The Physiology of Marathon Running: What Science Reveals About 26.2 Miles

The Physiology of Marathon Running

Author: Hüseyin Akbulut — BSc Sport Sciences (rowing), MSc Marmara University

The marathon occupies a peculiar position in human physiology. It is long enough to exhaust the primary carbohydrate stores that fuel high-intensity exercise, yet fast enough that athletes cannot simply shift to fat oxidation and coast. It demands sustained output at intensities that stress the cardiovascular and thermoregulatory systems simultaneously, over a timeframe that accumulates significant mechanical damage to muscles and connective tissue. It is, in a word, unforgiving — and understanding its physiology reveals why some runners make it look easy and others suffer dramatically in the final miles.

This article examines the science of marathon physiology from the ground up: the energy systems at work, the role of glycogen and fat, the debate between VO₂max and running economy as performance predictors, Eliud Kipchoge’s remarkable physiological profile, and the evidence behind optimal pacing.

Energy Demands of 42.2 Kilometres

The gross energy cost of running a marathon is approximately 2,500–3,000 kilocalories for a 70-kilogram runner, depending on speed, running economy, and terrain. This translates to roughly 60–70 kilocalories per kilometre — or, expressed as a metabolic flux, somewhere between 40 and 80 mL of oxygen per kilogram of body mass per minute, depending on pace.

At marathon race pace for a competitive runner (corresponding to approximately 75–85% of VO₂max), the oxidative system supplies the overwhelming majority of energy. Anaerobic glycolysis contributes most noticeably at the start of the race, during surges, and in the closing kilometres. The blend of fuels — glycogen and fat — shifts continuously throughout the race, and this shift is where many of marathon physiology’s most important insights lie.

Muscle glycogen stores in a well-fuelled, carbohydrate-loaded runner amount to approximately 300–500 grams, providing 1,200–2,000 kilocalories. Liver glycogen adds 80–100 grams. Blood glucose at any given moment contributes a negligible reserve — perhaps 5 grams. The mathematics make the problem clear: glycogen stores are insufficient to fuel an entire marathon at competitive intensity. Fat, stored in virtually unlimited quantities even in lean athletes, must bridge the gap.

Glycogen vs. Fat: The Fuel Mix

The proportion of energy derived from fat versus carbohydrate at marathon pace is one of the most studied questions in sports nutrition. At moderate intensities (roughly 65% VO₂max), fat can contribute 50% or more of total energy. As intensity rises toward 85% VO₂max, carbohydrate increasingly dominates — carbohydrate oxidation rates climb steeply, while fat oxidation rates plateau and then decline.

This intensity-dependence has a critical implication: running slightly too fast significantly accelerates glycogen depletion. A 1% increase in pace above the lactate threshold can dramatically shift the fuel mix toward carbohydrate, advancing the point of glycogen exhaustion by kilometres. This is the physiological explanation for why early pacing errors in a marathon are so costly — the glycogen penalty compounds over time.

Trained endurance athletes shift their crossover point — the exercise intensity at which fat and carbohydrate contribute equally — higher than untrained individuals. A well-trained marathon runner might oxidise fat effectively up to 70–75% VO₂max, sparing glycogen and sustaining pace longer before depletion. This fat oxidation capacity is partly genetic but substantially trainable, particularly through high-volume low-intensity training and strategic carbohydrate periodization during preparation.

VO₂max and Running Economy: Which Predicts Marathon Performance?

VO₂max — maximal oxygen uptake — was long considered the gold standard of endurance capacity. Higher VO₂max means more oxygen delivered to muscles per unit time, enabling higher ATP production rates via oxidative phosphorylation. Elite marathoners typically show VO₂max values of 70–85 mL/kg/min in men and 60–75 mL/kg/min in women — roughly twice the average sedentary value.

Yet among elite runners, VO₂max alone is a poor discriminator of marathon performance. Within a group of athletes all sharing VO₂max values in the 75–85 mL/kg/min range, finishing times can differ by 15–20 minutes. This is where running economy becomes decisive.

Running economy is defined as the oxygen cost of running at a given submaximal speed — expressed as mL O₂/kg/km or as energy cost per distance. A runner with superior economy uses less oxygen to maintain the same pace, effectively “getting more miles per gallon.” Research by Daniels and Daniels (1992) showed that running economy varied by up to 30% among runners with similar VO₂max values — a variation far larger than VO₂max variance within elite groups.

Running economy is influenced by biomechanical factors (stride mechanics, vertical oscillation, ground contact time), tendon stiffness (the Achilles and patellar tendons function as energy storage springs), and muscle fibre type distribution (higher Type I content reduces oxygen cost at given intensities). Interestingly, shoe technology — specifically carbon-fibre plated racing shoes — has been shown in studies by Barnes and Kilding (2019) to improve running economy by approximately 4%, which helps explain recent marathon world record improvements.

Eliud Kipchoge’s Physiology: A Case Study

Eliud Kipchoge represents what may be the most optimised marathon physiology documented in the scientific literature. His VO₂max has been measured at approximately 85 mL/kg/min — remarkable, but not unique among elite distance runners. What distinguishes him is a combination of exceptional running economy, a very high lactate threshold relative to VO₂max, and extraordinary psychological capacity to maintain optimal effort over extended periods.

Studies conducted in the context of the Breaking2 project (2017) and subsequent INEOS 1:59 Challenge (2019) revealed that Kipchoge could sustain close to his lactate threshold for the full marathon duration — a pace that would exhaust most runners within 30–40 minutes. His ability to oxidise fat efficiently at near-threshold intensities, combined with aggressive carbohydrate fuelling strategies, allowed glycogen reserves to be stretched across the full 42.2 kilometres.

Physiologists have noted his remarkably low vertical oscillation during running — minimal up-and-down movement that represents wasted energy — and excellent arm mechanics that reduce the energy cost of forward propulsion. These biomechanical efficiencies, compounded across tens of thousands of steps, translate to significant fuel savings over a marathon.

The Wall: Physiology of Miles 18–22

Hitting the wall is not metaphorical. Around kilometres 30–35 (miles 18–22), many recreational and competitive marathon runners experience a sudden, sometimes dramatic decline in pace accompanied by cognitive impairment, nausea, and what runners describe as a qualitative change in effort — legs that feel “made of concrete.” The physiological underpinning is well-established.

Costill’s foundational biopsy studies demonstrated that muscle glycogen approaches critically low levels in this phase of the race. When glycogen content in active muscle fibres falls below approximately 70–100 mmol/kg dry weight, contractile function is compromised even if the athlete makes no subjective sense of reduced capacity. The forced shift to fat oxidation alone cannot maintain the original pace because fat oxidation produces ATP more slowly and consumes more oxygen per mole of ATP than carbohydrate oxidation.

Simultaneously, if carbohydrate intake during the race has been insufficient, hepatic glycogen may be substantially depleted, and blood glucose can decline toward hypoglycaemic levels (below 3.5–4 mmol/L). Mild hypoglycaemia impairs central nervous system function, affecting perceived exertion, motivation, and coordination.

The practical implications are straightforward: carbohydrate intake during the race (30–60 grams per hour from gels, chews, or sports drinks) significantly delays glycogen depletion and the wall. The IAAF guidelines and subsequent research by Burke and colleagues suggest that trained runners can benefit from up to 90 grams per hour if using multi-transportable carbohydrates (glucose + fructose combinations) that use different intestinal transporters, avoiding the bottleneck that limits glucose absorption alone.

Thermoregulation Under Marathon Conditions

Heat production during marathon running is substantial. Metabolic efficiency during running is approximately 20–25%, meaning 75–80% of energy liberated by oxidative metabolism appears as heat. The body’s thermoregulatory response — increased skin blood flow and sweating — competes with working muscle for cardiac output, creating a cardiovascular strain that worsens as the race progresses and core temperature climbs.

Core body temperature in marathon runners typically reaches 39–40°C by the finish in temperate conditions. In warm or humid environments, temperatures of 40–41°C can occur, at which point heat illness risk increases substantially. The wet bulb globe temperature (WBGT) index combines ambient temperature, humidity, solar radiation, and wind speed into a single heat stress indicator; WBGT above 28°C is considered dangerous for endurance performance and safety.

Performance degradation in the heat follows a well-described pattern. Ely and colleagues (2007), analysing a large database of US marathon finishers, demonstrated a progressive performance decline with increasing temperature: approximately 1.6% degradation per 5°C rise in maximum race-day temperature above approximately 8°C. These effects were larger in slower runners (who spend more time on course) and in women than men, likely due to differences in surface area-to-mass ratios and acclimatisation status.

Optimal Pacing Strategy

Decades of performance data and physiological research converge on the same conclusion: even pacing, or very slight negative splitting (running the second half marginally faster than the first), produces the best marathon outcomes. Studies by Hanley (2013) examining World Marathon Major results showed that even among sub-elite finishers, those who ran even splits lost significantly less time in the final third of the race compared to positive splitters.

The physiological explanation is that exceeding lactate threshold early in the race causes disproportionate glycogen consumption and lactate accumulation. The “debt” incurred in the first half is paid back — with interest — in the second. Running the first half at 102–104% of sustainable race effort often leads to a death spiral in the final 10 kilometres, while running at 98–100% of sustainable effort allows for a controlled, often faster finish.

Conclusion

Marathon physiology is a masterclass in the intersection of fuel management, cardiovascular capacity, thermoregulation, and mechanics. VO₂max sets the ceiling; running economy and lactate threshold determine how close to that ceiling an athlete can operate for 42.2 kilometres. Glycogen management — through pacing, in-race fuelling, and training-induced metabolic adaptations — separates those who cross the finish in control from those who hit the wall.

For a rigorous, comprehensive treatment of endurance science — including marathon physiology, energy systems, and training principles — visit sporeus.com/threshold/ to explore THRESHOLD.