GI stress during endurance racing isn't bad luck, it's physiology. Here's what's actually happening in your gut and how to think about fueling around it.
Your gut doesn't fail you on race day because you chose the wrong gel flavour. It fails because your body has redirected up to 80% of its blood supply away from your digestive system. Most fueling products were never designed with that physiological reality in mind.
Yet the default response from athletes and coaches alike is to treat it as an inevitable cost of racing hard. It isn't. It's a predictable consequence of physiology, and once you understand the mechanism, you can make more informed decisions about your fueling approach.
This post covers what's happening in your gut during intense effort, why traditional fueling strategies break down under load, and what a more rational approach looks like.
What Actually Happens to Your Gut During Hard Exercise?
Blood is aggressively pulled away from your digestive system the moment you push into race intensity. This is called splanchnic vasoconstriction, and it's not subtle. Independent research has documented reductions in gut blood flow of up to 80% during high-intensity endurance exercise (Rowell, 1974, see References). Your body is prioritising working muscle, cardiac output, and thermoregulation. The gut drops to the bottom of the list.
The downstream consequences stack up fast. Gut motility slows, the muscular contractions that move food through your digestive tract become sluggish and unreliable. Intestinal permeability increases, meaning the gut lining becomes more reactive and vulnerable to irritants. Stress hormones like cortisol and adrenaline compound the problem by further suppressing digestive function. You are, in effect, trying to fuel a high-performance engine through a system that is simultaneously under significant physiological stress.
Why Do Most Energy Gels Cause Nausea and Bloating?
Most energy gels deliver a concentrated bolus of simple sugars into a gut that is already struggling to process anything. A standard gel contains 20–30g of carbohydrate in a thick, viscous format that sits in the gastric environment, demands water to dilute, and hits intestinal transporters in a sudden, steep concentration spike.
Carbohydrate absorption depends on specific intestinal transporters, SGLT1 handles glucose, GLUT5 handles fructose. These have an upper limit. For glucose alone, that ceiling sits at roughly 60g per hour. Combining glucose and fructose in a 2:1 ratio can push total absorption toward 90g per hour in trained athletes under optimal conditions - a finding supported by independent research (Jeukendrup, 2010, 2014, see References). When splanchnic blood flow is reduced and motility is impaired, even those thresholds become aspirational. Substrate backs up in the intestinal lumen. Osmotic pressure rises. Fluid is drawn in. The result is bloating, cramping, and the kind of urgency that derails race execution.
Does the Format of an Energy Gel Actually Matter?
Format is one of the most underexamined variables in sports fueling, and it has direct physiological consequences. Thick gels require more water to process, spend longer in the stomach, and create a steeper osmotic gradient in the gut. In a race context where hydration balance is already tight, a product that pulls additional fluid into the intestine to manage its own concentration creates a compounding problem.
From a physiological standpoint, a more fluid format moves through the gastric environment faster and creates a lower osmotic gradient than a viscous one. This difference affects the demands placed on a digestive system already operating under significant physiological stress during intense exercise. These are general principles of gastrointestinal physiology and apply regardless of the specific product being consumed.
What Is Liposomal Delivery and How Does It Work?
Liposomal delivery is a nutrient encapsulation method that uses microscopic phospholipid spheres, structurally similar to human cell membranes, to carry compounds through the gastrointestinal environment. Because of their membrane-mimicking structure, liposomes can interact with the gut lining through pathways that differ from conventional transporter-mediated uptake.
In research contexts, liposomal formats have been studied for their potential to reduce osmotic disruption and to maintain function under conditions where conventional digestion is less efficient. This is an active area of scientific inquiry. Claims about the performance of any specific liposomal product depend on that product's formulation and should be evaluated against the available evidence for that specific formulation.
Frequently Asked Questions
Why do I feel sick after taking a gel mid-race?
Nausea after mid-race gels is frequently attributed to osmotic overload in a gut with reduced blood flow. The concentrated sugars in a standard gel create a steep osmotic gradient, drawing fluid into the intestinal lumen faster than a compromised digestive system can process. From a physiological standpoint, a more dilute, fluid format creates a lower osmotic gradient, which is one reason format is considered an important variable in fueling strategy research.
How much carbohydrate can I actually absorb during a race?
In well-trained athletes using a 2:1 glucose-to-fructose ratio, peak absorption has been studied at around 90g of carbohydrate per hour under optimal laboratory conditions (Jeukendrup, 2010, see References). During intense racing, with gut blood flow significantly reduced, real-world absorption may be considerably lower. Fueling to the theoretical ceiling without accounting for gut function is a well-documented path to GI distress in the scientific literature.
Is GI distress during racing just something I have to accept?
GI distress during racing is widely described in sports science as a design and strategy problem rather than an unavoidable side effect of going hard. Product format, carbohydrate concentration, delivery mechanism, and fueling timing all influence gut tolerance. Athletes who treat it as inevitable may be missing addressable variables in their fueling approach.
Does training the gut actually work?
Regular exposure to carbohydrate intake during training has been shown in independent research to upregulate intestinal transporters and improve gut tolerance over time (Costa et al., 2017, see References). Gut training has limits, however it cannot override the fundamental physiological reality of reduced splanchnic blood flow at race intensity. Delivery format and timing remain relevant variables regardless of how well-trained the gut is.
Practical Takeaway
Audit your fueling strategy before your next race. If you are using thick, high-concentration gels and experiencing gut issues, product format is the most likely variable to address first. Prioritise a 2:1 glucose-to-fructose ratio, reduce bolus size, increase intake frequency, and consider a fluid format over a viscous one. Practice your fueling in training at race intensity, not just in easy sessions where your gut is fully functional.
The principle that connects all of this is straightforward: what you consume matters far less than what your body can actually absorb when it matters most. That gap between intake and absorption is where most fueling strategies quietly fail.
Science over hype. Every time.
For athletes looking to apply these physiological principles in practice, it is worth understanding how some products have been designed with these specific failure points in mind, not as a cure for GI distress, but as a considered attempt to reduce the variables that contribute to it.
SUPPLME Energy Gel is a liquid energy supplement that combines an ultra-fluid format with liposomal delivery technology. It delivers 32g of carbohydrate per 40ml serving in a 2:1 glucose-to-fructose ratio. The formulation reflects the physiological principles discussed above fluid format to reduce osmotic gradient, liposomal delivery as an encapsulation method, and a dual-transporter carbohydrate ratio, and is designed for use during endurance training and competition. As with any supplement, individual responses vary, and athletes are encouraged to trial any product in training before race-day use.
References
Rowell, L.B. (1974). Human cardiovascular adjustments to exercise and thermal stress. Physiological Reviews, 54(1), 75–159. This is an independent academic publication. The research was not conducted by or in association with SUPPLME.
Jeukendrup, A.E. (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care, 13(4), 452–457. This is an independent peer-reviewed publication. The research was not conducted by or in association with SUPPLME.
Jeukendrup, A.E. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), 25–33. This is an independent peer-reviewed publication. The research was not conducted by or in association with SUPPLME.
Costa, R.J.S., et al. (2017). Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism, 42(5), 547–557. This is an independent peer-reviewed publication. The research was not conducted by or in association with SUPPLME.
