7 Factors That Limit Your Carb Intake During Endurance Exercise

Introduction

You know the drill: you aim for 60–90 g carbohydrate per hour, but after a couple of gels the stomach starts to grumble and your bottle suddenly looks less appealing. Most endurance athletes can tolerate more carbohydrate with practice, yet many hit a ceiling well below the recommendations. This post unpacks the physiological bottlenecks that limit uptake and comfort, and how you can work around them in training and racing. Where relevant, we also highlight how Carb Accelerator can fit alongside smart fuelling to support carbohydrate absorption and gut comfort.

The problem

Guidelines suggest 30–60 g carbohydrate per hour for most sessions, increasing to about 90 g per hour in prolonged events when using mixed sugars. In practice, many athletes stall earlier because the gut cannot deliver what the legs are demanding. That gap matters: when exogenous carbohydrate delivery is suboptimal, perceived exertion increases, late-race pacing suffers, and GI symptoms can derail performance. The question is not “how much should I take,” but “what stops me absorbing more, and how do I remove those roadblocks.” (Jeukendrup, 2014).

Science deep dive – 7 limiting factors

1. Transporter saturation in the small intestine
   Glucose absorption relies mainly on the sodium-dependent SGLT1 transporter, which saturates at around 60 g per hour. Fructose uses a different gateway (GLUT5). Combining glucose- and fructose-containing sources enables parallel transport, increasing intestinal delivery and oxidation rates beyond glucose alone, towards ~90 g per hour when well practised. (Jeukendrup, 2014; Jeukendrup, 2010). 

2. Gastric emptying throttled by concentration and osmolality
   Your stomach meters fluid and carbohydrate into the intestine. High-carbohydrate concentrations and hyperosmolar drinks slow emptying and can increase nausea or sloshing. In hot conditions, where you need both fluid and carbohydrate, keeping beverage carbohydrate concentration at or below ~8% generally supports faster emptying and better comfort. (Marriott, 1994; Millard-Stafford & colleagues, 2024). 

3. Splanchnic blood flow falls with intensity and heat
   During hard exercise, blood is diverted away from the gut to working muscles and skin. This hypoperfusion impairs digestion and absorption, elevating GI symptom risk. Heat further reduces exogenous carbohydrate oxidation even when hydration is maintained, meaning the same intake may “feel” heavier in the heat. (Costa et al., 2017; Mougin et al., 2025). 

4. Formulation choices: carb type, fibre, fat and protein
   Glucose-only fuels hit the SGLT1 ceiling quickly. Mixed glucose-fructose blends raise exogenous oxidation and performance compared with isocaloric glucose, provided total concentration is managed. Added fibre, fat or protein before or during exercise slows gastric emptying and can push osmolality up, worsening tolerance at race intensity. (Jeukendrup, 2014; Currell & Jeukendrup, 2008, summarised). 

5. Electrolyte context for absorption
   SGLT1 co-transports glucose with sodium. Very low sodium availability in the intestinal lumen can reduce coupled glucose-sodium-water uptake, while excessive sodium without adequate fluid is not helpful either. Practically, pairing carbohydrate with appropriate sodium in fluids or foods supports co-transport and hydration without driving osmolality too high.  

6. Gut training status
   Like muscle, the gut adapts. Repeated exposure to higher intakes of mixed carbohydrates during training can upregulate transporter expression and improve gastric emptying and symptom tolerance, although effects vary and require consistency over weeks. Recent work shows gut-training protocols are promising, even if short-term interventions in already highly trained athletes yield modest changes. (Stellingwerff et al., 2014; King et al., 2022; Cao et al., 2025). 

7. Fermentable sugars and enzyme bottlenecks
   Some athletes are sensitive to rapidly fermentable oligosaccharides (FODMAPs) or large boluses of fructose, which can draw water into the intestine and increase gas. Specific digestive enzymes such as alpha-galactosidase target certain oligosaccharides, and carbohydrases like amylase support starch breakdown, potentially improving comfort for susceptible athletes when used judiciously alongside diet planning.

Practical application – from theory to your bottles and pockets

• Match fuel type to target rate
  If you aim for >60 g/h, use multiple transportable carbohydrates (e.g., glucose-fructose blends) rather than glucose alone to avoid SGLT1 saturation. Build up gradually in training runs and rides. (Jeukendrup, 2014). 

• Dial in concentration and volume
  Keep drink carbohydrate concentration ~6–8% when conditions require combined fluid and carbohydrate delivery. In cool weather or when relying on solid/gel fuels, consider separating fluids (plain water or electrolyte) from carbohydrates to control osmolality. (Millard-Stafford & colleagues, 2024). 

• Respect heat and intensity
  Expect lower tolerance on very hot days or during race surges. Pre-cooling, pacing surges, and taking smaller, more frequent sips or bites can help keep gastric emptying on track. (Costa et al., 2017; Mougin et al., 2025). 

• Support co-transport
  Include sodium sensibly across your plan, particularly in longer events, to aid glucose-sodium-water uptake while maintaining overall drink osmolality in the comfortable range. 

• Gut training is a session, not a slogan
  Progress your intake by 10–15 g/h per week across long sessions, rehearse race-like mixes and timing, and keep notes on symptoms. Combine with pre-exercise meals that are lower in fibre, fat and protein when high intake is planned. (King et al., 2022; Cao et al., 2025). 

• Manage fermentables and consider targeted enzymes
  Athletes with known sensitivity to FODMAP-rich pre-race meals can reduce that load in the 24 h pre-event. For some, targeted digestive enzymes, taken according to label directions, may help reduce gas from specific oligosaccharides when fuelling. 

How Carb Accelerator Helps

Carb Accelerator is positioned to work alongside evidence-based fuelling strategies. The inclusion of digestive enzymes relevant to carbohydrate and starch breakdown (for example, amylase and alpha-galactosidase) and soothing phytonutrients such as peppermint, fennel and ginger is designed to support comfort when you are progressively increasing carbohydrate intake in training. This is not a shortcut for poor planning; rather, it complements practices like mixed-carb fuelling, appropriate drink concentration and gut training to help reduce the common bottlenecks outlined above. 

You can also use Carb Accelerator during phases when you are rehearsing higher carbohydrate targets, with the aim of supporting carbohydrate absorption and reducing GI distress as your gut adapts. As ever, individual response varies, so log subjective comfort and adjust. (Jeukendrup, 2014; Costa et al., 2017). 

Conclusion

Your ceiling for carbohydrate intake is rarely fixed. It is usually set by a handful of modifiable factors: transporter saturation, gastric emptying, fluid-electrolyte context, exercise intensity and heat, formulation choices, gut-training status and how your gut handles fermentable substrates. Tackle these systematically in training, and you can often move from “I can’t get past 40 g/h” to “I can hold 70–90 g/h when it matters.” Use products and practices that align with the physiology, and rehearse them until they feel routine.

References

• Jeukendrup AE. Carbohydrate intake during exercise. Sports Medicine. 2014;44(Suppl 1):S25–S33. Springer

• Jeukendrup AE. Multiple transportable carbohydrates and their benefits. Curr Opin Clin Nutr Metab Care. 2010;13(4):452–457. PubMed

• Marriott BM (ed.). Food Components to Enhance Performance. National Academies Press; 1994. Chapter: Gastric Emptying During Exercise. NCBI

• Millard-Stafford M, et al. Perspectives on enhancing human performance in the heat. Temperature. 2024;11(1). PubMed Central

• Costa RJS, et al. Systematic review: exercise-induced gastrointestinal symptoms in endurance sports. Aliment Pharmacol Ther. 2017;46(3):246–265. PubMed

• Mougin L, et al. Heat stress impairs exogenous carbohydrate oxidation during running. J Appl Physiol. 2025. Physiology Journals

• King AJ, et al. Short-term very high carbohydrate diet and gut-training have minor effects in highly trained endurance athletes. Nutrients. 2022;14:1929. MDPI

• Cao W, et al. A review of carbohydrate supplementation approaches and gastrointestinal tolerance training. Nutrients. 2025;17(9). PubMed Central