Most athletes are fueling based on a number they heard somewhere, not a number they have ever tested. The 30 to 60 grams per hour rule has been repeated so often it is treated as fact, but it was derived from lab averages across mixed populations, not from your event, your pace, or your gut.
Carbohydrate needs during endurance exercise are not fixed. They scale with intensity, shift with duration, and are hard-capped by how much your intestines can actually absorb under race stress. Understanding these three levers does not just make you a smarter athlete - it is the difference between finishing strong and watching your race unravel somewhere around the 75% mark.
What Determines How Many Carbs You Need Per Hour?
Your carb requirement is set by how fast you are burning carbohydrate, which is directly tied to exercise intensity. At low aerobic intensities, around 55 to 65% of VO2max, fat oxidation covers the majority of your energy demands. Push above 75% VO2max and carbohydrates become the dominant fuel. This is why a 5-hour ultramarathon runner and a 3.5-hour marathon runner have fundamentally different hourly fueling requirements, even though both events are long.
Duration adds a second layer. Trained athletes carry roughly 400 to 500 grams of stored muscle glycogen, enough to sustain approximately 90 to 120 minutes of hard running before depletion becomes a performance threat. The longer your event, the earlier you need to begin fueling to stay ahead of that deficit. Waiting until you feel it is already too late.
How Much Carbohydrate Can the Gut Actually Absorb Per Hour?
The intestinal absorption ceiling for carbohydrates sits at around 60 grams per hour when using a single carbohydrate source, specifically glucose. That ceiling exists because the glucose transporter SGLT1 becomes saturated. Research by Jeukendrup and colleagues - independent research conducted without involvement of SUPPLME or its products - established that combining glucose and fructose, which use separate intestinal transporters, SGLT1 and GLUT5 respectively, raises that ceiling to approximately 90 grams per hour in trained athletes with a 2:1 ratio of glucose to fructose.
Critically, those upper limits assume a gut that has been trained to absorb under exercise stress. Untrained athletes attempting 60 to 90 grams per hour without progressive gut conditioning frequently experience cramping, nausea, and GI distress. Intestinal transporter proteins upregulate with consistent practice, which means gut training is not optional if you are targeting the upper end of the absorption range.
What Are the Right Carb Targets by Event Length?
Target ranges shift significantly depending on how long you are racing.
For efforts under 75 minutes, endogenous glycogen is typically sufficient with good pre-race nutrition. A single fueling hit around the 45 to 55 minute mark is worthwhile for hard-effort half marathons to blunt late-race fade. Target: 0 to 30 grams per hour.
For the marathon, 45 to 60 grams per hour is the evidence-supported target, introduced early, starting around 30 to 40 minutes in, not reactively when you feel flat. A four-hour marathoner needs roughly 160 to 170 grams of exogenous carbohydrate across the race to meaningfully defend glycogen and sustain pace through the final 10 kilometres.
For ultras and events over 5 hours, the target drops to 30 to 60 grams per hour, varied by terrain and intensity. Gut fatigue accumulates at ultra distances and nausea risk peaks in the final third of the event. A consistent, easily digestible fueling anchor becomes essential when appetite drops and solid food becomes harder to manage.
How Do You Build Gut Tolerance for Race-Day Fueling?
Gut training follows the same logic as any physiological adaptation: progressive overload over time. Start fueling on training runs longer than 60 minutes, even when you do not feel like you need to. Begin at 30 grams per hour for two to three weeks. Increase by 10 to 15 grams per hour every two to three weeks provided no GI distress occurs.
Practise at race-specific intensities. Gut behaviour at easy training pace does not reliably predict how your digestive system performs under race stress. Test your exact race-day products in conditions that mirror the event, including heat, technical terrain, and fatigue.
Using a product with a fixed, known carbohydrate dose is important here. If the dose changes between products, you cannot isolate what is working or identify what is causing problems.
Frequently Asked Questions
How many grams of carbs should I take per hour during a marathon?
For most trained marathon runners, 45 to 60 grams per hour is the target range, introduced at 30 to 40 minutes into the race. Starting early is essential because you need to replace glycogen progressively, not rescue a depleted system.
Can you absorb 90 grams of carbs per hour during running?
Yes, but only with a 2:1 glucose-to-fructose combination and a trained gut. Independent research by Jeukendrup et al. - conducted without involvement of SUPPLME or its products - demonstrated that multiple-transporter carbohydrate blends raise oxidation rates toward 1.7 grams per minute in trained athletes who have progressively conditioned their intestinal absorption.
Why do I get stomach cramps when I take gels during races?
Cramps during fueling are almost always a gut training deficit. The intestinal transporters required to absorb glucose and fructose at race pace are trainable, but they need consistent practice in training, not just deployment on race day.
Should I fuel differently for an ultramarathon versus a marathon?
Yes. Ultras require lower per-hour carb targets, typically 30 to 60 grams, adjusted dynamically by terrain and effort level. Gut fatigue and nausea risk are the primary constraints at ultra distances, so format and palatability matter as much as dose.
Practical Takeaway
Build your fueling protocol around three variables: your event duration, your intensity, and your current gut capacity. Start at 30 grams per hour and increase methodically. Practise fueling in training conditions that reflect race day. Log your energy and GI response after every long session, and adjust in 10 to 15 gram increments. Fueling is a trainable skill with a measurable ceiling, and getting there requires a systematic approach, not guesswork.
Science over hype. Every time.
The principles above - fixed dosing, multi-transporter carbohydrate ratios, and consistent format - informed the design of SUPPLME's Energy Gel as a supplement built to support a structured fueling protocol. Each 40ml gel delivers a fixed 32 grams of carbohydrate via a 2:1 glucose-to-fructose ratio, with liposomal delivery used as a formulation approach for the liquid format. It is designed to be a consistent, countable unit you can anchor a protocol to, whether you are targeting 30 grams per hour or working toward 90.
References
Jeukendrup, A.E., Moseley, L., Mainwaring, G.I., Samuels, S., Perry, S., & Mann, C.H. (2006). Exogenous carbohydrate oxidation during ultraendurance exercise. Journal of Applied Physiology, 100(4), 1134–1141. Independent research conducted without involvement of SUPPLME or its products.
Jeukendrup, A.E., & Jentjens, R. (2000). Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Medicine, 29(6), 407–424. Independent research conducted without involvement of SUPPLME or its products.
Jentjens, R.L., & Jeukendrup, A.E. (2005). High rates of exogenous carbohydrate oxidation from a mixture of glucose and fructose ingested during prolonged cycling exercise. British Journal of Nutrition, 93(4), 485–492. Independent research conducted without involvement of SUPPLME or its products.
