Nutrition is the fourth discipline of long-course triathlon. You can execute perfect pacing on the bike, hold your run form through kilometre 30, and still DNF because your gut staged a revolt at kilometre 80. GI distress, nausea, bloating, cramping, and the desperate search for a port-a-loo, is the most common performance-limiter in Ironman racing that athletes do nothing systematic about. They accept it as an immutable fact of racing rather than a trainable adaptation.
It is not immutable. The gut is a trainable organ. The science on this has matured considerably in the last decade, and what it tells us is that your ability to absorb, tolerate, and oxidise 90 grams of carbohydrate per hour during hard exercise is a function of training, not luck, genetics, or which brand of gel you choose on race morning.
This article covers the physiology behind gut training, the practical protocol for building to 90g/hour, the carbohydrate composition that makes high-rate fuelling feasible, and the common mistakes that keep athletes stuck at 60g and suffering.
Why GI Problems Happen in the First Place
To understand gut training, you need to understand why the gut fails under exercise stress.
During sustained aerobic effort, blood is preferentially directed toward working muscle and away from the splanchnic circulation, the vascular bed that supplies the gut. At intensities above roughly 70% of VO2 max, splanchnic blood flow can drop by 50–80%. This ischaemic environment compromises intestinal integrity, slowing motility and reducing absorptive capacity. Add mechanical stress from running (the colon takes a literal pounding at each footstrike), dehydration-induced reductions in gut blood flow, and the psychological load of racing, and you have a system under significant strain.
On top of this, when you push carbohydrate intake above what the small intestine can absorb, osmotically active carbohydrates draw water into the gut lumen. The result is bloating, cramping, and urgency. This is not a sign that you ate the wrong food, it is a sign that your absorptive capacity has been exceeded.
The key absorptive bottleneck is transporter availability. Glucose is absorbed via the SGLT1 transporter, which saturates at approximately 60g per hour. Fructose uses a separate transporter - GLUT5 - which runs in parallel. Combining glucose and fructose sources allows you to bypass the SGLT1 ceiling and push total carbohydrate absorption to 90g/hour or higher, provided the ratio is right and your gut is conditioned to handle the load.
The Science of Gut Adaptation
The transporter saturation model is well established, but the adaptation story is less commonly appreciated.
Research from the Netherlands, notably work from Asker Jeukendrup's group and subsequent studies, demonstrated that habitually consuming high carbohydrate intakes during training upregulates SGLT1 and GLUT5 expression in the intestinal epithelium. In simple terms: the more you train your gut to process carbohydrate at volume, the more transporter capacity you develop. This is a genuine physiological adaptation, not a placebo effect.
Studies using stable isotope tracers have confirmed that athletes who regularly train with high carbohydrate intakes show meaningfully greater exogenous carbohydrate oxidation rates than untrained controls given the same intake. The muscle is burning more of what the gut is absorbing because the absorptive step is no longer the rate-limiting constraint.
There is also an adaptation in gut permeability and tolerance. Repeated exposure to high carbohydrate loads during exercise reduces the degree of GI symptoms reported, even when the underlying physiology (exercise intensity, heat, duration) remains constant. The gut learns, in other words.
The practical implication: athletes who only practise high-rate fuelling in races are asking their gut to perform a task it has never trained for, under the worst possible conditions, peak fatigue, heat, race-day cortisol. This is a setup for failure.
The 2:1 Glucose-to-Fructose Ratio
Before discussing protocol, it is worth being precise about carbohydrate composition, because this is where a lot of athletes get it wrong.
To reach 90g/hour without overwhelming the SGLT1 transporter, you need a glucose-to-fructose ratio of approximately 2:1. This means for every 90g of carbohydrate consumed:
- ~60g should come from glucose sources: maltodextrin, glucose, rice, banana
- ~30g should come from fructose sources: sucrose (50% fructose), fruit, fructose-containing gels
Sucrose (standard table sugar) is itself a 1:1 glucose-fructose disaccharide, making it a convenient combined source. Many purpose-built sports nutrition products, gels, chews, and drink mixes designed for high-rate fuelling, are formulated around the 2:1 ratio specifically. If you are building your own fuelling strategy from whole foods, this matters: plain maltodextrin or dextrose alone will cap you at ~60g/hour regardless of quantity consumed.
Higher fructose ratios (1:0.8 or above) have been studied and may offer modest oxidation advantages in some athletes, but they also carry a higher GI risk in individuals sensitive to fructose malabsorption. The 2:1 ratio is the established, well-tolerated starting point for most athletes.
Building to 90g/Hour: The Practical Protocol
Gut training requires the same progressive overload logic you apply to physical training. You do not jump from 40g/hour to 90g/hour in a week. You build systematically over eight to twelve weeks, using long training sessions as the vehicle.
Phase 1 — Establish your baseline (weeks 1–2)
Start by quantifying what you are currently consuming during long sessions. Most age-group athletes who have never deliberately trained their gut are taking in 40–60g/hour, often inconsistently. Calculate this precisely using the label data on your current products. Also note any symptoms you experience: timing, type, severity.
Phase 2 — Increment up (weeks 3–8)
Add approximately 10–15g of carbohydrate per hour every 1–2 weeks, using a 2:1 glucose-fructose product or combination. Do this in long aerobic sessions, your Sunday long ride and your long run are the primary vehicles. The session needs to be at least 90 minutes; shorter sessions do not adequately stress the absorptive system.
Consume carbohydrate every 20–30 minutes rather than in large boluses. Spreading intake maintains a consistent absorptive load and avoids osmotic spikes. A 90g/hour target over a three-hour ride means roughly 30g every 20 minutes, or 45g every 30 minutes, plan your packaging accordingly.
Phase 3 — Stress-test the gut (weeks 9–12)
Once you are regularly hitting 75–90g/hour in aerobic sessions without symptoms, begin practising at race-relevant intensities. The gut is far more tolerant at Zone 2 than at tempo or threshold. If you can only manage 90g/hour when cruising, you have not yet trained the system for race demands.
Introduce brick sessions specifically for gut stress-testing: consume 90g/hour on the bike, then immediately test your tolerance on the first 20–30 minutes of the run. The bike-to-run transition is the highest-risk window for GI distress. Practise the handoff between liquid calories on the bike and more concentrated sources on the run.
Race rehearsal sessions
At least three times before your target race, complete a full nutrition rehearsal using race-day products at race-day volumes and intensities. This includes:
- Exact gels, chews, or drink mix you will use on race day
- Aid station simulation (consuming while moving, not slowing)
- Target weather conditions if possible
- Running volume post-bike sufficient to expose late-run gut stress
What to Eat: Practical Food Choices
The 90g/hour target can be achieved through commercial sports nutrition, whole food combinations, or a hybrid approach. What matters is hitting the ratio and the rate consistently.
Commercial sports nutrition
Purpose-formulated products from brands like SiS Beta Fuel, Maurten (Drink Mix 320), and similar 2:1 or high-carbohydrate products are designed specifically for this purpose. They have controlled ratios, known osmolality, and predictable GI profiles. The cost is high and palatability can degrade during a 10–12 hour Ironman, but the precision is useful in training phases.
Whole food and hybrid approaches
Many experienced long-course athletes use a hybrid model: liquid carbohydrates (drink mix in bottles) on the bike for volume and ease, with gels and more solid foods providing variety and palatability as the race extends. Bananas, dates, rice cakes, and boiled potato (a staple in IRONMAN aid stations) all contribute glucose. Paired with sucrose-containing gels or drinks, these can approximate the 2:1 ratio.
On the run, the stomach becomes progressively less tolerant of fats and proteins as blood flow to the gut is further compromised by running mechanics. Shift toward simpler, more liquid carbohydrate sources, flat cola (glucose plus caffeine), gels washed down with water, and isotonic drink mixes, as the run extends.
Hydration and Sodium: The Gut Training Multiplier
No discussion of GI tolerance is complete without addressing hydration and sodium. The two systems are deeply linked.
Dehydration accelerates the gut blood flow reduction that compromises absorptive capacity. At 2–3% body mass loss, gut tolerance degrades meaningfully even at moderate carbohydrate intakes. Staying ahead of sweat losses, not just reacting to thirst late in the event, maintains the gut environment that makes high-rate fuelling possible.
Sodium is critical for two reasons. First, SGLT1 is a sodium-coupled transporter, sodium is required for glucose absorption at the intestinal wall. Second, sodium-containing solutions maintain plasma osmolality and reduce the osmotic gradient that draws water into the gut when carbohydrate concentrations are high. Isotonic or slightly hypotonic carbohydrate-sodium solutions absorb faster and cause fewer osmotic symptoms than concentrated, sodium-free solutions.
Target sodium intakes of 800–1,200mg/hour are appropriate for most long-course athletes in moderate to hot conditions. This is well above what most standard gels or drink mixes provide at standard dilution, you will likely need dedicated sodium supplementation (capsules, electrolyte tabs, or high-sodium drink mixes) to reach this range.
Gut training sessions should also be performed in conditions that reflect race heat exposure. A gut that tolerates 90g/hour in 15°C training weather may not tolerate the same load in 28°C race conditions without deliberate heat-adaptation sessions.
Common Mistakes That Undermine Gut Training
Practising fuelling only on easy days. If your 90g/hour sessions are always at 65% of FTP, you have not trained the gut for race intensity. Gut tolerance degrades non-linearly with intensity. Some sessions need to replicate race-relevant power outputs.
Changing products close to race day. Race day is not the time to test a new gel, a new drink mix, or a new brand. Everything should be trialled in training. If your target race uses a specific nutrition sponsor on the course, train with those products.
Neglecting the run leg. Most gut training occurs on the bike because that is where the bulk of calories are consumed in an Ironman. But the run leg is where the consequences show up. Always include post-bike run gut training in your protocol.
Inconsistent feeding schedules. Taking 90g in one 30-minute window and nothing for the next 40 minutes is not the same as 90g/hour spread evenly. Set a timer. Eat to a schedule, not to hunger.
Under-hydrating during gut training sessions. The gut training protocol should include deliberate hydration practice. Drinking too little during training masks gut issues that will appear in race conditions.
Putting It Together: The Race-Day Execution
By race day, your gut should have logged dozens of hours at 75–90g/hour. The protocol should feel automatic. A few execution principles:
Start fuelling within the first 20–30 minutes of the bike. Athletes who wait until they feel hungry are already behind. Hunger is a poor proxy for carbohydrate depletion and an even poorer proxy for absorptive readiness.
Reduce concentration, not volume, in the heat. If conditions are hotter than expected, dilute your drink mix slightly and compensate with an extra gel rather than reducing total carbohydrate intake. Concentrated solutions are more likely to cause osmotic distress in heat.
On the run, transition smoothly from bike nutrition. You will likely shift toward gels and cola and away from bottles. Maintain the 20–30 minute feeding interval. Do not abandon fuelling because of mild nausea, a brief feeding gap often makes nausea worse, not better, as blood glucose drops.
Caffeine deserves a mention: 100–200mg in the latter half of the bike and into the run can enhance carbohydrate oxidation, maintain gut motility, and buffer perceived effort. Most athletes find cola a convenient delivery vehicle on the run.
The gut that absorbs 90g/hour without complaint on race day is not naturally gifted. It is trained. Build it the same way you build your swim, bike, and run, systematically, progressively, and with race specificity. The reward is a fourth discipline that works for you rather than against you.
Common Questions
How long does it take to train your gut for 90g of carbs per hour?
Most athletes require 8–12 weeks of consistent gut training to reach 90g/hour without GI symptoms. Progress in 10–15g/hour increments every 1–2 weeks during long aerobic sessions of at least 90 minutes. Rushing the adaptation — jumping more than 15g/hour in a single block — significantly increases the risk of GI distress and sets the process back.
Why do I need both glucose and fructose to hit 90g carbs per hour?
Glucose is absorbed via the SGLT1 intestinal transporter, which saturates at approximately 60g per hour. Fructose uses a separate transporter — GLUT5 — that operates in parallel. Combining both at a 2:1 glucose-to-fructose ratio allows you to bypass the SGLT1 ceiling and push total carbohydrate absorption to 90g/hour or higher. Consuming only glucose or maltodextrin sources, regardless of quantity, will cap absorption at around 60g/hour and increase osmotic GI symptoms above that threshold.
What is the best carbohydrate ratio for Ironman fuelling?
A 2:1 glucose-to-fructose ratio is the established standard for high-rate fuelling in long-course triathlon. This means roughly 60g from glucose sources (maltodextrin, glucose, banana, rice) and 30g from fructose sources (sucrose, fruit, fructose-containing gels) per hour. Some research supports slightly higher fructose ratios for trained athletes, but 2:1 remains the most widely tolerated and best-validated starting point.
Can I use real food instead of gels for Ironman nutrition?
Yes, and many experienced long-course athletes use a hybrid approach — liquid carbohydrates (drink mix) on the bike for volume and ease, supplemented with whole foods like bananas, dates, rice cakes, and boiled potato. What matters is hitting the 2:1 glucose-to-fructose ratio and the target intake rate consistently. On the run, shift toward simpler, more liquid carbohydrate sources as the gut becomes less tolerant of fats and solids under sustained running load.
Why does GI distress get worse on the run in an Ironman?
Multiple factors compound on the Ironman run. Splanchnic blood flow is further reduced at running intensities compared to cycling. Mechanical impact from footstrike physically stresses the colon. Dehydration accumulated over the bike leg degrades gut absorptive capacity. And cumulative fatigue reduces the body's ability to manage osmotic stress from carbohydrate intake. This is why brick sessions with deliberate gut stress-testing are critical — the run leg is where GI issues manifest, not just where they originate.
How much sodium should I take per hour in an Ironman?
Most long-course athletes in moderate to hot conditions should target 800–1,200mg of sodium per hour. Sodium is required for glucose absorption via the SGLT1 transporter and helps maintain the plasma osmolality that keeps fluid in circulation rather than drawing water into the gut. Standard gels and drink mixes at typical dilution rarely reach this range, so dedicated sodium supplementation — capsules or electrolyte tabs — is usually necessary to hit the upper end of this target.
Should I practise gut training in hot conditions?
Yes. Heat independently reduces splanchnic blood flow and accelerates dehydration, both of which compromise gut absorptive capacity. A gut that tolerates 90g/hour in cool training conditions may fail at the same intake in race-day heat. If your target race is in a warm environment, include deliberate heat-exposure gut training sessions — particularly in the final 4–6 weeks of your build — to stress-test your fuelling strategy under realistic thermal conditions.