113 vs 226 Energy Metabolism Matrix: Full Analysis of Glycogen Threshold, Fat Oxidation Limits, and Fueling Rate Differences

1. Introduction and Cutting-Edge Research Background

In triathlon, the 113 (i.e., 70.3, comprising a 1.9 km swim, 90 km bike, and 21.1 km run) and the 226 (i.e., 140.6, comprising a 3.8 km swim, 180 km bike, and 42.2 km run) are not merely multiples of each other in distance; the two events operate on fundamentally different energy metabolism logic. Over the past decade, sports science research on “endogenous glycogen stores” and “exogenous carbohydrate oxidation rates” has completely transformed the race pacing philosophy of elite athletes.

According to a meta-analysis published in Sports Medicine in 2021, total glycogen stores in the human liver and skeletal muscles range from approximately 400 to 600 grams (about 1,600 to 2,400 kcal). However, this “internal fuel tank” cannot be withdrawn from indefinitely. Research indicates that when muscle glycogen concentration falls below 150 mmol per kilogram of wet weight (mmol/kg ww), the body activates a “glycolysis inhibition” mechanism, causing a sharp decline in power output; when concentrations drop below 100 mmol/kg ww, athletic performance experiences an irreversible cliff-like decline.

This means that whether racing 113 or 226, athletes must find a dynamic equilibrium between “glycogen depletion” and “fat oxidation.” However, the equilibrium point differs drastically between the two. In a 4-to-6-hour 113 race, athletes can sustain blood glucose stability through 60 to 80 grams of exogenous carbohydrate per hour, relying on remaining glycogen for a “final showdown” in the last half marathon. But in an 8-to-14-hour 226 race, maintaining the same fueling rate and intensity output almost guarantees hitting the so-called “wall”—central nervous system fatigue and muscular metabolic failure—late in the bike leg or early in the marathon.

In recent years, sports nutrition researchers led by Jeukendrup have proposed the “Multiple Transportable Carbohydrates” (MTC) theory, confirming that when athletes ingest a mixture of glucose and fructose (at a ratio of approximately 1:0.8 to 1:1), the intestinal SGLT1 and GLUT5 transporter proteins can operate in parallel, raising the hourly oxidation ceiling from 60 grams for a single carbohydrate to 90–120 grams. However, achieving this physiological limit requires highly adapted gut training and rigorous osmolality management—it cannot be accomplished by “cramming” right before race day.

This article adopts the dual perspectives of exercise physiology and biomechanics to construct an energy metabolism mathematical matrix for the 113 and 226, deeply analyzing the essential differences in glycogen critical thresholds, fat oxidation rate limits, and fueling rates. It also provides periodized training plans and race-day strategies directly applicable to real-world courses such as the East/West Wuling ascent, IRONMAN Taiwan (Penghu), and the KONA World Championship.

2. Core Mechanisms of Exercise Physiology and Biomechanics

2.1 The “Crossroads” of Energy Metabolism: The Crossover Concept

The “Crossover Concept,” proposed by Brooks and Mercier in 1994, remains the classic model explaining the relationship between exercise intensity and fuel selection. The theory states that at rest, the oxidation ratio of fat to glucose is approximately 7:3. As exercise intensity increases, sympathetic nervous system activation and rising plasma catecholamine concentrations promote glycogenolysis and muscular glucose uptake, causing the proportion of carbohydrate oxidation to rise sharply.

At 113 race intensity (approximately 75%–85% of maximum heart rate, or 85%–95% of Functional Threshold Power), the energy contribution ratio of carbohydrate to fat is approximately 65% to 35%. In other words, in a 5-hour 113 race with a total energy expenditure of 3,500 kcal, approximately 2,275 kcal would come from carbohydrate (equivalent to 568 grams)—approaching or even exceeding the physical limit of the body’s glycogen stores. Therefore, the key to winning a 113 lies in “delaying glycogen depletion” and preserving a “critical minority” of fast-burning fuel for the final run segment.

In contrast, at 226 race intensity (approximately 65%–75% of maximum heart rate, or 65%–80% of FTP), the energy contribution ratio reverses to 55% fat versus 45% carbohydrate. Taking an 11-hour race with a total expenditure of 8,000 kcal as an example, carbohydrate consumption would be approximately 3,600 kcal (900 grams)—far exceeding endogenous stores. This means more than 300 grams of carbohydrate must be filled by “exogenous fueling.” If the fueling rate cannot keep pace with the consumption rate, the body is forced to accelerate muscle protein breakdown for gluconeogenesis, leading to muscle loss and immune function decline.

2.2 A Biomechanical Model of the Glycogen Critical Threshold

We can establish a simplified energy balance equation to describe glycogen dynamics:

[
\frac{dG}{dt} = R_{\text{exogenous carbohydrate}} + R_{\text{gluconeogenesis}} - R_{\text{carbohydrate oxidation}} - R_{\text{glycogen synthesis}}
]

Under steady-state conditions, the net glycogen depletion rate (( -dG/dt )) must be less than the exogenous carbohydrate absorption rate; otherwise, the athlete will hit the glycogen critical threshold at a specific time point (( T_{\text{critical}} )). ( T_{\text{critical}} ) can be estimated using the following formula:

[
T_{\text{critical}} = \frac{G_{\text{initial}} - G_{\text{critical}}}{(R_{\text{carbohydrate oxidation}} - R_{\text{exogenous carbohydrate}} - R_{\text{gluconeogenesis}})}
]

Assume a 70 kg male athlete with an initial muscle glycogen of 500 grams, a critical threshold of 100 grams, and a gluconeogenesis rate of approximately 15 grams per hour. At 113 intensity (carbohydrate oxidation rate of 110 g/h), if exogenous fueling reaches only 60 g/h, then:

[
T_{\text{critical}} = \frac{500 - 100}{110 - 60 - 15} = \frac{400}{35} \approx 11.4 \text{ hours}
]

This appears safe, but the formula ignores the variation in “local muscle glycogen.” During the bike leg, the quadriceps’ glycogen consumption rate is 3 to 5 times that of the upper body. By the time the athlete transitions to the run, local glycogen in the quadriceps may have dropped below 80 mmol/kg ww. Even if total body glycogen remains high, power output will collapse due to local energy crisis. This is the physiological root cause of why many 113 athletes who push “too hard on the bike” experience “cramping legs and an inability to walk” during the run segment.

2.3 The “Ceiling” of Fat Oxidation Rate

Although fat oxidation is the primary energy source in ultra-endurance events, its rate has a strict physiological ceiling. Research shows that highly trained endurance athletes achieve a maximal fat oxidation rate (Fatmax) of approximately 0.5 to 1.0 grams per minute (i.e., 30 to 60 grams per hour), and this rate is only attainable within the “fat-burning zone” of 55%–65% of maximum heart rate. When intensity exceeds 75% of maximum heart rate, fat oxidation rates decline sharply due to “carbohydrate-induced suppression of fat oxidation.”

In a 226 race, if an athlete can ride at 65% of maximum heart rate, fat oxidation can contribute approximately 45 grams of energy per hour (about 405 kcal), representing 50%–60% of total energy expenditure. However, when encountering headwinds or climbs (such as the crosswinds on the Penghu跨海大橋 or the long gradual climbs on KONA’s Queen K Highway), power demands spike instantaneously. Fat oxidation rates cannot keep pace synchronously, forcing the body to shift toward carbohydrate and accelerating glycogen depletion. Therefore, 226 pacing strategy is essentially a delicate coordination between the “fat engine” and the “carbohydrate turbocharger.”

3. Key Parameter Measurements and Comparative Analysis

To provide concrete scientific evidence, the following comparison of key energy metabolism parameters between the 113 and 226 is compiled from multiple empirical studies published in the Journal of Applied Physiology and Medicine & Science in Sports & Exercise:

Table 1: Comparison of Key Energy Metabolism Parameters: 113 (70.3) vs. 226 (140.6)

Table 2: Simulated Hourly Energy Deficit for Athletes of Different Body Weights in Both Distances

Table 2 clearly shows that heavier athletes face more severe energy deficits in 113 races. This is because carbohydrate oxidation rates correlate positively with body weight at high intensities, but intestinal absorption rates have an absolute ceiling. Therefore, heavier athletes’ 113 strategy must emphasize “lowering intensity for greater efficiency”; otherwise, they are highly susceptible to a serious energy crisis during the run segment.

4. Periodized Training Plans and Equipment Setup & Adjustment Guide

4.1 The 113 Athlete’s “Lactate Tolerance and Fueling Sprint” Plan

The 113 is essentially an “extended high-intensity aerobic race.” Training focus should be on increasing muscle glycogen storage capacity and intestinal absorption efficiency.

Phase 1 (Base Period, Weeks 1–4): Glycogen Supercompensation and Fatmax Optimization

• Long slow distance (LSD) rides: Once per week, 3–4 hours, heart rate Zone 2 (65–72% LTHR). Deliberately reduce fueling to 30–40 g/h of carbohydrate to promote fat oxidation enzymes (CPT-1) and mitochondrial density.
• Run strength training: Twice per week, including single-leg deadlifts and Bulgarian split squats, emphasizing quadriceps eccentric strength to handle the glycogen-depleted state in the final run segment.

Phase 2 (Race-Specific Period, Weeks 5–10): Simulated Race Intensity and Gut Training

• Bike tempo sessions: Twice per week, 3 x 20 minutes at 85–90% FTP with 5-minute recoveries. During the ride, ingest 20 grams of a glucose+fructose mixture every 15 minutes to train gut tolerance.
• Brick transition runs: Once per week, ride 2.5 hours (Zone 3) followed immediately by a 40-minute run at 85% of your 10K best pace, simulating the “heavy legs” sensation of the 113 run segment.

4.2 The 226 Athlete’s “Fat Engine and Power Management” Plan

The training philosophy for the 226 is to “reduce hourly energy expenditure and increase the proportion of fat contribution.” This requires a large volume of low-intensity aerobic base work and strict power management discipline.

Phase 1 (Base Period, Weeks 1–8): Building “Fat Adaptation”

• Morning fasted aerobic sessions: Three times per week, 60–90 minutes of Zone 1–2 running or cycling in a completely fasted state (water only), promoting upregulation of fat oxidation pathways.
• Long rides: Every two weeks, 5–6 hours, heart rate strictly controlled in Zone 2 (65–72% LTHR), power output maintained at 65–75% FTP, fueling limited primarily to electrolytes and small amounts of amino acids.

Phase 2 (Pre-Race Specific Period, Weeks 9–16): Simulating the 226’s “Fueling Rate Ceiling”

• Long ride + long run double-day: Saturday, ride 5 hours (Zone 2) with a fueling training goal of 90–120 g/h of MTC; Sunday, run 2.5–3 hours (Zone 2–3a), continuing the same fueling rhythm.
• Bike “intensity variations”: During long rides, include a 5-minute Zone 3 climb every 30 minutes (simulating the Tianxiang to Bilu Shenmu section of the East Wuling ascent), training the ability to maintain fueling rhythm amidst power fluctuations.

4.3 Equipment Setup and Power Distribution Recommendations

• 113 Race: An aero bike or time trial bike is recommended, with a more aggressive riding position. However, be mindful of “iliopsoas tightness” issues during the run segment. Power distribution should prioritize “steady output,” with the difference between NP (Normalized Power) and AP (Average Power) kept within 5% to avoid excessive surges that could trigger an energy crisis in the later run.
• 226 Race: A more comfortable endurance road bike or triathlon bike is recommended. Cockpit setup should prioritize “reducing lower back pressure and maintaining pedaling smoothness.” The power strategy employs a “descending intensity approach”: maintain 75% FTP for the first third, drop to 70% in the middle, and if perceived exertion allows, increase back to 75% in the final third to preserve quadriceps glycogen for the run.

5. Race Fueling, Environmental Adaptation, and Race-Day Strategies

5.1 The “Quality” and “Quantity” of Carbohydrate Fueling

In a 113 race, the fueling rate target should be 60–80 grams per hour, primarily using a single carbohydrate (glucose), because at higher intensities, intestinal blood flow decreases, and complex MTC formulas may increase osmolality and the risk of gastrointestinal discomfort. During the bike leg, aim to ingest 15–20 grams of carbohydrate every 15 minutes (equivalent to 60–80 g/h), paired with 500–750 ml of fluid. Once on the run, as gastrointestinal blood flow decreases further, switch to 20 grams of energy gel or liquid fuel every 20 minutes.

In a 226 race, the fueling rate must be increased to 90–120 grams per hour, and it is essential to use an MTC formula of “glucose + fructose” (ratio 1:0.8). Taking IRONMAN Taiwan (Penghu) as an example, the hot, humid conditions on the bike course can cause sweat rates as high as 1.2 to 1.5 liters per hour. In this case, electrolyte replacement should be prioritized, with 800–1,000 mg of sodium per hour to prevent hyponatremia.

5.2 Environmental Adaptation: From Yangmingshan’s Windy Sword to KONA’s Dry Heat

Taiwan’s 113 races (such as Taitung’s Living Water Lake and Kenting IRONMAN 70.3) are predominantly characterized by high humidity and high temperatures, which significantly increase cardiovascular strain, raising heart rate by 5–10 beats per minute compared to normal conditions at the same power output. Athletes are advised to undergo “passive heat adaptation” 7–10 days before the race, consisting of 30–40 minutes of hot water baths at 40°C daily, or 60 minutes of low-intensity cycling in a hot environment, to increase plasma volume and skin blood flow efficiency.

For 226 athletes heading to KONA or Penghu, the dual challenges of “dry heat” and “crosswinds” must be addressed head-on. KONA’s Queen K Highway frequently experiences crosswinds of 20–30 km/h, which can increase aerodynamic drag power consumption by 15% to 25%. In such conditions, make good use of “legal drafting zones” (except in non-drafting races), reduce frontal area in your riding position, and adjust your power pacing to reserve a 5% power buffer for wind variations.

6. Common Operational Mistakes and Scientific Myth-Busting

Myth 1: “Just eat more energy gels and you’ll avoid hitting the wall”

This is the most common fatal error among 113 athletes. As mentioned earlier, the intestinal carbohydrate absorption rate has an absolute ceiling (60 g/h for a single sugar). When intake exceeds the transporter’s saturation rate, excess carbohydrate remains in the gut, causing osmotic diarrhea and severe bloating, which accelerates dehydration and electrolyte imbalance. The scientific approach is: perform glycogen supercompensation in the 3 days before the race (10–12 g/kg body weight of carbohydrate per day), strictly control hourly intake during the race, and repeatedly test gut tolerance in training.

Myth 2: “The 226 only requires endurance training, not speed work”

Although the average intensity of a 226 is lower, climbs, headwinds, and the “test of will” in the final marathon all require a certain speed reserve. If you only train in Zone 2 for extended periods, the oxidative capacity of Type IIa muscle fibers will decline, making you more prone to prematurely tapping into glycogen when facing power fluctuations. It is recommended that 226 athletes still include one weekly Zone 3–4 “tempo ride” or “hill intervals” to maintain neuromuscular recruitment efficiency.

Myth 3: “Once fat-adapted, you don’t need to fuel with carbohydrates”

Some proponents of extreme “keto adaptation” argue that once the body adapts to a high-fat diet, it can rely entirely on fat for fuel. However, scientific evidence shows that even in a highly fat-adapted state, the central nervous system still requires glucose (approximately 120 grams per day). In a 226 race, when intensity rises above Zone 3, carbohydrate oxidation demand instantly exceeds the upper limit of fat oxidation. If endogenous glycogen is severely depleted, it can lead to severe central fatigue and impaired decision-making. The pragmatic approach is to “use fat adaptation as a training tool, but still supplement with carbohydrates during the race.”

Myth 4: “At the start of the run segment, you should immediately accelerate to make up time”

In the transition area of a 113 or 226, many athletes who underperformed on the bike will “sprint” at the start of the run to make up time. However, transitioning from the seated cycling position to the upright impact of running involves entirely different muscle recruitment patterns. At this point, glycogen stores in the quadriceps and calf muscles are already at low levels. Immediately pushing intensity into Zone 4 will accelerate glycogen depletion and trigger severe cramping. The correct strategy is to “run the first 3 kilometers after transition 5–10 seconds per kilometer slower than your target pace,” allowing the cardiovascular system and muscle metabolism to gradually adapt.

7. Expert FAQ

Q1: What is the core difference in “bike power pacing” between the 113 and the 226?

A: For the 113, power pacing should center on “maintaining a stable output above threshold,” with a recommended riding intensity of 85–90% of FTP and a power coefficient of variation (CV) kept within 5% to avoid excessive surges that cause lactate accumulation. For the 226, the priority is “efficiency,” with a recommended riding intensity of 65–75% of FTP. Short bursts up to 80% FTP are acceptable on climbs, but on descents or tailwind sections, you must actively “back off” to keep average power within the target range. Overall, power management in the 226 is more conservative than in the 113, because every watt of excessive output will be repaid with “run slowdown” ten hours later.

Q2: How do you train the gut to tolerate 120 grams of carbohydrate per hour?

A: Intestinal transporter proteins (SGLT1 and GLUT5) can be upregulated through training. Starting 8–12 weeks before the race, perform 2–3 “fueling simulation sessions” per week: during long rides or runs, deliberately increase hourly carbohydrate intake from 60 grams progressively to 90, then to 120 grams, while recording the degree of gastrointestinal discomfort (bloating, nausea, urge to defecate). Simultaneously, simulate the race’s fluid intake rhythm (750 ml per hour) during training, as adequate fluid is a prerequisite for carbohydrate absorption. Do not suddenly increase fueling volume two weeks before the race; this will only cause severe intestinal distress.

Q3: What is the optimal strategy for caffeine and electrolyte supplementation in a 226 race?

A: Caffeine has been shown to reduce ratings of perceived exertion (RPE) and enhance fat oxidation efficiency. It is recommended to ingest 200 mg of caffeine (approximately one cup of American coffee) in the early part of the race (hours 1–2), and an additional 100–200 mg mid-race (hours 5–6). However, total intake should not exceed 6 mg per kilogram of body weight, as higher doses may cause heart palpitations and diuretic effects. For electrolytes, aim for 600–900 mg of sodium per hour, paired with 100–200 mg of potassium. In hot race conditions (such as a Penghu summer), increase sodium intake to 1,000 mg/hour and consider using hypertonic salt tablets.

Q4: What are the unique energy metabolism challenges of the East Wuling ascent (from Puli to Wuling, elevation 2,800 meters)?

A: Although the Wuling course is short (approximately 55 km), the average gradient exceeds 8%, making it a “high-intensity climbing” event. Above 2,000 meters of elevation, the oxygen content of the air drops to about 80% of sea level, causing maximal aerobic capacity (VO2max) to decline by 10–15%. This forces the body to shift toward anaerobic glycolysis earlier, with glycogen consumption rates 20–30% higher than at sea level for the same power output. Therefore, even for 113 or 226 athletes, power pacing should be reduced by 5–8% in the Wuling race, and fueling frequency should be increased (15–20 grams of carbohydrate every 15 minutes) to cope with the metabolic stress of high altitude.

Q5: For a beginner about to tackle their first 226, what are the three most critical pre-race preparations?

A: First, you must establish gut tolerance for “90–100 grams of carbohydrate per hour.” This requires repeated practice over at least 8 weeks of long rides, not just willpower on race day. Second, power pacing on the bike must be “extremely conservative.” It is recommended to stay below 70% of FTP for the first 90 kilometers. It is far better to spend an extra 10–15 minutes on the bike to ensure sufficient quadriceps glycogen reserves for the run segment. Third, glycogen supercompensation in the 3 days before the race and the “final meal” 12 hours before the start are crucial. Aim for 8–10 g/kg body weight of carbohydrate, choosing low-fiber, low-fat, easily digestible foods to ensure glycogen stores are maximized.

Conclusion: From 113 to 226—The Ultimate Test of “Energy Management Wisdom”

The distance difference between the 113 and the 226 is, on the surface, merely a multiple; in essence, it represents two entirely distinct exercise physiology propositions. The 113 tests the explosive power management of “delaying glycogen depletion” at high intensity, while the 226 is a marathon-like game of dancing with the “limits of fat oxidation” and “intestinal absorption rates.” Only through scientific data monitoring, rigorous periodized training, and flexible race-day adaptation can your body’s energy systems achieve perfect harmony at the moment you cross the finish line. Whether you are preparing to challenge the 113 at Taitung’s Living Water Lake or the 226 in Penghu, remember: true speed comes from a deep understanding of, and humble adherence to, your body’s metabolic rhythms.