Breaking the 90g-Per-Hour Barrier: Dual-Source Carbohydrate Oxidation Dynamics and Precision Fueling Strategies for Ultra-Endurance Events

1. Introduction and Cutting-Edge Research Background

In the realm of ultra-endurance sports, whether it is tackling the 3,275-meter climb of the East Route to Wuling, the relentless rolling hills of Yangmingshan’s “Wind and Sword” course, or the 226-kilometer ordeal of IRONMAN KONA, the ultimate limiting factor of athletic performance often lies not in the muscles, but in the resilience of the “energy supply chain.” Over the past two decades, one of the most significant breakthroughs in sports nutrition science has been the complete rewriting of our understanding of the upper limits of carbohydrate (CHO) absorption.

Historical Evolution: The Cognitive Leap from 30g to 120g per Hour

As early as the 1980s, the sports science community generally believed that the human body could only oxidize approximately 30 to 60 grams of exogenous carbohydrate per hour during exercise. This view was primarily based on experimental data using glucose alone or glucose polymers (such as maltodextrin). The physiological explanation at the time was that intestinal glucose absorption depends on the sodium-glucose cotransporter 1 (SGLT1), and the saturation rate of this transport pathway limited the flow of glucose into the bloodstream. However, with a series of pioneering studies published by researchers such as Jentjens and Jeukendrup in the early 2000s, a new perspective emerged: the human intestine possesses two independent and parallel carbohydrate absorption pathways.

The Birth and Empirical Validation of the Dual-Channel Theory

In addition to SGLT1, fructose is primarily absorbed via facilitated diffusion through glucose transporter 5 (GLUT5). More importantly, after fructose is absorbed into the enterocytes, its subsequent metabolic pathways do not completely overlap with those of glucose, and the two are processed differently in the liver. This means that by ingesting glucose and fructose simultaneously, athletes can “divert” the intestinal transport load, bypassing the saturation limits of a single channel. Data from the Jeukendrup laboratory showed that when athletes ingested glucose and fructose at a rate of 1.8 g/kg body weight per hour (at a ratio of approximately 2:1), exogenous carbohydrate oxidation rates reached 1.2 to 1.5 g/min (72 to 90 g/hr), significantly higher than the 1.0 to 1.1 g/min (60 to 66 g/hr) observed with glucose alone. Further research indicated that increasing the ingestion rate to 144 g/hr (at a 1.8:1 ratio) could push oxidation rates close to 2.0 g/min (120 g/hr), although oxidation efficiency (oxidized/ingested) would slightly decrease. Nevertheless, the substantial increase in absolute oxidation is decisive for prolonged, high-output events.

The Specific Demands of Taiwan’s Race Environment

In Taiwan’s ultra-endurance scene, whether facing the grueling 55-kilometer, 2,800-meter climb of the “West Route to Wuling” from the Geographic Center Monument to the Wuling parking lot, or the “One-Day Double Tower” (Danta) spanning 520 kilometers with riding times often reaching 15 to 20 hours, athletes confront not only muscle fatigue but also a glucose supply crisis for the central nervous system. When liver glycogen is depleted and exogenous glucose supply is insufficient, the brain emits strong fatigue signals, forcing the body to slow down or even stop. Therefore, establishing a precise fueling strategy based on dual-carbohydrate source kinetics has become an essential discipline for top amateur athletes and professional cyclists aiming to break their personal bests.

2. Core Mechanisms of Exercise Physiology and Biomechanics

Molecular Dynamics of Intestinal Absorption

To understand how dual carbohydrate sources break through oxidation limits, one must first grasp the molecular-level mechanisms of intestinal absorption. Glucose and maltodextrin (composed of multiple glucose units) are primarily actively transported across the small intestinal brush border membrane via SGLT1. This process depends on the sodium ion concentration gradient across the cell membrane, which is maintained by the Na⁺/K⁺-ATPase on the basolateral membrane. The transport rate (Vmax) of SGLT1 is approximately 0.5 to 1.0 µmol per minute per centimeter of intestine and is easily saturated.

Fructose, on the other hand, undergoes facilitated diffusion via GLUT5. This process does not directly consume ATP but relies on the fructose concentration gradient in the intestinal lumen. The transport capacity of GLUT5 was originally thought to be low, but with sustained fructose ingestion, GLUT5 expression in enterocytes increases significantly (this is the molecular basis of intestinal adaptation). More critically, fructose is rapidly phosphorylated to fructose-1-phosphate within the enterocyte. This step consumes ATP but also accelerates the intracellular metabolic clearance of fructose, maintaining the concentration gradient from the lumen into the cell, thereby promoting continuous absorption.

Hepatic Metabolic Partitioning and Oxidative Energy Supply

After absorption, glucose and fructose enter the liver via the portal vein. Glucose can directly enter the systemic circulation for muscle utilization or be converted into glycogen for storage. Fructose is primarily taken up by the liver, and its metabolic pathway bypasses the regulatory checkpoint of phosphofructokinase (PFK-1), meaning its breakdown is not tightly inhibited by cellular energy status. In the liver, fructose is converted into glucose, lactate, or free fatty acids, with approximately 20% to 30% of fructose being converted to lactate and released into the bloodstream. Lactate itself is a high-quality energy substrate that muscles can utilize immediately, and during exercise, it is preferentially oxidized by the myocardium and slow-twitch muscle fibers.

The calculation model for exogenous carbohydrate oxidation (ExoCHOox) is as follows:

[
ExoCHOox (g/min) = \frac{Ra_{exo} + Ra_{endo} \times (1 - HGP)}{V_{CO2} \times 20.2}
]

In practical application, we more commonly use isotope tracer methods (such as ¹³C-labeled glucose and fructose) to distinguish between exogenous and endogenous (liver glycogen) oxidation ratios. Research shows that when total carbohydrate ingestion reaches 120 g/hr (glucose:fructose = 2:1), peak exogenous oxidation rates can reach 1.7 to 2.0 g/min, and the glycogen-sparing effect can reach approximately 20% to 30%.

Why the 1:0.8 to 2:1 Ratio Range?

This ratio range is not arbitrary but is calculated based on the “maximum synergistic transport capacity” of SGLT1 and GLUT5. Assuming the maximum transport rate of SGLT1 is approximately 60 g/hr of glucose, and the maximum transport rate of GLUT5 after adaptation is approximately 40 to 50 g/hr of fructose, the total absorption capacity is approximately 100 to 110 g/hr. If the glucose proportion is too high (>2:1), SGLT1 will be overloaded, and unabsorbed glucose will remain in the intestinal lumen, causing osmotic diarrhea and gastrointestinal distress. If the fructose proportion is too high (<1:0.8), GLUT5 becomes overloaded, and the rate of excess fructose conversion to lipids in the liver increases, potentially causing hepatic metabolic stress and blood lipid fluctuations.

The kinetic ratio formula can be expressed as:

[
R_{total} = \frac{R_{glucose}}{K_{m, SGLT1} + R_{glucose}} + \frac{R_{fructose}}{K_{m, GLUT5} + R_{fructose}}
]

When (R_{glucose} : R_{fructose}) falls between 1:0.8 and 2:1, the saturation levels of both channels and the total absorption rate achieve an optimal balance.

3. Key Parameter Measurements and Comparative Analysis

Laboratory Data vs. Real-World Comparison

The following comparison table, compiled from literature and simulated measured data, presents exogenous oxidation rates and gastrointestinal distress risk under different carbohydrate source ratios and ingestion rates:

Absorption Rate Changes Under Different Temperatures and Environments

4. Periodized Training Plans and Gut Adaptation Tuning Guide

The Scientific Basis of Gut Training

The gut is not a static conduit; through regular training, the efficiency and tolerance of intestinal carbohydrate absorption can be significantly enhanced. Research shows that 4 to 6 consecutive weeks of “high-carbohydrate fueling training,” performed 3 to 4 times per week, can increase GLUT5 and SGLT1 expression by approximately 40% and 20%, respectively. This adaptation process must be progressive; otherwise, gastrointestinal distress is highly likely.

Eight-Week Gut Adaptation and Fueling Practice Plan

The Interaction Between Intensity and Fueling

It is important to note that intestinal absorption rate decreases as exercise intensity increases. When intensity exceeds the threshold (e.g., above 85% of FTP), intestinal blood flow is significantly shunted to working muscles, leading to reduced absorption efficiency. Therefore, in training plan design, the main fueling should be completed 30 minutes before high-intensity intervals, avoiding large intakes during high-intensity periods.

5. Race Fueling, Environmental Adaptation, and Practical Strategies

Precise Fueling Schedule for 5 to 15 Hour Events

Using the “One-Day Double Tower” (Danta) and the “KONA bike leg” as examples, here is a practical strategy for ingesting 90 to 120 grams per hour:

Special Fueling Strategy for the East Route to Wuling

For long-climb events like the East Route to Wuling (approximately 4 to 6 hours), where intensity is continuously maintained at the Zone 3 to Zone 4 boundary, intestinal blood flow is restricted. It is recommended to reduce hourly intake to 75 to 90 grams, primarily using liquid energy drinks to avoid gel retention in the stomach. Additionally, since the altitude gain exceeds 2,500 meters, attention should be paid to the risk of intestinal edema in hypoxic environments; fuel solution concentration should not exceed 8%.

Coping with High-Heat Environments (e.g., KONA Bike Leg)

The KONA bike leg often involves high temperatures and strong winds, where fluid loss rates can exceed 1.5 liters per hour. It is recommended to increase total hourly fluid intake to 800 to 1,000ml, and reduce carbohydrate concentration to 6% to 7% to ensure gastric emptying rate. Simultaneously, sodium intake should be increased to 600 to 800mg per hour to maintain plasma volume and intestinal absorption efficiency.

6. Common Operational Mistakes and Scientific Myth-Busting

Myth 1: “If dual carbohydrate sources work, wouldn’t triple or quadruple sources work even better?”

Products containing a third sugar (such as isomaltulose) or a fourth sugar (such as agave syrup) have appeared on the market. However, current scientific evidence shows that additional sugar sources that do not provide a transport pathway independent of SGLT1 and GLUT5 only increase osmotic pressure and gastrointestinal burden. Although isomaltulose has a low glycemic index, its oxidation rate is far lower than that of glucose and fructose. In high-intensity events, it occupies stomach capacity without providing immediate energy. It is recommended to limit total sugar sources to two and strictly control the ratio.

Myth 2: “If 120g per hour works in races, I should train that way too.”

Gut adaptation takes time. Attempting 120 g/hr directly in high-intensity training without prior adaptation is highly likely to cause severe diarrhea and dehydration, ruining training quality. Training intake should be progressively increased according to the training plan phase, and high intake (>100g/hr) is only recommended for long-distance training sessions exceeding 4 hours or race simulations. For daily training within 2 hours, maintaining 60 to 80 g/hr is sufficient.

Myth 3: “Natural fructose from fruit is better than artificial fructose?”

This is a serious misconception. Whether fructose comes from fruit or is industrially produced, the absorption mechanism in the intestine is identical. Fruit also contains fiber, water, and other organic acids, which can delay gastric emptying and increase satiety, but this also means it cannot provide sufficient carbohydrate density per unit of time. Taking a banana (containing approximately 12 to 15 grams of carbohydrate, of which about 5 grams is fructose) as an example, to reach an intake of 90 grams per hour, one would need to eat 6 to 8 bananas. This is clearly impractical and likely to cause bloating. During exercise, precisely formulated gels and energy drinks should be the primary fuel source, with fruit reserved for post-race recovery.

Myth 4: “Consuming fructose causes liver fat accumulation and is harmful to health?”

This myth originates from studies linking high fructose consumption at rest (exceeding 100g per day) to non-alcoholic fatty liver disease. However, during prolonged endurance exercise, fructose is rapidly directed toward oxidative pathways or converted to lactate for muscle utilization, and does not activate significant de novo lipogenesis pathways. The hepatic processing of fructose during exercise is fundamentally different from that at rest. According to isotope studies, the proportion of fructose converted to free fatty acids during exercise is less than 2%.

7. Expert FAQ

Q1: How do I determine whether a 1:0.8 or 2:1 ratio is right for me?

This depends on your event duration and gastrointestinal tolerance. The principle is: the longer the event, the higher the fructose proportion should be (trending toward 1:0.8 or even 1:1), because prolonged exercise depletes liver glycogen, and fructose can more directly replenish glycogen precursors. It is recommended to conduct A/B testing during training: use a 2:1 ratio for weeks 1-2, recording subjective GI scores (1-10) and performance; switch to a 1:0.8 ratio for weeks 3-4 and compare the differences. If 1:0.8 does not cause significant bloating or diarrhea and power output is better maintained, use this ratio for the race.

Q2: When consuming energy gels and energy drinks simultaneously, how do I calculate total carbohydrate intake?

Precise calculation is essential. For example, if each gel pack contains 25g of carbohydrate (10g fructose, 15g maltodextrin) and each 500ml bottle of energy drink contains 40g of carbohydrate (maltodextrin:fructose = 2:1): if you consume 2 gel packs and 1 bottle of energy drink per hour, total carbohydrate is 25×2 + 40 = 90g, with total fructose at 10×2 + 13.3 ≈ 33.3g and maltodextrin at 15×2 + 26.7 ≈ 56.7g, giving an actual ratio of approximately 1.7:1, which falls within the recommended range. It is recommended to use a spreadsheet or sports app to track hourly intake composition.

Q3: How should fueling be adjusted during climbs (such as Wuling)?

During climbs, due to increased intensity and respiratory rate, the swallowing action may trigger the gag reflex. It is recommended to complete the main fueling on flat or gentle sections before the gradient steepens, and during the climb, only use small, frequent liquid intake. Additionally, pre-squeeze gels into a soft flask and consume by “sipping” to avoid the action of tearing open packaging interfering with bike handling. If the gradient exceeds 8%, it is recommended to reduce hourly intake by 20% to reduce gastrointestinal pressure.

Q4: Will blood glucose fluctuate dramatically after consuming dual carbohydrate sources? Should I worry about insulin response?

During exercise, because muscle contraction promotes the translocation of GLUT4 transporters to the cell membrane, muscle glucose uptake is in a “non-insulin-dependent” state. Therefore, consuming dual carbohydrate sources during exercise does not cause significant blood glucose fluctuations. However, consuming high-glycemic-index carbohydrates (including maltodextrin and fructose) immediately after exercise actually accelerates muscle glycogen resynthesis, and the insulin response at this time is desirable. The key point is that within 30 minutes after the event ends, you should consume 1.2g of carbohydrate per kilogram of body weight (with a 1:1 dual-source ratio) to maximize recovery efficiency.

Q5: If I experience gastrointestinal distress during a race, what should I do?

First, immediately stop consuming any solid or hypertonic fluids and switch to small sips of plain water. If symptoms persist for more than 20 minutes, reduce intensity to Zone 1 (recovery zone) to allow intestinal blood flow to return. Do not consume high-concentration sugar solutions or caffeine at this time, as this will exacerbate intestinal irritation. Prepare emergency GI medications (such as anti-diarrheal agents) before the race and discuss personal allergies and medication contraindications with your physician beforehand. If symptoms are severe, immediately seek assistance from the race medical station. Remember, a severe gastrointestinal collapse can lead to dehydration and electrolyte imbalance; it is better to lose 15 minutes than to push through and risk a DNF.

Key References: Jeukendrup, A.E. (2010) “Carbohydrate and exercise performance: the role of multiple transportable carbohydrates” published in Sports Medicine; Rowlands, D.S. et al. (2015) “Effect of graded fructose coingestion with maltodextrin on exogenous carbohydrate oxidation and exercise performance” published in Medicine & Science in Sports & Exercise.