The Impact of Low-Carb High-Fat (LCHF) Diets on Ultramarathon Performance: A Balanced Assessment of Systematic Reviews

Based on peer-reviewed research from international sports science journals, this article provides an in-depth analysis of the effects of the “LCHF diet” on athletic performance, combined with Taiwan-specific riding and race scenarios, offering actionable training nutrition recommendations.

The low-carbohydrate, high-fat (LCHF) diet claims to turn the body into a “fat-burning machine,” freeing athletes from dependence on carbohydrate supplementation. However, for endurance sports requiring high-intensity output, the scientific evidence presents a complex, two-sided picture.

In Taiwan’s endurance sports community—whether it’s the climbing enthusiasts heading west up Wuling, the long-distance riders heading east through the Huatung Valley, or participants in the Sun Moon Lake loop, Taroko Marathon, or 226 km Ironman triathlons—the topic of the “LCHF diet” matters because it directly determines whether you can maintain your 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 one’s nutritional strategy is often the key variable separating finishing from dropping out, and a personal best from a meltdown. This article will guide 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 that your fueling strategy is truly built on science.

Review of Academic Research

Regarding the scientific exploration of the “LCHF diet,” 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 form our current understanding of this topic:

1. Volek JS et al. (2016, Metabolism)—the FASTER study found that long-term LCHF ultramarathon runners achieved fat oxidation rates as high as 1.5 g/min.

2. Burke LM et al. (2017, Journal of Physiology)—the race-walking study showed that while LCHF increased fat oxidation, it impaired exercise economy, with high-intensity performance declining.

3. Burke LM (2021, JAP)—subsequent research confirmed that LCHF impairs performance in high-intensity events.

4. McSwiney FT et al. (2018, Metabolism)—showed that the adaptation period requires more than 3–4 weeks.

Taken together, these studies reveal that the scientific picture of the “LCHF diet” is not a single, fixed conclusion but rather one that has been continuously revised and deepened as research methods have advanced. Early studies predominantly used laboratory-controlled time trials or exhaustion tests, while subsequent research progressively incorporated stable isotope tracing, 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 employed a randomized crossover design, where each subject serves as both the experimental and control group, greatly reducing noise from individual differences. However, extrapolating research conclusions still requires caution: the responses of 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, one must also distinguish between “statistically significant” and “practically meaningful”—a 1% improvement might decide medal placement in elite competition, but its significance is relatively limited for recreational riders.

Core Mechanisms

The LCHF diet upregulates fat oxidation-related enzymes (such as CPT-1, HSL) through long-term carbohydrate restriction, increasing the body’s capacity to use fat for energy. However, fat oxidation produces approximately 8% less ATP per unit of oxygen than carbohydrate, and at high intensities it cannot supply ATP quickly enough, leading to reduced exercise economy. The downregulation of the glycolytic system is the core mechanism behind impaired high-intensity performance.

To truly understand how the “LCHF diet” affects athletic performance, one must return to physiology at the cellular and systemic levels. Athletic performance is the result of multi-system coordination: the cardiovascular system handles oxygen and fuel delivery, 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 effects of this topic across different physiological levels, 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 the efficiency of energy conversion and the direction of adaptation |

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

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

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

Two dimensions deserve particular emphasis: “dose-response” and “temporal dynamics.” The same nutritional intervention, at different intake levels and different timing, can produce vastly different—even opposite—effects. This is precisely why many mainstream recommendations are one-sided. Only by understanding the mechanisms can we judge “under what circumstances 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, limitations often stem from the phosphagen system and the accumulation of glycolytic byproducts; in multi-hour endurance events, limitations shift toward the combined effects of glycogen depletion, rising core temperature, fluid and electrolyte imbalance, and central fatigue. The reason the “LCHF diet” deserves in-depth discussion is precisely because it can specifically target certain of these limiting factors. This also reminds us that no nutritional strategy can be evaluated in isolation from the “exercise context”—a fueling rhythm suited to a 40-minute criterium may not apply to a 6-hour climbing epic, and vice versa. The more thoroughly you understand the mechanisms, the more flexibly you can adjust across different event formats, rather than rigidly adhering to a single fixed formula. This ability to “adjust according to context” is precisely the dividing line between amateur athletes and those who truly understand sports science.

Dose-Response Relationship

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

| Dose / Condition | Effect Description |

|—|—|

| Adaptation period 3–4 weeks | Fat oxidation begins to upregulate |

| Peak fat oxidation 1.5 g/min | Far higher than high-carbohydrate individuals |

| Above 85% VO2max | LCHF performance disadvantage is pronounced |

| Ultra-slow ultramarathon | May be applicable |

As the table shows, benefits often follow an “inverted U-shaped” or “threshold-plateau” curve: effects increase with dose until the 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 that “finding your own 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 individual’s body weight, metabolic rate, gut tolerance, and genetic background will cause the optimal dose to shift in a personalized way.

Differences Across Populations

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

| Population Aspect | Response Characteristics | Practical Recommendations |

|—|—|—|

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

| Male vs. Female | Differences in body weight, hormonal cycles, and sweat composition affect dosage and requirements | Females should individualize based on body weight and pay attention to iron and energy availability |

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

| Body Size Differences | Body weight directly affects the absolute amount 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 pitfall: studies mostly report “group average responses,” but beneath the average often lies enormous individual variability. In the same intervention, some may be strong responders, some non-responders, and some even negative responders. This is why even if a study shows “average effectiveness,” you still need to confirm through your own experimentation which category you fall into. The recommended approach is to conduct personalized A/B testing: in two training sessions with conditions as similar as possible, adopt and then not adopt the strategy respectively, 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 essential path to translating group science into a personal prescription.

Taking the common amateur endurance population in Taiwan as an example, many are middle-aged cyclists over 35 who train in their spare time around work. 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 bring relatively greater room for improvement. Female athletes need to pay special attention to the impact of the menstrual cycle on metabolism and needs, as well as whether energy availability is sufficient, to avoid falling into the trap of low energy availability (LEA) while pursuing weight reduction. After understanding population differences, you will realize: truly professional nutritional advice is always an individualized prescription that “varies from person to person,” not a one-size-fits-all slogan.

Practical Training Application

Theory must ultimately be implemented in training plans and on the race course. The following provides a practical framework for translating the “LCHF diet” into concrete training and competition 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. Gastrointestinal tolerance to new fueling products takes time to build.

• Periodization Mindset: Align nutritional strategies with the training cycle—the base phase can focus on adaptation-oriented strategies, while the pre-season shifts to performance-oriented fueling optimization.

• Progressive Introduction: Start with low doses and low frequency, adjust gradually based on bodily responses, and build a personalized dosage and timing profile.

• Data Tracking: Combine records from power meters, heart rate, subjective fatigue (RPE), and gastrointestinal comfort to objectively evaluate whether the intervention is truly effective.

• Overall Context: Nutrition is one part of training, sleep, recovery, and psychology; a single supplement cannot compensate for sleep deprivation or flaws in training design.

Taking a one-week training schedule as an example, it is recommended to rehearse different fueling scenarios in the key high-intensity sessions during the week (such as threshold intervals, hill repeats) and the long weekend rides: high-intensity days focus on rapid energy supply and central nervous system activation, while long-distance days focus on sustained energy supply, gastrointestinal tolerance, and recovery. Through repeated rehearsal, the body can execute the optimal fueling rhythm in an “automated” manner on race day, leaving mental resources for pacing and tactical decisions. Remember, the goal of a nutritional strategy is not to pursue theoretical perfection, but to remain stable and reliable 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, but eating carelessly during regular training.” This is precisely putting the cart before the horse: regular training is the best laboratory for building gut tolerance, testing dosages, and cultivating fueling rhythm. If you hope to execute a fueling plan of 80 grams of carbohydrates per hour on race day, you must rehearse it repeatedly in training until your body becomes accustomed to it; if you want to rely on a certain supplement, you must confirm in training that it is truly effective for you and has no side effects. It is recommended to integrate a nutrition log with a training log, recording the 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 aspect of “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 part of training and take it seriously, rather than as an afterthought you scramble for before races—your improvement curve will be noticeably different.

Local Application in Taiwan

Taiwan’s unique climate, terrain, and race culture add localization considerations to the application of the “LCHF diet.” Taiwan’s summer heat and humidity often push the perceived temperature above 35°C, with sweat rates and fluid-electrolyte losses far exceeding the research conditions of temperate countries. This means that hydration and fueling recommendations from foreign literature often need to be “adjusted upward.” In events like the Westbound Wuling climb, which ascends from sea level to 3,275 meters, the appetite suppression at high altitude, cold temperatures, and prolonged exercise pose a severe test for energy planning.

Regarding 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 cyclists planning classic routes such as Sun Moon Lake, Wuling, Beiyi, Buyanting, and the East Coast Crossing inventory fueling points along the way in advance, and strengthen sodium and fluid supplementation in response to Taiwan’s humid and hot environment. Athletes in the Taroko Marathon, Taipei Marathon, and various local triathlon events should likewise incorporate the above local climate factors into their personalized nutrition plans to perform at their best under subtropical conditions.

Common Myth-Busting

Myth: The myth is that “LCHF is suitable for all endurance sports.” In fact, it may only be beneficial in very low-intensity, ultra-long-duration events; for events requiring sprinting or high intensity, it is actually a burden.

This type of myth spreads widely often because it “sounds reasonable,” is easy to pass along by word of mouth, or is amplified by marketing rhetoric. Yet the value of science lies in testing intuition with rigorous evidence: many seemingly obvious notions fail to hold up under the scrutiny of randomized controlled trials. The field of sports nutrition is especially full of oversimplified “panacea” style promotion, compressing complex dosages, timing, and individual differences into a slogan. The next time you hear a categorical nutritional claim, it is worth asking: “What is the level of evidence for this claim? Who is the target population? Are the dosage and timing clear?” 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

The “LCHF diet” 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 dosage, appropriate timing, individualized adjustment, and synergy with overall training, recovery, and sleep. For endurance sports enthusiasts in Taiwan, while mastering the scientific principles, it is even more important to combine local climate, terrain, and race characteristics to transform general rules into a personalized prescription suited to oneself. May every cyclist sweating on Wuling, in the rift valleys, and on round-island routes break through their limits through scientific nutritional strategies and enjoy the pure joy that sport brings. Before you next step onto the race course, do not forget—your bottle holds not just water and sugar, but an entire system of validated sports science.

Related Reading

• Gastrointestinal Tolerance of Energy-Dense Foods in Ultramarathon Races: A Study on Trail Running Event Nutrition
• Low-Carb High-Fat and Ketogenic Diets: The Biggest Controversy in Endurance Sports, a Triathlon Coach’s Fifteen-Year Observation
• Fat Oxidation Capacity Training: A Study on Mitochondrial Adaptations from Low-Carb High-Fat Diets Combined with Training
• The Athlete’s Guide to Low-Carb and Ketogenic Diets: The Real Evidence for LCHF, the Cost of Fat Adaptation, and Who It’s Actually For