Marathon Gel Strategy: Timing, Amount, and Absorption

*This article was produced with sponsorship from DM Mitsui Sugar Co., Ltd.

When should you take gels during a marathon, and how many? Behind that question there is usually an experience like this: you took gels and still ran out of energy late in the race.

In most cases, the cause is that the carbohydrate in the gels was not absorbed as well as you thought. There are two reasons. First, there is a ceiling on how fast your gut can absorb carbohydrate. Second, during exercise your stomach and gut both become slower at moving gels along and taking them in.

Gels made only of glucose plateau in absorption at around 60 g per hour※1. Commercial running gels have an osmolality more than ten times that of sports drinks, and taken without water they tend to stay in the stomach※2. With water, however, absorption speed is the same as with a liquid drink※3.

This article explains those two reasons and shows how to build a plan for gel type, number, timing, and water. How many grams per hour to aim for, and why running out of carbohydrate late in the race makes you slow down, are covered in How Many Carbs Per Hour for a Marathon?. Hydration itself is covered in the Marathon Hydration Guide.

Reason 1: Your Gut Can Only Absorb Carbohydrates So Fast

Glucose is absorbed through the SGLT1 transporter in the small intestine (a protein that carries sugar into cells), and this transporter saturates at around 60 g per hour※1※4. Any glucose beyond that is not absorbed. It stays in the gut and causes bloating and discomfort.

A typical energy gel contains 20–25 g of carbohydrate. With glucose-only gels, you hit the ceiling at about three gels per hour.

Fructose is absorbed through a different transporter, GLUT5. Because the route is different, a gel that contains both glucose and fructose raises the absorption ceiling to 90 g per hour※4. In one study, a drink with a glucose–fructose blend improved time-trial performance by about 8% over glucose alone※5.

This means how many gels to carry depends on the type of carbohydrate in the gel. For the target amount itself (60–90 g/h) and the total you need for each finish time, see How Many Carbs Per Hour for a Marathon?.

Let me share my own experience. In races, I took one gel at every aid station. Each gel contains 25 g of carbohydrate, and at about 2.5 gels per hour that already exceeds 60 g.

At that pace, you would carry about seven gels for a full marathon. Looking around, not many runners carry that many. I carried six at most myself and figured that was enough.

But when I rebuilt my plan around 60–90 g per hour, I found there was still room to increase if I chose a blend of glucose and fructose. Even though I felt I was fueling well, I had not reached the upper end of the target.

Reason 2: At Higher Intensity, Your Stomach Empties and Absorbs Less

Gels Have an Extremely High Osmolality

How fast the stomach passes its contents to the small intestine (the gastric emptying rate) depends on the energy density and volume of what you eat and drink, and on exercise intensity※6. Osmolality is another major factor※2※7.

Up to moderate intensity, running itself actually speeds up gastric emptying※7. The problem comes late in the race, when intensity and duration have built up.

Osmolality is a measure of how concentrated the dissolved particles are. A study of 51 commercial running gels found an average osmolality of 4,424 mmol per kg, with a range of 303 to 10,135※2. An isotonic sports drink is around 280–330, so the average gel is about 15 times higher, and the highest gels are more than 30 times higher.

When a high-osmolality liquid enters the stomach, the stomach draws in water to dilute it. During that time, the contents stay in the stomach. In a study that had runners drink solutions of different concentrations while running, the higher the carbohydrate concentration, the more remained in the stomach, and water emptied fastest※7.

Viscosity (thickness) itself also slows gastric emptying. One study reported that adding gel-forming dietary fiber to raise viscosity significantly slowed gastric emptying※8.

However, that was measured with a meal at rest, and osmolality was not measured. For gels, absorption speed is the same as with a liquid drink when they are taken with water※3※9, which suggests that how you take the gel matters more than viscosity itself.

Late in the Race, Absorption Itself Drops

As exercise intensity and duration increase, blood is sent preferentially to the muscles and less reaches the stomach and gut. The gut wall becomes damaged, and digestion and absorption decline. About 60% VO2 max and 2 hours or more is considered a rough threshold where these changes become likely※10.

The feeling that you cannot take gels or water late in the race comes from delayed gastric emptying combined with this drop in blood flow.

But With Water, the Form Doesn’t Matter

A study that compared glucose and fructose at 108 g per hour as a gel versus a drink found no difference in how the carbohydrate was used※3.

A newer study had athletes take 120 g per hour as a drink, a gel, a chew, or a mix, and found no difference in how the carbohydrate was used or in stomach issues※9. In all of these studies, the carbohydrate was taken with water.

A review that pooled several studies concluded that even a carbohydrate drink processed to gel inside the stomach (a hydrogel made with alginate and pectin) did not change how the carbohydrate was used or stomach issues, compared with a conventional carbohydrate drink※11.

In other words, the gel itself is not the problem. The problem is taking gels without water, before your gut is used to them, and in quick succession. Taking 150–200 mL (about 5–7 fl oz) of water with each gel dilutes the osmolality and brings you closer to the conditions in these studies.

All of these studies were done on bikes, though. Running, with its up-and-down motion, puts more strain on the stomach and gut, so it calls for more careful checking.

Why Many Runners End Up Taking Gels Without Water

In a survey of 171 recreational runners in Japan, 91.2% thought fluid intake during a race is important, while 77.2% thought nutrition during a race is important※12. Taking gels and water together as a set is not yet an established habit.

A half marathon study compared drinking a 6% carbohydrate drink according to guidelines with taking glucose gels while drinking water at one’s own pace※13. Race times did not differ, but in the gel condition fluid intake fell to less than a third and body weight loss was greater. In other words, switching to gels tends to reduce how much water you take in.

In fact, comments like these stand out among the questions recreational runners bring to me: “The gel is thick and makes me feel sick in one gulp,” or “I drank water at an aid station, got heartburn, and have run with no fuel and no fluids ever since.”

Moderate or worse GI symptoms during a race affected just under 30% of participants in a recreational marathon※14. In a survey of 805 recreational runners in China, GI symptoms were reported by 26.1%, and runners who used gels or sports drinks were more likely to have symptoms※15.

Symptoms also hurt your time. In a half marathon with 18 well-trained runners, the 3 who reported GI discomfort while taking gels were 2.4% (105 seconds) slower than in the no-gel condition※16. The number of runners is small, but it shows how costly symptoms can be.

Deciding to skip water and fuel because you feel sick leaves you with the delayed gastric emptying explained above, and ends up making you slow down. I once failed to grab water at an aid station, forced down a gel on its own, and felt sick for a long time afterward. My experience is that gels only work as a set with water.

How to Build a Gel Plan Around Absorption

Choose a Product That Contains Both Glucose and Fructose

Check the ingredient list for fructose, or for labels such as “two types of carbohydrate” or “dual source”※4※5. A blend can support up to 90 g per hour.

Take 150–200 mL of Water With Each Gel

When you take a gel, drink water with it. The studies above all used conditions with plenty of fluid (867 mL per hour in Pfeiffer 2010)※3※9. A practical guide is 150–200 mL (about 5–7 fl oz), the amount in one cup at an aid station.

It also helps to set aside the water at aid stations for gels, separate from the water you drink to quench thirst.

Every 20–30 Minutes, at Even Intervals From Early On

A good interval is once every 20–30 minutes, or every 5 km (about 3 miles). This splits your hourly target (60–90 g) evenly and matches the spacing of aid stations.

Waiting until you feel hungry or sluggish is too late. For roughly the first 30 minutes, carbohydrate from your pre-race meal is still available, so there is no need to rush.

Personally, to take gels with water every 20–30 minutes, I take one gel at every aid station.

Test It on a Long Run Before Race Day

Taking a high amount for the first time in a race (say 75 g per hour) makes stomach issues more likely because your gut is not used to it. Test the same product, amount, and timing as race day on a long run. Your gut can get used to fueling. Studies found that repeatedly taking carbohydrate during training for several weeks reduced stomach issues and malabsorption (→ Gut Training for Runners).

Summary

Deciding how many gels to take and when is easier if you work backward from how much you can absorb, rather than from how many gels you can carry. Start by checking the blend, take gels with water, and space them evenly from early in the race. Check your target amount in How Many Carbs Per Hour for a Marathon? and how to get your gut used to fueling in Gut Training for Runners, then try it all on a long run before your race.

References

※1 Jeukendrup AE, Jentjens R (2000) “Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research” Sports Med

※2 Zhang X, O’Kennedy N, Morton JP (2015) “Extreme Variation of Nutritional Composition and Osmolality of Commercially Available Carbohydrate Energy Gels” Int J Sport Nutr Exerc Metab

※3 Pfeiffer B, Stellingwerff T, Zaltas E, Jeukendrup AE (2010) “Carbohydrate oxidation from a carbohydrate gel compared with a drink during exercise” Med Sci Sports Exerc

※4 Fuchs CJ, Gonzalez JT, van Loon LJC (2019) “Fructose co-ingestion to increase carbohydrate availability in athletes” J Physiol

※5 Currell K, Jeukendrup AE (2008) “Superior endurance performance with ingestion of multiple transportable carbohydrates” Med Sci Sports Exerc

※6 Costill DL, Saltin B (1974) “Factors limiting gastric emptying during rest and exercise” J Appl Physiol

※7 Neufer PD, Costill DL, Fink WJ, Kirwan JP, Fielding RA, Flynn MG (1986) “Effects of exercise and carbohydrate composition on gastric emptying” Med Sci Sports Exerc

※8 Darwiche G, Björgell O, Almér LO (2003) “The addition of locust bean gum but not water delayed the gastric emptying rate of a nutrient semisolid meal in healthy subjects” BMC Gastroenterol

※9 Hearris MA, Pugh JN, Langan-Evans C, Mann SJ, Burke L, Stellingwerff T, Gonzalez JT, Morton JP (2022) “13C-glucose-fructose labeling reveals comparable exogenous CHO oxidation during exercise when consuming 120 g/h in fluid, gel, jelly chew, or coingestion” J Appl Physiol

※10 Costa RJS, Snipe RMJ, Kitic CM, Gibson PR (2017) “Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease” Aliment Pharmacol Ther

※11 King AJ, Rowe JT, Burke LM (2020) “Carbohydrate Hydrogel Products Do Not Improve Performance or Gastrointestinal Distress During Moderate-Intensity Endurance Exercise” Int J Sport Nutr Exerc Metab

※12 田井勇毅, 真木瑛, 木戸康博 (2021) 「市民ランナーを対象とした食行動と食意識に関する調査」 [Dietary behavior and dietary consciousness among competitive amateur runners; in Japanese] 日本スポーツ栄養研究誌 (Jpn J Sports Nutr)

※13 Lee MJ, Hammond KM, Vasdev A, Poole KL, Impey SG, Close GL, Morton JP (2014) “Self-selecting fluid intake while maintaining high carbohydrate availability does not impair half-marathon performance” Int J Sports Med

※14 Pugh JN, Kirk B, Fearn R, Morton JP, Close GL (2018) “Prevalence, Severity and Potential Nutritional Causes of Gastrointestinal Symptoms during a Marathon in Recreational Runners” Nutrients

※15 Zhao X, Chen Y, Li X, Wen W, Zhang J, Qiu J (2025) “Gastrointestinal symptoms among recreational long distance runners in China: prevalence, severity, and contributing factors” Front Nutr

※16 Burke LM, Wood C, Pyne DB, Telford DR, Saunders PU (2005) “Effect of carbohydrate intake on half-marathon performance of well-trained runners” Int J Sport Nutr Exerc Metab