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Carbohydrate fuelling for runners: what's being used and what the research says

We report what runners are using and what the research says. We don't recommend taking any supplement. Talk to your GP or a registered sports dietitian before trying one.

Quick answer

Research suggests it does in longer races. In a systematic review, 82% of 61 trials found carbohydrate improved endurance performance, with bigger gains in longer efforts[1]. Guidance links the amount to event length: a mouth rinse or small amounts for about an hour, about 60 g an hour for 2 to 3 hours, and about 90 g for ultra-endurance events[2].

Research
Strong (proposed)Clearest in efforts of over about 2 hours. Mixed for runs of around an hour.
Proposed by our editor; not yet confirmed by an expert reviewer.
WADA status
Permitted
Distances in the research
800 m–5K: limited research10K–half: studiedMarathon: studiedUltra: limited research

What is carbohydrate fuelling?

Carbohydrate fuelling means eating carbohydrate-rich food in the days and hours before a race, often called carb loading, and taking carbohydrate during longer races as drinks, gels or food[3]. Reviews also include refuelling afterwards[4].

The Australian Institute of Sport puts sports drinks, gels and chews in Group A of its supplement framework, meaning strong scientific evidence for use in specific situations in sport[5]. The World Athletics consensus puts food first, with sports foods for when eating ordinary food is impractical[6].

Glucose, and maltodextrin, a chain of glucose units, are absorbed through a gut transporter called SGLT1, and fructose through another, GLUT5; blends of the two are called multiple transportable carbohydrates[7][2]. Hydrogel drinks add sodium alginate and pectin to the carbohydrate[8]. Ordinary food counts too: in the ISSN's ultra-marathon examples, a 150 g banana gives 30 g of carbohydrate and a 40 g gel 23 g[9].

Also called: energy gels, sports drinks, carb loading, carbohydrate loading, race fuelling.

Why is carbohydrate fuelling getting attention?

In a 2024 survey of 5,668 marathon runners in China, 57% carb-loaded, men and women alike, 84% took carbohydrate during races or training, and more than 90% of supplement users used energy gels[10].

Guidance has moved from a single target to amounts that depend on how long the event lasts[2]. A 2026 review traces how suggested intakes rose over two decades from 16 to 120 g an hour, and argues that the highest rates lack firm evidence of better ultra-endurance performance[11].

Actual intakes vary widely: in one study of endurance races, marathon runners averaged 35 g an hour, and individual athletes across the events ranged from 6 to 136 g[12]. Low-carbohydrate, high-fat eating has drawn interest, and has been tested in elite race walkers[13].

The claims, checked against the research

  • Faster marathon timesSupported

    In a treadmill marathon, 7 trained runners were about 4 minutes faster with a 5.5% carbohydrate drink than with water[14]. A planned gel strategy, with some caffeine, was 4.7% faster at the Copenhagen Marathon in a non-randomised comparison[15].

  • Holding pace late in long racesSupported

    Runners held pace longer with a carbohydrate drink in a treadmill marathon[14], and a 5 km time trial after 2 hours of running was about 6 to 8% faster with 90 g an hour than with placebo[8].

  • Faster 10K and half marathon timesMixed

    Across sports, mouth rinses gave a small benefit in a meta-analysis, borderline in its most cautious model and not significant in running alone[16]. A rinse made no significant difference for 15 women in a 1-hour race[17], and a running review found gels did not change 16 to 21 km performance, though several studies found benefits in runs of 90 to 120 minutes[18].

  • Carb loading makes you fasterMixed

    In a classic 1971 study, some runners ran 30 km up to 15 minutes faster after 3 or more days of high-carbohydrate eating[11]. In trained runners, loading raised glycogen without changing 20.9 km times[19].

  • Fewer stomach problems in racesSupported

    In two small randomised trials from one research group, 2 weeks of taking carbohydrate on runs reduced gut symptoms in a later test, while a placebo did not[20][21]. The ISSN says the evidence supports gut training to ease gut distress in racing[9].

  • More carbs an hour, faster raceMixed

    In cycling, one trial suggested performance improved as glucose intake rose from 15 to 60 g an hour, and in another a glucose-fructose drink gave 8% more time-trial power than a glucose drink taken at 1.8 g a minute[2]. But in a mountain marathon, 120 g an hour did not significantly change race times compared with 60 or 90 g[22], and it brought elite marathoners the worst nausea[23].

  • Faster than a low-carb, high-fat dietMixed

    In elite race walkers, training improved 10 km times by 6.6% on a high-carbohydrate diet but not on a ketogenic diet[13], and a repeat study agreed[24]. Walkers chose their diets; neither study was randomised. But a 2026 review reports a trial in which 6 weeks on a low- or high-carbohydrate diet gave similar endurance, and argues fat-adapted athletes can perform as well[11].

  • Quicker refuelling after hard sessionsSupported

    In an early study in cyclists, glycogen was restored about 50% faster over 4 hours when carbohydrate came within 30 minutes rather than after 2 hours[25]. It measured glycogen, not performance.

How does carbohydrate fuelling work?

The body stores carbohydrate as glycogen in muscle and liver, a main fuel in moderate to hard endurance exercise[25]. Carb loading aims to store enough for the event's fuel needs[3], and trained runners who started a 20.9 km run with more muscle glycogen used more of it[19].

In efforts of about an hour, the benefit may come from carbohydrate sensed in the mouth acting on the brain. In efforts of over about two hours, it comes mainly from supplying fuel[1].

The gut sets a limit. Glucose uptake through SGLT1 appears to saturate at about 60 g an hour; adding fructose, which uses another route, raises how much of the ingested carbohydrate is burned[7]. Until 2004, the ceiling was thought to be about 1 g a minute; glucose plus fructose raised it to 1.26 g[2].

The gut also adapts. In animals, a high-carbohydrate diet increased SGLT1 within days, though direct human studies are lacking[7]. In 16 endurance-trained cyclists, 28 days of a higher-carbohydrate diet raised the rate at which ingested carbohydrate was burned[7][2].

Does carbohydrate fuelling improve running performance?

Yes, for longer races. In a systematic review of 61 trials, 82% found carbohydrate improved endurance performance, and the gains grew with the length of the exercise[1]. A review of running studies found that 13 of 17 comparing carbohydrate drinks with water or placebo showed a benefit, most likely in events over 2 hours[18]. For races of about an hour, the evidence is mixed[16][17].

800 m to 5K (Limited research)

We found no trials of carbohydrate swallowed during races this short. Mouth rinses have been tested: a meta-analysis included a 5 km running time trial and a run to exhaustion lasting about 3 minutes, and neither found a significant effect[16]. The World Athletics consensus describes competition nutrition as eating before, within and between events[6].

10K to half marathon (Studied)

For mouth rinses, a meta-analysis of 34 studies, most not in running, found a small benefit, borderline in its most cautious analysis; in the running studies alone the effect was not significant[16]. In 15 women racing for an hour on a track after an 8-hour fast, a rinse made no significant difference[17].

Over 16 to 21 km, a review of running studies found that carbohydrate gels did not change performance[18]. In 9 men drinking carbohydrate during a 21 km run, the type of pre-run meal, or a sugar-free control, made no significant difference to time[26], and carb loading did not change 20.9 km times in trained runners[19].

Marathon (Studied)

In a treadmill marathon, 7 trained runners were about 4 minutes faster with a 5.5% carbohydrate drink than with water, and held their pace for longer; a 6.9% drink made no significant difference[14]. In 11 trained male runners, a 5 km time trial after 2 hours of running was 5.6% faster with 90 g an hour of glucose and fructose as a standard drink, and 7.6% faster as a hydrogel, than with a placebo[8].

At the Copenhagen Marathon, runners with a planned strategy of gels and water, about 60 g of carbohydrate an hour plus some caffeine, finished 4.7% faster than matched runners who fuelled as they chose[15]. Among 257 London Marathon runners, those who ate more than 7 g of carbohydrate per kg of body mass the day before ran faster and held their pace better, though this was an observational study[27].

In a randomised trial in 20 elite male runners in a 42 km mountain marathon with about 2,000 m of climbing, 120 g an hour lowered exercise load and next-day blood markers of muscle damage compared with 60 or 90 g, but race times did not differ significantly[22].

A review notes that a runner finishing in 5 hours is unlikely to need 90 g an hour, as that would be close to, or above, their total carbohydrate use at that pace[2].

Ultra (Limited research)

Research is mostly observational. In one 161 km race, finishers averaged 65.8 g of carbohydrate an hour against 41.5 g in non-finishers, and intakes varied widely[9]. A 2026 review calls reports of very high intakes in ultra-marathon running outliers, or rates of 120 g an hour set by study protocols[11].

The ISSN gives 30 to 50 g an hour and says 90 g may be unrealistic in races over 6 hours[9].

At the 161 km Western States race, 96% of 272 runners had gut symptoms, and 36% of non-finishers gave them as a reason for dropping out[28].

Study by study

The main studies on this page. What they took is what the study gave, not advice for any individual.
StudyWho took partWhat they tookWhat was measuredWhat they found
Stellingwerff 2014 [1]Systematic review61 placebo-controlled studiesCarbohydrate during exercise v placeboEndurance performance or capacity82% found a benefit; gains grew with the length of the exercise
Tsintzas 1995 [14]Randomised controlled trial7 trained runners5.5% or 6.9% carbohydrate drink, or water, in three treadmill marathonsMarathon timeAbout 4 minutes faster with the 5.5% drink than water; 6.9% made no significant difference
Rowe 2022 [8]Randomised controlled trial11 trained male runners90 g an hour of 2:1 glucose-fructose (hydrogel or standard drink) or placebo for 2 hours; randomised, double-blind5 km time trial after the 2-hour run7.6% (hydrogel) and 5.6% (standard) faster than placebo; hydrogel 2.1% faster than standard
Hansen 2014 [15]Non-randomised trial28 non-elite marathon runners in 14 matched pairsPlanned gels (about 60 g an hour, some caffeine) v own choice; not described as randomisedCopenhagen Marathon time4.7% faster with the planned strategy
Atkinson 2011 [27]Observational study257 sub-elite London Marathon runnersCarbohydrate eaten the day before, recorded (observational)Marathon time and paceThose eating more than 7 g/kg ran faster and held pace better
Chryssanthopoulos 2018 [17]Randomised controlled trial15 recreational women runnersCarbohydrate or placebo mouth rinse after an 8-hour fastDistance in a 1-hour track raceNo significant difference (10.62 v 10.45 km)
Wong 2009 [26]Randomised controlled trial9 menHigh- or low-glycaemic meal or sugar-free control 2 hours before; carbohydrate drink during21 km treadmill run timeNo significant difference (91.1, 91.8 and 92.9 minutes)
Sherman 1981 [19]Randomised controlled trialTrained runners (numbers not given in the abstract)6-day diets with tapering runs: very low then high carbohydrate, moderate then high, or moderateMuscle glycogen and 20.9 km run timeGlycogen 207 and 203 v 159 mmol/kg; run times did not differ
Viribay 2020 [22]Randomised controlled trial20 elite male runners, previously gut-trained60, 90 or 120 g of carbohydrate an hour during a mountain marathon; randomisedExercise load, next-day muscle damage markers, race timeLower load and damage markers at 120 g; race times did not differ significantly
Costa 2017 [20]Randomised controlled trial25 endurance runners2 weeks of carbohydrate or placebo on runs, then a 2-hour run at 90 g an hourGut symptoms and a 1-hour distance testSymptoms down 60-63%, distance up 4-5% with carbohydrate; placebo distance down 2%
Miall 2018 [21]Randomised controlled trial18 endurance runnersDaily 1-hour runs for 2 weeks at 90 g an hour or placebo; randomised, blindedGut symptoms, malabsorption and a 1-hour effort after a 2-hour runFewer symptoms, less breath hydrogen; distance 11.7 to 12.3 km with carbohydrate only
Burke 2017 [13]Non-randomised trial21 elite male race walkers3 weeks of hard training on high, periodised or ketogenic diets; allocated by preference10 km race walk time and economyHigh and periodised carbohydrate improved 6.6% and 5.3%; ketogenic did not improve
Burke 2020 [24]Non-randomised trial26 elite male and female race walkers25 days on the same three diets, allocated by belief, then 2.5 weeks of high carbohydrate10,000 m race and a later 20 km raceHigh carbohydrate improved 4.8%; ketogenic 2.3% slower, with no rebound gain over 20 km

The key studies, in order

  1. 1981TrialTrained runners: two carb-loading regimens raised muscle glycogen similarly, but neither changed 20.9 km run times. [19]
  2. 1995Trial7 trained runners: about 4 minutes faster in a treadmill marathon with a 5.5% carbohydrate drink than water; a 6.9% drink made no significant difference. [14]
  3. 2001Non-randomised trialTrained women and men eating 12 g/kg of lean body mass a day for 3 days raised muscle glycogen similarly. [33]
  4. 2001Trial6 men and 6 women: women raised muscle glycogen on a high-carbohydrate diet only when extra energy was added; men did so without it. [32]
  5. 2009Trial9 men, 21 km run with a carbohydrate drink: a high- or low-glycaemic meal 2 hours before made no significant difference to time. [26]
  6. 2011Observational study257 London Marathon runners: eating more than 7 g/kg of carbohydrate the day before went with faster times. [27]
  7. 2011Expert reviewIntake scaled to the event: a rinse or small amounts for about 1 hour, 30-60 g an hour for longer events, up to 90 g beyond 2.5 hours. [4]
  8. 2012Observational study221 endurance athletes: intakes ranged from 6 to 136 g an hour; serious gut symptoms hit 4% of marathoners and about 31% in Ironman races. [12]
  9. 2014Expert review30-50% of athletes report gut complaints during exercise, mostly mild, with physiological, mechanical and nutritional causes. [38]
  10. 2014Non-randomised trial28 non-elite marathoners in matched pairs: a planned gel strategy with some caffeine finished 4.7% faster than free choice. [15]
  11. 2014Expert reviewIntake by event length: a rinse or small amounts for about 1 hour, up to about 60 g an hour for 2-3 hours, and about 90 g for ultra-endurance. [2]
  12. 2014Systematic review61 placebo-controlled studies: 82% found carbohydrate improved performance, with bigger gains in longer exercise. [1]
  13. 2015Observational study272 runners in a 161 km race: 96% had gut symptoms, and 36% of non-finishers gave them as a reason for dropping out. [28]
  14. 2016ConsensusJoint ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada position on the type, amount and timing of intake for sport. [37]
  15. 2016Expert review30 running studies: 13 of 17 found carbohydrate drinks beat water or placebo, with benefits most likely beyond 2 hours; gels did not change 16-21 km results. [18]
  16. 2017Non-randomised trial21 elite male race walkers: training improved 10 km times by 6.6% and 5.3% on high and periodised carbohydrate diets, but not on a ketogenic diet. [13]
  17. 2017Trial25 endurance runners: 2 weeks of gut training with carbohydrate cut gut symptoms by 60-63% and improved a 1-hour test; placebo did not. [20]
  18. 2017Expert reviewThe gut adapts: more dietary carbohydrate increases the transporters that absorb it, and practice can improve stomach comfort. [7]
  19. 2017ConsensusISSN nutrient timing: 8-12 g/kg a day maximises glycogen; about 30-60 g an hour in long, hard exercise; 1.2 g/kg an hour for quick recovery. [25]
  20. 2018Trial15 recreational women in a 1-hour track race after an 8-hour fast: a carbohydrate mouth rinse made no significant difference. [17]
  21. 2018ConsensusSports foods and supplements can help in specific situations. Contamination is a known anti-doping risk of supplement products. [41]
  22. 2018Trial18 endurance runners: 2 weeks of daily runs taking 90 g an hour reduced gut symptoms and malabsorption; placebo did not. [21]
  23. 2019ConsensusWorld Athletics consensus: food first, periodised guidance, eating before, within and between events, and a warning on energy deficiency. [6]
  24. 2019Expert reviewDistance runners: carbohydrate-rich eating before races, and during them from a mouth rinse to 75-90 g an hour in the longest races. [3]
  25. 2019ConsensusISSN ultra-marathon stand: 30-50 g of carbohydrate an hour with some protein, and about 10 g/kg a day for the 48 hours before a race. [9]
  26. 2020Non-randomised trial26 elite male and female race walkers: 10,000 m times 4.8% faster on high carbohydrate and 2.3% slower on a ketogenic diet. [24]
  27. 2020Trial9 trained male runners, 3 hours at 90 g an hour: a hydrogel drink made no difference to fuel use, gut symptoms or endurance. [35]
  28. 2020Meta-analysis78 studies: performance may be trivially lower in the early follicular phase of the menstrual cycle, on low-quality evidence. [30]
  29. 2020Trial20 elite male runners in a mountain marathon: 120 g an hour lowered muscle damage markers against 60 or 90 g; race times did not differ significantly. [22]
  30. 2021Non-randomised trial18 elite race walkers: a 5-day ketogenic diet plus a ketone ester slowed 10,000 m times by about 6%. Walkers chose their group. [42]
  31. 2022Meta-analysis34 studies, mostly not in running: a maltodextrin mouth rinse gave a small performance benefit, borderline in the most conservative analysis and not significant in running alone. [16]
  32. 2022Trial11 trained male runners: 90 g an hour of glucose-fructose made a 5 km time trial after 2 hours 7.6% (hydrogel) and 5.6% (standard drink) faster than placebo. [8]
  33. 2022Meta-analysis10 studies reviewed, 7 pooled: adding sodium alginate to a carbohydrate drink did not change performance, carbohydrate use or blood glucose. [36]
  34. 2023ConsensusIOC consensus: low energy availability harms health and performance in women and men; low carbohydrate availability plays a growing role. [31]
  35. 2023ConsensusISSN on female athletes: energy availability first, and carbohydrate needs met in every menstrual phase. Female-specific research is scarce. [29]
  36. 2025Survey5,668 marathon runners in 2024: 57% carb-loaded, over 90% of supplement users used gels, and about half had had gut symptoms. [10]
  37. 2025Non-randomised trial8 elite male marathoners: 120 g an hour burned the most ingested carbohydrate and aided economy but caused the worst nausea. No performance test; order not described as randomised. [23]
  38. 2026Expert reviewReview of more than 160 studies: carbohydrate improves performance, mainly, the authors argue, by preventing low blood sugar beyond 2-3 hours. [11]

Who has the research been done in?

Most trials are small and in men. A review of 30 running studies counted 505 men and 76 women, mostly young men tested after fasting, which limits what can be said about women, adolescents, older runners and runners who have eaten[18].

Field studies include recreational runners: non-elite marathoners in Copenhagen[15], sub-elite runners in London[27], and runners of all levels in the Chinese survey, 22% of them women[10]. The race-walking trials of low-carbohydrate diets were in world-class athletes[13].

Amateur ultra runners have been reported to average about 30 g an hour[9]. World Athletics flags women, young and masters athletes for special consideration[6].

What we know about women

Research in women is thin. The ISSN says female-specific evidence is lacking; it describes sex differences in carbohydrate and fat use, and advises meeting carbohydrate needs in every menstrual phase, with more in the luteal phase and the active-pill weeks of oral contraceptive users[29]. A meta-analysis of 78 studies found performance may be trivially lower in the early follicular phase, on low-quality evidence[30]. Low energy availability harms health and performance in women and men, and the IOC notes a growing role for low carbohydrate availability[31].

Women store extra glycogen as men do when they eat enough. Trained women raised muscle glycogen only when extra energy was added to a high-carbohydrate diet, while men did so on the high-carbohydrate diet alone[32]. When carbohydrate was matched to lean body mass, women and men stored similar amounts, in both menstrual phases tested[33]. For pregnancy, the NHS says starchy foods are an important source of energy and should make up just over a third of the food eaten, and that there is no need to 'eat for 2'[34].

What the research can't tell us yet

Researchers still disagree on why carbohydrate helps. A 2026 review argues the main benefit is preventing low blood sugar in efforts longer than 2 to 3 hours, and says this matters for how much athletes are told to take[11].

Many running studies are small: the treadmill marathon trial had 7 runners[14], and the marathon field study compared 14 matched pairs without describing the allocation as randomised[15].

Some dose studies measured fuel use only: a 2022 study of 90 v 120 g an hour had no performance test[11], and a 2025 study in elite marathoners says the performance effects of 120 g an hour are still to be determined[23].

Hydrogel results conflict: one running trial found a benefit over a standard drink[8], another found none[35], and a meta-analysis found no difference in performance[36].

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How and when is carbohydrate fuelling used in studies?

The joint position of the ACSM, the Academy of Nutrition and Dietetics and Dietitians of Canada sets out the type, amount and timing of intake[37]. A 2011 review gives small amounts or a mouth rinse for about an hour, 30 to 60 g an hour for longer events, and up to 90 g an hour beyond about 2.5 hours[4]. A 2014 review gives up to about 60 g an hour for 2 to 3 hours and about 90 g for ultra-endurance events, in grams per hour rather than per kg, and says amounts may need to be lower at easier paces[2].

The ISSN gives about 30 to 60 g an hour, as a 6 to 8% drink every 10 to 15 minutes, for hard exercise over an hour[25], and 30 to 50 g an hour with 5 to 10 g of protein for ultra-marathons[9]. A review of distance runners puts intakes in the longest races at 75 to 90 g an hour[3].

Above about 60 g an hour, studies mix sugars. Blends tested include 2:1 glucose and fructose at 90 g an hour[8], and 1:1 maltodextrin and fructose at 120 g an hour in elite marathoners[23]. One study gave 120 g an hour as a drink, gel, jelly chew or a mix of these[11].

Practice changes what the gut can handle. In 25 runners, 2 weeks of taking carbohydrate on runs, as gel-discs or food, cut gut symptoms by 60 to 63%[20]; in 18 runners, daily runs with 90 g an hour reduced symptoms and malabsorption[21]. The ISSN advises practising nutrition strategies in training, well before racing[9].

When studies gave it

The classic 1970s loading protocol paired 3 to 4 days of hard training on little carbohydrate with a switch to about 8 to 10 g per kg of body mass a day while cutting training sharply[25]. Later work dropped the depletion phase: a gentler regimen reached similar glycogen levels in trained runners[19], and one study nearly doubled glycogen with a single day of 10 g/kg[25]. The ISSN ultra-marathon stand describes about 10 g/kg a day for the 48 hours before a race[9].

An overnight fast can cut liver glycogen by as much as 80%[9]. A high-carbohydrate meal 4 hours before cycling raised muscle and liver glycogen, and some but not all studies of pre-exercise meals found better performance[25]. In 9 men drinking carbohydrate during a 21 km run, a 1.5 g/kg meal 2 hours before made no significant difference compared with a sugar-free control[26]. Fears of a blood-sugar dip from carbohydrate in the final hour trace to one early study; a warm-up, or eating 15 rather than 75 minutes before, lessened it[25].

During exercise, intake is spread out: a drink every 10 to 15 minutes in ISSN guidance[25], and 30 g every 20 minutes in the gut-training trials[20]. With under 4 hours to recover, the ISSN gives 1.2 g/kg an hour, or 0.8 g/kg an hour with 0.2 to 0.4 g/kg an hour of protein[25].

What studies used. These are research protocols, not advice for any individual.
What studies usedWhenRef.
A carbohydrate mouth rinse or small amounts of carbohydrateDuring exercise lasting about an hour (guidance)[2]
About 30 to 60 g of carbohydrate an hour, as a 6 to 8% drink every 10 to 15 minutesDuring hard exercise lasting more than an hour (ISSN guidance)[25]
Up to 90 g of carbohydrate an hour, from blends of carbohydrate typesDuring events longer than about 2.5 hours (review guidance)[4]
30 to 50 g of carbohydrate an hour, with 5 to 10 g of proteinDuring ultra-marathons (ISSN guidance)[9]
About 10 g of carbohydrate per kg of body mass a dayThe 48 hours before a race (ISSN ultra-marathon guidance)[9]
1.2 g of carbohydrate per kg of body mass an hourAfter exercise, when recovery time is under 4 hours (ISSN guidance)[25]

What are the side effects, and who should be careful?

Gut problems are common: 30 to 50% of athletes report them during exercise, and estimates for distance runners range from 30 to 90%. Most are mild, but some can be serious[38].

About half the Chinese marathon runners had had a gut symptom while running, most often bloating, loose stools, stitches and belching[10]. Serious symptoms affected 4% of marathon runners and about 31% of Ironman triathletes, and were more likely in athletes with a history of gut problems[12].

Higher intakes went with more nausea and flatulence[12], and the ISSN names high carbohydrate intake, especially concentrated drinks above 8%, as the main nutritional cause of gut upset in ultra-marathons[9]. Fibre, fat, protein, fructose and dehydration are also linked[38]. In running, about 45 g an hour appears well tolerated, and intakes near 90 g less so[11].

Glucose and fructose together seem to cause fewer symptoms than the same large amount of glucose alone[38]. In one running trial, a hydrogel caused fewer symptoms than a standard drink[8]; in another, every runner had symptoms with both[35].

Medicines and health conditions

The NHS says low blood sugar usually affects people with diabetes who take insulin or some other diabetes medicines, and lists doing a lot of exercise without eating more carbohydrate or reducing insulin as one cause[39]. It points people with diabetes to their diabetes care team for advice[39].

Anti-inflammatory painkillers (NSAIDs) such as ibuprofen have been linked with a leakier gut and gut symptoms in prolonged exercise, and a review advises against them before races, mainly for athletes with a history of gut problems[38]. Some race strategies add caffeine[15], which can cause gut distress, headaches and anxiety[9].

Be careful if

  • You often get stomach problems on runs: gut complaints affect 30 to 50% of endurance athletes[38], and practice in training reduced them in one trial[20].
  • You're planning a new gel or drink for race day: guidance stresses a race plan that has been practised in training[3].
  • You're aiming for very high intakes: in running, amounts near 90 g an hour appear harder to tolerate than about 45 g[11].
  • You have diabetes and take insulin or some other diabetes medicines: the NHS lists a lot of exercise without eating more carbohydrate or reducing insulin as a cause of low blood sugar[39].
  • You're eating too little overall, or cutting carbohydrate hard: low energy availability can harm health and performance in women and men[31], and a ketogenic diet held back race-walk performance in elite athletes[13].
  • You take painkillers such as ibuprofen around races: NSAIDs have been linked with gut symptoms during prolonged exercise[38].

Is carbohydrate fuelling allowed in competition?

Carbohydrate is a food and is not on the World Anti-Doping Agency's Prohibited List[40].

The anti-doping risk with any supplement product is contamination: products can contain prohibited substances that aren't on the label, and athletes are responsible for anything found in their sample[41].

Independent quality-assurance schemes, often called batch testing, check supplement products for prohibited substances[41]. WADA says using a product from one of these schemes can reduce, but not eliminate, the risk of an anti-doping violation[40], and the IOC says no product can be guaranteed entirely safe[41].

Myth or evidence?

5 things runners hear about carbohydrate fuelling. Pick an answer to see what the research says.

  1. 1.Taking carbohydrate during a marathon can make you faster.

    Show the answer

    Evidence. In a treadmill marathon, runners were about 4 minutes faster with a 5.5% carbohydrate drink than with water[14], and a planned gel strategy with some caffeine was 4.7% faster in a non-randomised field study[15].

  2. 2.The more carbohydrate you take in a race, the faster you'll run.

    Show the answer

    Myth. Guidance scales with the length of the event[2], and in running, amounts near 90 g an hour appear harder to tolerate than about 45 g[11].

  3. 3.Gut problems are rare in endurance athletes.

    Show the answer

    Myth. 30 to 50% of athletes report gut complaints during exercise[38].

  4. 4.You can train your gut to handle carbohydrate on the run.

    Show the answer

    Evidence. In a randomised trial, two weeks of practice cut gut symptoms by about 60%[20].

  5. 5.A carbohydrate mouth rinse helps in a 1-hour run.

    Show the answer

    Mixed. A review found it can help in efforts of about an hour[2], but it made no clear difference for women racing for an hour[17].

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Frequently asked questions

Does carbohydrate fuelling improve running performance?
Research suggests it does in longer races: in a systematic review, 82% of 61 trials found a benefit, and the gains grew with the length of the exercise[1].
How much carbohydrate an hour does published guidance give?
It depends on the event: a mouth rinse or small amounts for about an hour, up to about 60 g an hour for 2 to 3 hours, and about 90 g an hour for ultra-endurance events[2]. The ISSN gives a lower 30 to 50 g an hour for ultra-marathons[9].
Does carb loading help in a marathon?
The evidence points that way, though it is observational: London Marathon runners who ate more than 7 g/kg of carbohydrate the day before ran faster and held their pace better[27].
How many days before a race do runners carb load?
Newer protocols described by the ISSN use 1 to 3 days of about 8 to 10 g of carbohydrate per kg of body mass a day, with reduced training[25].
What should I eat before a race?
Studies have tested high-carbohydrate meals in the hours before exercise, with mixed results for performance[25]. In one 21 km run with a carbohydrate drink throughout, the pre-run meal made no significant difference[26].
Is 120 g of carbs an hour better than 90?
Not clearly. In a mountain marathon, race times did not differ significantly between 60, 90 and 120 g an hour[22]. In elite marathoners, 120 g an hour improved running economy compared with 60 g but brought the worst nausea, and performance was not tested[23].
Do hydrogel drinks work better than normal sports drinks?
The evidence conflicts. One trial in 11 runners found a faster 5 km time trial with a hydrogel[8], but another in 9 runners found no difference[35], and a meta-analysis found no performance difference[36].
Can you train your gut to take more carbohydrate?
Research suggests so. In two small randomised trials, 2 weeks of taking carbohydrate on runs cut gut symptoms by about 60%[20] and reduced malabsorption[21].
Does a carbohydrate mouth rinse work for runners?
Not clearly. A meta-analysis found a small benefit across types of exercise, but not a significant one in the running studies alone[16], and in 15 women racing for an hour on a track, a rinse made no significant difference[17].
Is a low-carb, high-fat diet better for endurance performance?
Not in the elite race-walking studies: a ketogenic diet made walking less economical and cancelled out the gains that training brought on high-carbohydrate diets[13][24]. A 5-day version with a ketone drink slowed 10,000 m times by about 6%[42]. A 2026 review, though, reports a trial in which 6 weeks on either diet gave similar endurance[11].
How soon after a long run do runners refuel?
Studies have given carbohydrate soon after exercise: in an early study in cyclists, glycogen was restored about 50% faster when carbohydrate came within 30 minutes rather than 2 hours later[25].
Do women need to fuel differently from men?
There is too little research to say for sure. The ISSN advises meeting carbohydrate needs in every phase of the menstrual cycle, with more in the luteal phase[29], and women raised muscle glycogen in carb loading only when they also ate more energy[32].
Are energy gels allowed in competition?
Yes. Carbohydrate is not on the WADA Prohibited List[40], though any supplement product can be contaminated[41].

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Written by Grant MacMillan, Editor, RunV. Not yet reviewed by an expert.

Last checked against its references . Next review due . How pages are reviewed · How we grade research

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