Glycogen availability remains the single most manipulable substrate-level determinant of endurance competition performance. Research consistently demonstrates that athletes who systematically manage carbohydrate intake across the full competition timeline—from 48 hours pre-event through the final kilometer—outperform those relying on generalized fueling habits. Yet even at elite levels, carbohydrate mismanagement on race day accounts for a disproportionate share of underperformance and preventable DNFs.

The challenge is not total carbohydrate quantity. It is the temporal architecture of intake. Liver and muscle glycogen respond to different loading timelines. Gastric emptying rates shift under sympathetic nervous system activation. Exogenous carbohydrate oxidation has hard physiological ceilings governed by intestinal transporter saturation. Each variable demands a distinct strategic response at a specific point in the competition window, and misaligning any single element cascades through the entire fueling system.

What follows is a three-phase framework for competition-day carbohydrate management: the pre-competition glycogen supercompensation window, the morning-of liver glycogen restoration protocol, and the during-event exogenous fueling strategy. Each phase is calibrated to event duration and intensity, because the metabolic demands of a 90-minute criterium differ fundamentally from those of an ultra-distance triathlon. This is not a one-size prescription but a protocol architecture—a decision framework for constructing individualized race-day nutrition grounded in current substrate metabolism research.

Classical glycogen supercompensation, as described by Bergström and Hultman, required a depletion phase followed by aggressive carbohydrate refeeding. The modified Sherman protocol eliminated that depletion requirement, demonstrating that trained athletes achieve supercompensation through 36-48 hours of elevated carbohydrate intake paired with a training taper alone. Current consensus targets 10-12 g/kg/day during this loading window, pushing muscle glycogen stores toward 150-250 mmol/kg dry weight—roughly double resting concentrations in well-trained endurance athletes.

Whether full supercompensation is warranted depends entirely on event demands. Competitions under 90 minutes at sub-maximal intensity rarely require it—normal glycogen reserves of 80-120 mmol/kg dry weight typically suffice. For events exceeding 90 minutes, or those involving repeated supramaximal efforts like criterium racing and intermittent team sports, the return becomes substantial. Hawley's group demonstrated 2-3% performance improvements in prolonged events when glycogen stores were maximized—a margin that frequently determines podium positions at the elite level.

Source selection during loading matters more than many practitioners acknowledge. High-glycemic-index carbohydrates—white rice, refined cereals, sports drinks, low-fiber breads—produce glycogen synthesis rates of 5-8 mmol/kg dry weight per hour. Adding protein in a 4:1 carbohydrate-to-protein ratio further accelerates storage through insulin-mediated GLUT4 translocation. The common instinct to eat clean during taper actively works against athletes here, as fiber-rich whole grains slow gastric transit and reduce net absorption rates during the critical loading window.

Residual fiber load represents the most commonly mismanaged variable in pre-competition nutrition. Athletes maintaining high-fiber diets through the loading phase frequently present with gastrointestinal distress on race morning. Shifting toward low-residue, easily digestible carbohydrate sources 48 hours before competition simultaneously optimizes glycogen synthesis kinetics and reduces GI risk. This is nutritional periodization applied directly to competition preparation—the days before a race are categorically not the time for whole-grain commitment.

One frequently overlooked consequence of effective loading: each gram of glycogen stores with approximately 3 grams of water. An athlete supercompensating by 600-800 grams of additional glycogen should anticipate 1.8-2.4 kg of associated weight gain. This is physiologically advantageous—it creates an endogenous fluid reservoir supporting thermoregulation and delaying dehydration during competition. Athletes unfamiliar with the protocol often interpret this weight gain negatively, so communicating expectations before the loading phase begins is essential for both compliance and psychological readiness on race morning.

Takeaway

Glycogen supercompensation is a time-limited investment with diminishing returns below 90 minutes of competition—match the magnitude of your loading protocol to the metabolic demands of your specific event, and shift to low-residue carbohydrate sources 48 hours out to optimize both synthesis and GI tolerance.

The pre-competition morning meal serves a fundamentally different physiological purpose than the loading phase. Muscle glycogen, once supercompensated, remains relatively stable overnight. Liver glycogen, however, depletes at approximately 5-6 grams per hour during sleep to maintain blood glucose homeostasis via hepatic glycogenolysis and gluconeogenesis. After an 8-hour sleep, liver stores may be reduced by 40-50%. The morning meal's primary objective is liver glycogen restoration—not further muscle glycogen augmentation.

Timing is the critical variable. Consensus recommends consuming the pre-event meal 3-4 hours before competition start, providing sufficient time for gastric emptying and absorption while avoiding the transient reactive hypoglycemia that can occur when high-glycemic-index carbohydrates are consumed 30-60 minutes before exercise onset. This insulin-mediated blood glucose dip—driven by the exogenous insulin surge coinciding with exercise-induced GLUT4 translocation—is generally self-correcting, but it impairs perception of readiness and can disrupt early pacing strategy.

Composition should target 1-4 g/kg of carbohydrate, scaled inversely with proximity to start time. An athlete eating 4 hours out can tolerate the upper range. Two hours out demands a considerably lighter approach. Fat and fiber should be minimized to accelerate gastric emptying. Moderate protein at 0.15-0.25 g/kg can be included for satiety without significantly slowing absorption. The non-negotiable rule applies here without exception: familiar, well-tolerated foods only. Competition morning is never the time for nutritional experimentation.

Glycemic index of the morning meal warrants nuanced consideration. While high-GI foods are optimal during the loading phase for glycogen synthesis, research by Wee and colleagues suggests that a moderate-GI pre-event meal produces a more stable blood glucose profile during the first competition hour. Their work demonstrated higher fat oxidation rates during subsequent exercise following low-GI pre-event meals, potentially sparing glycogen in early race stages. The practical significance of this effect scales with event duration and the athlete's individual degree of metabolic flexibility.

For athletes with early start times who cannot achieve the full 3-4 hour window, a two-phase approach provides a practical solution. A smaller meal of 1-2 g/kg carbohydrate consumed 2 hours before start, supplemented by a liquid carbohydrate top-up—sports drink or gel with water—15-20 minutes before the gun. This late-stage intake bypasses the reactive hypoglycemia window entirely and delivers readily available exogenous glucose at exercise onset. The prerequisite is rehearsal: this exact protocol must be practiced across multiple training sessions to calibrate individual tolerance and establish genuine confidence in the approach.

Takeaway

The morning meal restores liver glycogen, not muscle glycogen—its design should prioritize rapid absorption and gastrointestinal tolerance over caloric volume, because on race morning what you cannot absorb becomes a liability rather than an asset.

During-event carbohydrate strategy is perhaps the most extensively researched—and most frequently misapplied—component of competition nutrition. The governing principle is direct: exogenous carbohydrate oxidation follows dose-response kinetics up to a hard ceiling determined by intestinal transporter capacity. Single-source glucose maximizes oxidation at approximately 60 g/hour, limited by SGLT1 transporter saturation in the intestinal brush border. Adding fructose, absorbed via the independent GLUT5 transporter, enables total oxidation rates of 90-110 g/hour in trained, gut-adapted individuals.

Event duration dictates fueling magnitude. For events lasting 1-2 hours, 30-60 g/hour provides a performance benefit primarily through central nervous system mechanisms—oral glucose sensing activates reward centers and reduces perceived exertion independent of actual substrate delivery to working muscle. In the 2-3 hour range, a full 60 g/hour from glucose-based sources becomes appropriate as endogenous glycogen depletion becomes increasingly rate-limiting. Beyond 3 hours, multiple transportable carbohydrate formulations targeting 80-100+ g/hour represent the current evidence-based performance ceiling.

The 2:1 glucose-to-fructose ratio has emerged as the evidence-supported standard for multiple transportable carbohydrate formulations, though recent work suggests ratios approaching 1:0.8 may be equally effective with improved gastric tolerance. The non-negotiable point is that fructose must be present to engage the independent GLUT5 absorption pathway. Athletes consuming only glucose-based products above 60 g/hour will experience GI distress from unabsorbed carbohydrate accumulating in the intestinal lumen—regardless of gut training status. This transporter ceiling is physiological, not psychological.

Gut trainability is a genuine and significant physiological adaptation. Research by Cox and colleagues demonstrated that as little as two weeks of elevated carbohydrate intake during training increased intestinal absorption capacity and reduced GI symptoms during subsequent exercise. SGLT1 transporter protein expression upregulates in response to repeated luminal carbohydrate exposure. The practical implication is critical: race-day fueling strategies must be rehearsed systematically across training blocks—not merely for psychological familiarity, but for measurable physiological adaptation of the absorptive apparatus itself.

Delivery format matters under race conditions. Liquid carbohydrate sources—sports drinks, gels consumed with adequate water—generally offer faster gastric emptying than solids, particularly at intensities above 70% VO₂max where splanchnic blood flow is substantially reduced. In ultra-endurance events at lower intensities, solid and semi-solid sources provide psychological variety and contribute to satiety. The optimal competition approach typically combines formats: liquid-dominant during high-intensity segments, with solids introduced during lower-intensity phases or aid station windows where reduced pace allows consumption and GI tolerance permits digestion.

Takeaway

Exogenous carbohydrate delivery is constrained by intestinal transporter biology, not willpower—the 60 g/hour single-source ceiling is a physiological fact, and exceeding it without engaging the fructose pathway guarantees GI failure regardless of mental toughness or training volume.

Competition-day carbohydrate management operates as a three-phase system, each phase targeting distinct physiological mechanisms with different optimization parameters. The loading phase fills the substrate reservoir. The morning meal restores the hepatic ignition system. During-event fueling provides the sustained exogenous supply line. Treating these as a unified sequential protocol rather than isolated ad hoc decisions is what separates systematic performance nutrition from hopeful improvisation.

Implementation demands both individualization and rigorous rehearsal. The ranges provided—10-12 g/kg/day for loading, 1-4 g/kg morning-of, 30-100+ g/hour during competition—are evidence-based frameworks, not universal prescriptions. Every athlete must calibrate these parameters against individual GI tolerance, specific event demands, and anticipated environmental conditions through deliberate, repeated training-day practice.

The athletes who execute race-day nutrition with precision are those who have tested every variable before competition arrives. Supercompensation timelines, morning meal composition, carbohydrate delivery formats, and hourly intake targets should all be established certainties by the time the starting gun fires. In competition nutrition, the fastest adaptation is always the one you have already made.