We had previously discussed the impacts of heat on fueling and hydration in our two part series that can be found on our blog page.
As we are now entering the main heat of summer, we have been receiving lots of questions from customers and the athletes we work with. Aaron has also been boots on the ground at gravel events interacting closely with athletes competing during those races. Interestingly, we have been receiving lots of questions on gastrointestinal (GI) distress as of late. Usually, the athletes will say they have no problem with high carb fueling during their preparation (when the weather is temperate and cooler), but will experience GI distress as the season progresses, during races, and typically when they are in 3+ hours into their event, despite having trained their gut in the off-season in preparation for this high carb fueling for races.
In this blog, we will do a deep dive into why you may be experiencing an absence of gut distress during your preparation and lead-up for these events, but suddenly experience GI distress during your summer events.
We must preface that everyone’s physiology is different, some have higher GI tolerance than others, so this might not be a challenge for you. Nonetheless, the physiology remains interesting and we highlight the significant influence of heat on fueling and hydration, despite the lack of integrative discussion among these variables.
First, let’s understand what happens when you ingest a gel or high carb solution.
Prior to being absorbed by the body, the carbohydrates contained in a gel or drink mix must first empty from the stomach and enter the small intestine, where absorption occurs. The rate at which stomach contents empty is influenced by factors including nutrient and energy density, fluid volume, osmolality, environmental conditions (e.g. heat stress), and exercise intensity. Because the gel or carb drink mix has a higher osmolality (600-1400+ mOsm/kg, although dilution can occur in the stomach) compared to the body’s physiological range (275 to 296 mOsm/kg), water will temporarily move into the intestinal lumen (space in the intestine that contains the consumed product) as a result of osmosis. At the same time, carbohydrates are absorbed through their respective transporters (SGLT1, GLUT5, etc.). As carbohydrates are absorbed, the osmotic gradient progressively changes, favoring net water absorption back into the body. The water absorbed across the intestinal wall can carry dissolved solutes (e.g. electrolytes) with it through paracellular pathways. This is known as solvent drag. This mechanism enhances the movement of water and electrolytes, while carbohydrate absorption remains primarily dependent on transporter absorption. Importantly, these processes occur simultaneously rather than sequentially. Carbohydrate absorption, water movement, and shifts in osmolality begin almost immediately and continuously influence one another.
Now, what happens when you do a long race in the heat?
The efficiency of gastric emptying and intestinal absorption of carbohydrates depends heavily on hydration status, GI blood flow, and GI function. During prolonged exercise at race-intensity in the heat, dehydration, heat stress, and reductions in GI blood flow can progressively reduce the gut's ability to process and absorb the ingested carbohydrates. So why is this the case?
Let’s walk through the physiology.
First, during exercise blood flow is primarily directed to the working muscles. Because there is a finite amount of blood in our bodies, this means that certain regions (i.e. the GI tract) of the body will receive less blood flow during exercise. This can be further exacerbated as intensity of exercise increases. Blood flow to the GI tract decreases between 30-80% during low to high intensity exercise (Qamar et al., 1987; Perko et al., 1998; Steege et al., 2012; Van Wijck et al., 2012; Oliveira et al., 2014). Second, in hot conditions, more blood flow is directed to the skin to facilitate heat dissipation, which further exacerbates the reduced blood flow to the GI tract. Third, as the duration of exercise in hot conditions increases, plasma volume progressively declines due to sweating leading to even less available blood volume to flow to the gut. This trifecta compromises the gut's ability to efficiently process and absorb carbohydrates compared to cooler conditions. Furthermore, reductions in hydration status and GI blood flow impair the mechanisms that drive water and electrolyte absorption, reducing the gut's capacity to absorb fluid alongside carbohydrates.
As the gut's ability to process and absorb nutrients becomes increasingly challenged, the rate at which nutrients are delivered from the stomach to the intestine may begin to exceed the intestine's ability to process and absorb them. This accumulation of unabsorbed nutrients within the small intestine can activate feedback mechanisms that slow gastric (stomach) emptying, a phenomenon commonly referred to as the duodenal brake (Rønnestad et al., 2016). The combination of these factors results in the feeling of heaviness and general gut discomfort due to unabsorbed carbohydrates both in the intestine and in the stomach. Interestingly, studies have shown that exogenous carbohydrate oxidation is reduced during exercise in the heat, even when hydration is maintained. The precise mechanism remains unclear, but impairments somewhere along the pathway from ingestion to muscle oxidation are thought to contribute (Jentjens et al., 2006; Mougin et al., 2025).
The Take Away.
All in all, while the gut is still inherently capable of absorbing high amounts of carbohydrates, it is the conditions that the gut is operating under which makes GI distress more likely. It is not that your gut training has suddenly disappeared, rather, the physiological environment that supports digestion and absorption has changed. A fueling strategy that works well in cool training conditions may become increasingly difficult to tolerate during a long and hot race as dehydration, heat stress, and reductions in GI blood flow progressively reduce the gut's ability to absorb carbohydrates.
With this in mind, the goal in hot races is not necessarily to maximize carbohydrate intake, but rather to match carbohydrate intake to the gut's current physiological capacity. Thus, the mind set needs to shift from “I have consistently been fueling with x grams of carbohydrates per hour, so I should do this for all my races” to “my maximum capacity is x grams of carbohydrates per hour in ideal conditions, how should I adjust my strategy when conditions are no longer ideal?”
We introduce a new approach to thinking about fueling called Maximum Tolerable Carbohydrate Intake (MTCI). Use this framework to help you plan and practice fueling your training and racing in the heat. In the corresponding blog, we discuss how to use this new framework and practical recommendations of how to avoid GI distress in the heat.
