Gut Training for Race Day: Why Runners Cramp
- Shrey Aggarwal
- Aug 4
- 7 min read
A runner I worked with last season threw up at 32km of her first marathon. She'd done eighteen weeks of training, hit every long run, and never once taken a gel while running. Race morning she took three in ninety minutes because the running group told her to. Her gut had no idea what to do with them.
That's the pattern behind most race-day stomach trouble I see. Not bad luck, not a weak stomach, not something the athlete was born with. An untrained organ asked to do a job it has never practised.
Quick Overview
What happens to your gut when you run?
Blood is finite. During hard running, your working muscles and your skin take priority, and far less of it reaches the intestine. Every stride jolts the gut on top of that. The combination damages the intestinal lining, so the barrier leaks, the stomach empties slower, and carbohydrate stops being absorbed properly (Costa et al., Aliment Pharmacol Ther, 2017).
Undigested carbohydrate sitting in the small intestine pulls water in behind it. That fluid shift is what produces the bloating, the sloshing, the cramp, and eventually the sprint to the side of the road. The carbohydrate never reached your muscles, so you also slowed down.
Heat multiplies all of it. The rise in core body temperature during exercise in the heat predicts the degree of intestinal damage and symptom severity reasonably well (Snipe et al., Temperature, 2023). This is the variable that breaks the most gut plans, whether that’s a June morning in Delhi, a July race in Singapore, or an unusually warm London Marathon. A fuelling plan that worked all winter can fall apart in summer at the same pace.
Why cramping is usually a fuelling problem
Abdominal cramping and skeletal muscle cramping are two different events and get confused constantly. Calf and hamstring cramps late in a race relate to neuromuscular fatigue and pacing. Stomach cramps relate to what’s in your stomach and how much blood is available to process it.
Roughly a quarter of recreational marathon runners report moderate or worse gut symptoms during the race (Pugh et al., Nutrients, 2018), and in 161km ultras almost everyone does (Stuempfle & Hoffman, J Sports Sci, 2015). This is normal, not a personal defect. It’s also largely preventable.
Three variables drive most cramping: the total grams per hour, the concentration of the drink or gel, and the type of carbohydrate. A gel taken with 200ml of water behaves very differently from the same gel taken dry. Concentrated solutions above roughly 8 percent sit in the stomach longer and drag fluid into the gut.
Dehydration makes symptoms worse, but drinking more water rarely fixes cramping on its own. Athletes who drink large volumes of plain water alongside gels often feel worse, because they’ve diluted sodium while still leaving carbohydrate unabsorbed.
The gut adapts, and there’s evidence
The intestine responds to a repeated stimulus the way muscle does. The transporters that carry glucose across the gut wall increase in number, gastric emptying speeds up, and symptoms fall.
Endurance runners who did two weeks of repeated carbohydrate feeding during running reduced gastrointestinal symptoms and malabsorption, improved blood glucose availability, and ran faster in a performance bout compared with placebo (Miall et al., Scand J Med Sci Sports, 2018). Two weeks. The runners weren’t doing anything exotic, just taking carbohydrate during training runs on a schedule instead of at random.
One caveat: I’ll state plainly, since most articles on this don’t. Nearly all the gut-training trials come from a single research group at Monash, and their own systematic review is the best summary of the evidence (Scrivin et al., Sports Med, 2023). The mechanism is sound and the effect sizes are consistent, but independent replication is thin. Treat the direction as reliable and the exact numbers as provisional.
The type of carbohydrate matters as much as the amount. A single source saturates its transporter at around 60g per hour. Adding fructose alongside glucose or maltodextrin opens a second transport pathway, raises how much you actually burn, and improves gut comfort at the same intake (O’Brien & Rowlands, Am J Physiol Gastrointest Liver Physiol, 2011). Ratios anywhere between 1:1 and 2:1 glucose to fructose work (Rowlands et al., Sports Med, 2015). The exact ratio matters far less than not relying on glucose alone, so read the ingredient panel rather than the front of the sachet.
Building an eight-week gut training for runners
Start eight to ten weeks out and write it into the plan the way you’d write intervals.
Begin at 30g of carbohydrate per hour on every long run, taken on a fixed twenty-minute schedule with 150 to 200ml of fluid, not whenever you feel like it. After two weeks, move to 60g per hour using a glucose-fructose product. This is where symptoms usually show up, and nausea or fullness at this stage means the stimulus is working. Hold the dose until it settles rather than progressing on schedule.
In the final three weeks, push toward 75 to 90g per hour if your race is longer than two and a half hours. If you’re racing a half marathon, 60g is plenty and there’s no reason to chase more.
Two rules matter more than the numbers. Rehearse at race pace, because gut blood flow at 5:00/km is not gut blood flow at 6:30/km. And rehearse in race conditions, with at least two long runs in the heat and humidity you’ll actually face. Use the exact products the race is handing out, or carry your own.
If symptoms are already there
Before adding more training stress, take out the obvious irritants.
Fermentable carbohydrates are the first thing I look at. A short low-FODMAP period reduced daily gut symptoms in runners with a history of exercise-associated distress, including flatulence, loose stool and urgency (Lis et al., MSSE, 2018). The common triggers are pulses, onion, garlic, wheat and milk in the 24 to 36 hours before hard sessions. In practice that means pulling back rajma and chana, or lentil soup and baked beans, depending on where you eat. This is a temporary pre-race tool, not a way to eat year-round. Fibre and fermentable carbohydrate are good for the gut the other 340 days.
Then look at fibre timing. Cut total fibre in the 24 hours before a race without cutting it in training. Move the last pre-race dinner from a whole-grain, bean-heavy, salad-loaded plate to something plainer: white rice with a light dal, or pasta with a simple sauce and eggs or paneer.
Painkillers make it worse. Ibuprofen taken before or during a long race damages the gut lining on top of the damage running already causes. If you’re racing through a niggle, take the tablet after, not before.
If symptoms persist after all of this, particularly if they show up at rest as well as during running, that needs a medical review rather than a nutrition tweak. Coeliac disease, IBS and lactose malabsorption all present this way, and no amount of gut training resolves them.
Race week and race morning
Nothing new. That advice is worn out because people keep ignoring it.
The pre-race meal should be one you’ve eaten before at least six times, three to four hours before the gun, low in fibre and fat, with 1 to 4g of carbohydrate per kg of bodyweight depending on how much time you have and how your stomach handles food. White rice, poha, plain idli, white toast with jam, or oats made with water all do the same job, which is carbohydrate with little fibre or fat. Practice it on your long-run mornings so race morning is a repeat, not a debut.
Keep your first feed early, around 20 minutes in, while gut blood flow is still reasonable. Waiting until you feel low means feeding a gut that’s already compromised.
Lemon water before a run does nothing for digestion. The idea is that citric acid primes the stomach or flushes something out, and neither holds up. Your stomach already sits at a pH well below what lemon juice can add, and a squeeze of lemon in 300ml of water changes nothing about how fast you empty a meal or how well you absorb it. On an empty stomach before a hard session, the acid load is mildly irritating for some people and can worsen reflux. Warm water is the useful version. It goes down easily, adds no fermentable sugar, and is what I’d give an athlete who wants something before an early run.
The Fueletics Perspective on gut training for runners
I’d rather see a runner take 60g per hour comfortably than 90g per hour badly. The number on the plan is worthless if it comes back up at 30km.
Gut training for runners also costs nothing. It needs no supplement, no equipment and no extra training load, only the discipline to take fuel on schedule during runs you’re already doing. In India it’s the highest-return change available to most marathon runners, because the standard approach here is still eighteen weeks of unfuelled long runs followed by improvising at the aid station. If you train somewhere else, you’ll recognise your own version of that.
If you have one race this season and eight weeks to prepare, take our sports nutrition assessment to figure out your customised plan.
Research References
Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome — implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics, 2017;46(3):246–265. doi:10.1111/apt.14157
Miall A, Khoo A, Rauch C, et al. Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption. Scandinavian Journal of Medicine & Science in Sports, 2018;28(2):630–640. doi:10.1111/sms.12912
Scrivin R, Costa RJS, et al. The effect of gut-training and feeding-challenge on markers of gastrointestinal status in response to endurance exercise: a systematic literature review. Sports Medicine, 2023. doi:10.1007/s40279-023-01841-0
Pugh JN, Kirk B, Fearn R, Morton JP, Close GL. Prevalence, severity and potential nutritional causes of gastrointestinal symptoms during a marathon in recreational runners. Nutrients, 2018;10(7):811. doi:10.3390/nu10070811
Stuempfle KJ, Hoffman MD. Gastrointestinal distress is common during a 161-km ultramarathon. Journal of Sports Sciences, 2015;33(17):1814–1821. doi:10.1080/02640414.2015.1012104
Lis DM, Stellingwerff T, Kitic CM, et al. Low FODMAP: a preliminary strategy to reduce gastrointestinal distress in athletes. Medicine & Science in Sports & Exercise, 2018;50(1):116–123. doi:10.1249/MSS.0000000000001419
Snipe RMJ, et al. The increase in core body temperature in response to exertional-heat stress can predict exercise-induced gastrointestinal syndrome. Temperature, 2023. doi:10.1080/23328940.2023.2213625
O’Brien WJ, Rowlands DS. Fructose-maltodextrin ratio in a carbohydrate-electrolyte solution differentially affects exogenous carbohydrate oxidation rate, gut comfort, and performance. American Journal of Physiology-Gastrointestinal and Liver Physiology, 2011;300(1):G181–G189. doi:10.1152/ajpgi.00419.2010
Rowlands DS, Houltham S, Musa-Veloso K, et al. Fructose–glucose composite carbohydrates and endurance performance: critical review and future perspectives. Sports Medicine, 2015;45:1561–1576. doi:10.1007/s40279-015-0381-0
