Gastrointestinal Tolerance of Energy-Dense Foods in Ultramarathon Racing: A Study on Nutrition for Trail Running Events

This article is based on peer-reviewed research from international sports science journals, providing an in-depth analysis of the impact of “ultramarathon fueling” on athletic performance, combined with Taiwan’s local riding and race scenarios, to offer actionable training nutrition advice.

Ultramarathons and trail races last dozens of hours, and relying solely on energy gels is unsustainable. High-energy-density real foods must be introduced, but gastrointestinal tolerance is the biggest challenge.

In Taiwan’s endurance sports community—whether it’s climbing enthusiasts tackling the Wuling ascent, long-distance riders crossing the East Rift Valley, or participants in Sun Moon Lake laps, Taroko Marathon, or 226 km Ironman triathlons—the topic of “ultramarathon fueling” matters because it directly determines whether you can maintain pace in the latter stages of an event, avoid cramping and hitting the wall, and recover effectively between consecutive training days. Many amateur athletes pour all their effort into power training and equipment upgrades while overlooking nutrition—this “free margin for improvement.” In fact, when training volume and equipment are comparable, the quality of nutritional strategy is often the key variable separating finishers from DNFs, and personal bests from blow-ups. This article will walk you through the complete context—from cellular molecular mechanisms and randomized controlled trial evidence to dose-response curves and practical application—debunking long-standing myths so your fueling strategy is truly built on science.

Academic Research Review

On the science of “ultramarathon fueling,” top international journals have accumulated rich and rigorous evidence. Below are several representative studies selected for their value in methodological design, sample populations, and strength of conclusions, which together build our current understanding of this topic:

1. Costa RJS et al. (2019, IJSNEM) reviewed gastrointestinal and nutritional strategies in ultra-endurance events.

2. Stellingwerff T (2016, Appl Physiol Nutr Metab) studied energy intake patterns in trail ultramarathons.

3. Tiller NB et al. (2019, JISSN) published a position statement on ultramarathon nutrition.

4. Costa RJS et al. (2017, Sports Medicine) reviewed exercise-induced gastrointestinal syndrome.

Taken together, these studies show that the scientific picture of “ultramarathon fueling” is not a single fixed conclusion, but is continually revised and deepened as research methods advance. Early studies mostly used laboratory-controlled time trials or exhaustion tests; subsequent research gradually introduced stable isotope tracers, muscle biopsies, functional magnetic resonance imaging (fMRI), and molecular biomarkers, allowing us to move from “observing phenomena” to “explaining mechanisms.” Notably, most high-quality studies employ randomized crossover designs, where each subject serves as both experimental and control, greatly reducing noise from individual differences. However, extrapolation of findings still requires caution: responses from well-trained laboratory subjects may not fully apply to general amateur athletes, and the effects of a single acute intervention may not equal long-term chronic adaptation. When reading the “effect sizes” and “statistical significance” of these studies, we must also distinguish between “statistically significant” and “practically meaningful”—a 1% improvement may decide medal placement in elite competition, but its significance is relatively limited for a recreational rider.

Core Mechanisms

During ultramarathons, intestinal blood flow decreases by 60–70%, triggering exercise-induced gastrointestinal syndrome (EIGS) and reducing absorption and tolerance. Low-intensity, long-duration events can incorporate savory foods, solids, and fat/protein to provide energy density and satiety, alleviating the monotony of sugar-based fueling. Gut training and pre-race rehearsal are prerequisites for tolerating real foods.

To truly understand how “ultramarathon fueling” affects athletic performance, we must return to physiology at the cellular and systemic levels. Athletic performance is the result of multi-system coordination: the cardiovascular system delivers oxygen and fuel, muscle cells handle energy conversion and mechanical contraction, the central nervous system regulates motor unit recruitment and perceived fatigue, and the gut and liver form the hub of nutrient absorption and metabolism. The reason these mechanisms translate into measurable performance differences is precisely because they act on one (or more) critical links in this chain. The table below organizes the key points of action at different physiological levels for this topic, helping you build a complete mechanistic picture:

| Level of Action | Key Mechanism | Significance for Athletic Performance |

|—|—|—|

| Cellular/Molecular | Affects mitochondrial efficiency, enzyme activity, and signal transduction | Determines efficiency of energy conversion and direction of adaptation |

| Muscle Tissue | Modulates substrate utilization, buffering capacity, and contractile function | Affects sustainable power output and onset of fatigue |

| Systemic Integration | Alters blood flow distribution, thermoregulation, and hormonal environment | Determines stability and safety during prolonged exercise |

| Central Nervous System | Modulates perceived fatigue, drive, and motor unit recruitment | Affects “how hard it feels” and the ability to persist |

Two dimensions deserve special emphasis: “dose-response” and “temporal dynamics.” The same nutritional intervention, at different doses and different timing, can produce vastly different or even opposite effects—this is precisely why many popular recommendations are one-sided. Only by understanding mechanisms can we judge “when to use it, how much, and when,” rather than blindly following trends.

Furthermore, the limiting factors of athletic performance shift dynamically with exercise intensity and duration: in short, high-intensity efforts, limits often come from the phosphagen system and accumulation of glycolytic byproducts; in multi-hour endurance events, limits shift to a combined effect of glycogen depletion, rising core temperature, fluid and electrolyte imbalance, and central fatigue. “Ultramarathon fueling” deserves in-depth exploration precisely because it can specifically target some of these limiting factors. This also reminds us that no nutritional strategy can be evaluated in isolation from the “exercise context”—a fueling rhythm suitable for a 40-minute criterium may not apply to a 6-hour climbing epic, and vice versa. The more thoroughly you understand mechanisms, the more flexibly you can adjust across different event formats, rather than clinging to a fixed formula. This ability to “adjust by context” is precisely the dividing line between amateur athletes and those who truly understand sports science.

Dose and Effect Relationships

In sports nutrition, “the dose determines both toxicity and benefit.” Too low a dose fails to reach the physiological threshold and is futile; too high a dose may trigger side effects, gastrointestinal distress, or even interfere with training adaptations. The table below summarizes the dose-effect correspondence for “ultramarathon fueling” and serves as the most important quantitative reference when developing a personal fueling plan:

| Dose / Condition | Effect Description |

|—|—|

| Low-intensity segment | Can incorporate solids/savory foods |

| Energy density | Supplement with fat and protein |

| EIGS | Intestinal blood flow drops 60–70% |

| Pre-race rehearsal | Test tolerance |

From the table above, benefits often follow an “inverted U” or “threshold-plateau” curve: effects increase with dose until an effective threshold is reached, but beyond a certain plateau, there is no additional benefit while marginal costs (side effects, gastrointestinal burden, financial expense) rise sharply. This means “finding your optimal dose” matters far more than “eating as much as possible.” It is recommended to progressively test different doses during training (not on race day), recording subjective feelings, gastrointestinal responses, and power data to build your own dose profile. Remember: the laboratory average is a starting point, not the endpoint; each person’s body weight, metabolic rate, gut tolerance, and genetic background will shift the optimal dose individually.

Differences Across Populations

The benefits of “ultramarathon fueling” are not equal for everyone. Age, sex, training status, body size, and genetic background all significantly modulate individual response magnitude. Ignoring these differences and applying a one-size-fits-all recommendation is one of the most common errors in sports nutrition.

| Population Dimension | Response Characteristics | Practical Recommendations |

|—|—|—|

| Beginners vs. Advanced | Advanced athletes have more mature physiological adaptations; responses are often more stable but with smaller marginal gains | Beginners should start conservatively at low doses and build tolerance first |

| Male vs. Female | Differences in body weight, hormonal cycles, and sweat composition affect dosing and needs | Females should individualize by body weight and pay attention to iron and energy availability |

| Young vs. Older | Older individuals often have reduced absorption efficiency and anabolic resistance | Older individuals may need higher doses or better timing |

| Body Size Differences | Body weight directly affects absolute amounts calculated per mg/kg or g/kg | Always convert to a dose corresponding to individual body weight; avoid copying general rules |

When interpreting “individual differences,” one must also be wary of a common statistical trap: studies mostly report “group mean responses,” but beneath the mean often lies enormous individual variability. In the same intervention, there may be strong responders, non-responders, and even negative responders. This is why even when a study shows “average effectiveness,” you still need to confirm through your own trials which category you fall into. The recommended approach is personalized A/B testing: in two training sessions with conditions as similar as possible, with and without the strategy, compare power, heart rate, and subjective feelings, and repeat several times before drawing conclusions. This empirical spirit of “using yourself as the sample” is the necessary path to translating group science into a personal prescription.

Taking Taiwan’s common amateur endurance population as an example, many are riders over 35 who train around work commitments. This group simultaneously faces declining recovery speed, insufficient sleep, and time pressure, so the “return on investment” of nutritional strategies is often higher than for young elites—that is, correct nutritional intervention can yield relatively greater room for improvement. Female athletes need to pay particular attention to the effects of the menstrual cycle on metabolism and needs, as well as whether energy availability is sufficient, avoiding the trap of low energy availability (LEA) in the pursuit of lightness. After understanding population differences, you will realize: truly professional nutritional advice is always an individualized prescription that “varies by person,” not a one-size-fits-all slogan.

Practical Training Application

Theory must ultimately translate to training plans and race courses. Below is a practical framework for converting “ultramarathon fueling” into concrete training and race operations:

• Pre-race testing principle: All nutritional strategies must be rehearsed in training first; “never try anything new on race day” is an iron rule. Gut tolerance to new fuels takes time to build.

• Periodization mindset: Align nutritional strategies with training cycles—base periods can emphasize adaptation-oriented strategies, while pre-season shifts to performance-oriented fueling optimization.

• Progressive introduction: Start with low doses and low frequency, adjust gradually based on body responses, and build a personalized dose and timing profile.

• Data tracking: Combine power meter, heart rate, subjective fatigue (RPE), and gastrointestinal comfort records to objectively evaluate whether an intervention is truly effective.

• Holistic context: Nutrition is one part of training, sleep, recovery, and psychology; no single fuel can compensate for sleep deprivation or flawed training design.

Using a one-week training schedule as an example, it is recommended to rehearse different fueling scenarios in the week’s key high-intensity sessions (such as threshold intervals or repeated climbs) and the weekend’s long ride: high-intensity days emphasize rapid energy supply and central activation, while long-distance days emphasize sustained energy, gut tolerance, and recovery. Through repeated rehearsal, your body can execute the optimal fueling rhythm “automatically” on race day, leaving mental resources for pacing and tactical decisions. Remember, the goal of a nutritional strategy is not theoretical perfection, but stable reliability under the fatigue, heat, and pressure of a real race course.

The most common mistake many people make when executing a nutrition plan is “being serious only on race day and eating casually during regular training.” This is precisely putting the cart before the horse: regular training is the ideal laboratory for building gut tolerance, testing doses, and cultivating fueling rhythm. If you hope to execute an 80 g/h carbohydrate fueling plan on race day, you must rehearse it repeatedly in training until your body is accustomed to it; if you want to rely on a supplement, you must confirm in training that it is truly effective and free of side effects for you. It is recommended to integrate a nutrition log with your training log, recording fueling content, intake timing, gastrointestinal responses, and performance data for each key session. After weeks to months of accumulation, the value of this personalized database will far exceed any generic nutrition guide. Additionally, do not overlook the often-underestimated “post-training recovery fueling”—the quality of recovery between consecutive training days often determines whether you can steadily accumulate training volume without injury, and training volume is the most fundamental engine of long-term progress. Treat nutrition as an integral part of training rather than a last-minute accessory before races, and your improvement curve will be noticeably different.

Local Application in Taiwan

Taiwan’s unique climate, terrain, and race culture add localized considerations to the application of “ultramarathon fueling.” Taiwan’s summer is hot and humid, with apparent temperatures often exceeding 35°C, and sweat rates and fluid/electrolyte losses far exceed the research conditions of temperate countries—meaning hydration and fueling recommendations from foreign literature often need to be “revised upward.” In events like the Wuling ascent, which climbs from sea level to 3,275 meters, high-altitude appetite suppression, low temperatures, and prolonged exercise pose a severe test for energy planning.

For local fueling options, Taiwan’s abundant bananas, sweet potatoes, pineapples, sports drinks, and convenience store ready-to-eat foods can all be incorporated into fueling strategies; the extremely high density of convenience stores also makes mid-ride refueling on long rides relatively easy. It is recommended that Taiwanese riders planning classic routes such as Sun Moon Lake, Wuling, Beiyi, Buyanting, and the East Coast crossing inventory refueling points along the way in advance, and strengthen sodium and fluid supplementation given Taiwan’s humid and hot environment. Athletes in the Taroko Marathon, Taipei Marathon, and triathlons across the island should likewise incorporate these local climatic factors into their individualized nutrition plans to perform at their best under subtropical conditions.

Debunking Common Myths

Myth: The myth is that “ultramarathons also require constant gel intake.” In reality, long-duration, low-intensity events require diverse real foods; pure sugar fueling easily causes taste fatigue and gastrointestinal distress.

Such myths spread widely because they “sound reasonable,” are easy to pass by word of mouth, or are amplified by marketing rhetoric. Yet the value of science lies in testing intuition with rigorous evidence: many seemingly obvious notions fail under the scrutiny of randomized controlled trials. Sports nutrition is especially rife with oversimplified “panacea” marketing that compresses complex dosing, timing, and individual differences into a slogan. The next time you hear a dogmatic nutritional claim, it is worth asking: “What is the level of evidence for this claim? Who is the target population? Are the dose and timing clearly specified?” Cultivating this evidence-based critical thinking is more valuable than memorizing any single conclusion, and it is a key step for amateur athletes moving toward scientific training.

Conclusion

“Ultramarathon fueling” is a topic in sports nutrition with both theoretical depth and practical value. From the academic evidence reviewed in this article, its benefits are real, but it is by no means an unconditional panacea—the key lies in correct dosing, appropriate timing, individualized adjustment, and synergy with overall training, recovery, and sleep. For Taiwan’s endurance sports enthusiasts, while mastering scientific principles, it is equally important to combine local climate, terrain, and race characteristics to transform general rules into a personalized prescription suited to you. May every rider sweating on Wuling, in the rift valley, or on the island-circumnavigation route break through their limits through science-based nutritional strategies and enjoy the pure joy of sport. Before you next step onto the race course, remember—your fuel bottle holds not just water and sugar, but an entire validated body of sports science.

Related Reading

• Comparative Study of Gastrointestinal Absorption Rates: Gel-Based vs. Liquid Fueling
• Ultramarathon Fueling Strategies: Nutrition Plans and Gut Training for 100K+ Events
• Gastrointestinal Risks in Marathon Fueling Strategies: Carbohydrate Type and Gut Adaptation Research
• Dietary Survey of Taiwanese Professional Cyclists: Energy Intake and Macronutrient Analysis