations.
• Pre-race food mistakes. A race-morning breakfast that's too high in fat or fibre, or that includes a food the athlete hasn't rehearsed, is a common cause of race-day GI distress that gets blamed on in-race fueling. The fix is to rehearse the exact breakfast during long runs and not introduce novelty on race morning.
• Running at a pace too close to threshold. Splanchnic blood flow drops more severely at higher intensities, and GI tolerance falls with it. Athletes who go out too fast often develop GI distress at mile 15 or 16 because the gut is oxygen-starved. The fix is to pace the race properly (see the pacing article).
• NSAIDs. Ibuprofen and related drugs taken before or during a race are a direct cause of gut damage and are associated with significantly higher rates of GI distress, GI bleeding, and hyponatremia. The fix is to not take NSAIDs around races, period.

Ricardo Costa's research on exercise-induced gastrointestinal syndrome shows that a meaningful fraction of GI distress in endurance events is driven by these preventable factors rather than by pure carb-intake intolerance. Fixing the obvious mistakes often eliminates what the athlete thought was a 'gut tolerance' problem.

How long does gut training take and how long do the adaptations last?

The practical timeline for meaningful gut adaptation is 6 to 8 weeks of consistent exposure, with the biggest gains usually showing up in weeks 3 to 5. Athletes who start cold and try to hit 90 g/h in the first week will usually develop symptoms; athletes who build up gradually across a month or two usually reach the target rate without distress.

The adaptations detrain if you stop practising. Transporter upregulation is not a permanent change — it requires continued use to maintain. An athlete who trains their gut for a marathon, finishes the race, and then stops taking carbs on long runs for three months will probably need to re-train the gut before their next race. This is why elite endurance athletes tend to fuel during all their long sessions year-round rather than cycling in and out of high-carb fueling.

The practical rule is that the gut should be exposed to race-day fueling rates at least once a week during the 8 weeks before a race, and at least every 2 weeks during base training to maintain the adaptation. You don't need to fuel every ride or every run — just the long ones, consistently.

What are the most common gut training mistakes?

Five mistakes catch most athletes who attempt gut training for the first time.

• Skipping the protocol entirely and trying to hit the race-day rate for the first time on race day. The single worst mistake. The predictable outcome is GI distress exactly when you can least afford it. If you have not rehearsed 90 g/h in training, do not attempt 90 g/h on race day.
• Using different products in training and racing. The gut adapts to specific products — flavor, texture, concentration, and transporter ratio all matter. If you train on SIS Beta Fuel and race on Maurten 320, the gut has not specifically adapted to the race-day product, and tolerance can drop significantly.
• Trying to reach 120 g/h when 90 g/h would do. Not every athlete needs 120 g/h, and pushing the upper limit when it's not necessary increases GI distress risk without a proportional performance benefit. For most marathoners and 70.3 athletes, 75 to 90 g/h is the right target.
• Taking carbs without adequate water. The gut tolerates concentrated carbs poorly without matching fluid intake. The rule of thumb is roughly 300 to 500 ml of water per 60 g of carbs, adjusted for conditions. Skipping the water is a fast route to cramping.
• Introducing novelty in the final two weeks before a race. The last two weeks should be gut-training consolidation, not experimentation. This is not the time to try a new brand, a new flavor, or a new concentration. Whatever you have been rehearsing all block is what you should race on.