Energy gels are highly concentrated carbohydrate sources engineered to provide rapid fuel for athletes during endurance exercise, offering a quick influx of energy to help prevent fatigue. They often contain sugars like glucose and fructose, along with electrolytes. However, a common side effect for many endurance athletes is gastrointestinal (GI) distress, including the sudden onset of diarrhea, sometimes called “runner’s trots.” This issue results from complex physiological changes during intense activity combined with the concentrated nature of the fuel. Understanding the underlying mechanisms and ingredient triggers is the first step in managing this common athletic complaint.
The Physiological Reasons for Gut Distress
The physical stress of intense endurance exercise makes the digestive system highly vulnerable to irritation. A primary mechanism is visceral ischemia, which involves the body redirecting blood flow. During high-intensity efforts, the body prioritizes sending oxygen-rich blood to the working muscles and skin for cooling, diverting blood away from the gastrointestinal tract.
This reduction in blood flow to the gut, known as splanchnic hypoperfusion, can be significant, sometimes decreasing by up to 80% during maximal exercise. Impaired blood flow limits the gut’s normal function, potentially making the lining more permeable and hindering nutrient absorption. This state of reduced digestive capacity makes the gut less able to handle the sudden load of concentrated fuel from an energy gel.
A second major factor is the osmotic gradient created by the gel’s high concentration. Because gels are potent sources of carbohydrates, they have a high osmolarity when they reach the small intestine. To dilute this concentrated mixture, the body rapidly pulls large amounts of water from the bloodstream into the intestinal lumen. This influx of fluid can lead to sloshing, increased intestinal volume, and accelerated transit time, which directly contributes to osmotic diarrhea.
Ingredients That Aggravate the Gut
Beyond the body’s physiological state, specific ingredients within energy gels can intensify the digestive challenge. Fructose is a common carbohydrate found in many gels, but its absorption is limited by a specific transporter protein in the small intestine. If the amount of fructose consumed exceeds the capacity of this transporter, the unabsorbed sugar continues down the digestive tract.
Once in the large intestine, the bacteria ferment the unabsorbed fructose, producing gas, bloating, and drawing extra water into the colon, which results in osmotic diarrhea. This is why many manufacturers aim for a glucose-to-fructose ratio of 2:1 or higher, as glucose uses a different, more efficient transporter, allowing for a greater overall carbohydrate intake without overloading the fructose pathway.
Another significant trigger is the inclusion of sugar alcohols, such as sorbitol, xylitol, or maltitol, often found in “sugar-free” or low-calorie products. These compounds are poorly absorbed by the small intestine and function as potent osmotic agents. Remaining in the gut, they draw substantial water into the colon, exhibiting a laxative effect. High levels of caffeine, sometimes added to gels, can also stimulate gut motility, accelerating the movement of contents and increasing the urgency for a bowel movement.
Strategies for Preventing GI Issues
The most effective strategy to prevent gel-induced diarrhea is to manage the osmotic concentration of the fuel immediately upon consumption. Athletes should always consume energy gels with a substantial amount of plain water, ideally 8 to 12 ounces. This dilution significantly lowers the osmolarity of the gel before it reaches the small intestine, minimizing the water-drawing effect. Taking a gel with a sports drink, which is already a concentrated solution, can compound the problem by increasing the total osmotic load.
Athletes can also improve their digestive tolerance through a process known as “gut training”. This involves gradually and repeatedly practicing the consumption of race-day nutrition, including gels, during training sessions. This consistent exposure allows the digestive system to adapt and potentially increase the efficiency of carbohydrate absorption. Furthermore, it is wise to avoid high-intensity efforts immediately following gel consumption, as intense exercise exacerbates the blood shunting effect, making the gut less capable of processing the fuel.
Careful product testing and ingredient selection are necessary steps in minimizing discomfort. Athletes should test different brands and carbohydrate ratios during training, rather than on race day, to determine individual tolerance. Opting for gels with a higher ratio of glucose or maltodextrin to fructose may be beneficial, as this blend utilizes multiple transport pathways for more efficient absorption. It is recommended to use smaller, more frequent doses of fuel, rather than large single boluses, to reduce the sudden load on the digestive system.