Fuelling Strategies in Marathon Running

Completing a standard marathon, a distance of 42.195 kilometres, places extraordinary metabolic demands on the human body. During such prolonged endurance exercise, skeletal muscles depend primarily on two fuel substrates: fatty acids and glycogen, the stored form of glucose found in muscle tissue and the liver. Although lipid reserves are virtually inexhaustible even in lean athletes, fatty acid oxidation is a biochemically slower process that yields fewer energy units per litre of oxygen consumed than the breakdown of carbohydrates. Consequently, as muscular and hepatic glycogen stores become depleted—typically occurring between the thirtieth and thirty-fifth kilometre—runners often experience a sudden, debilitating drop in pace known colloquially as ‘hitting the wall’. Understanding and delaying this substrate exhaustion has driven decades of exercise physiology research.

In the early decades of modern competitive distance running, the prevailing nutritional orthodoxy was remarkably austere. During the first half of the twentieth century, coaches and sports commentators widely believed that consuming liquids during a race weakened athletes, induced digestive distress, or reduced overall speed. Many competitors deliberately restricted fluid intake, viewing thirst tolerance as a mark of discipline. It was not until the late 1960s and 1970s that physiological studies began to demonstrate that dehydration compromised cardiovascular function and accelerated hyperthermia. As researchers highlighted the dangers of fluid loss, the sporting community underwent a dramatic shift in attitude, culminating in guidelines that urged runners to drink as much fluid as possible during endurance events.

However, this enthusiastic embrace of unrestrained fluid intake produced an unforeseen clinical risk: exercise-associated hyponatraemia (EAH). This condition occurs when excessive consumption of plain water or hypotonic beverages dilutes blood sodium levels to dangerously low concentrations. Rather than dehydration, it was overhydration that caused several high-profile medical emergencies at major city marathons during the 1980s and 1990s. When blood sodium falls sharply, water moves by osmosis into cells, causing tissue swelling; in severe instances, this leads to cerebral oedema, seizures, and even fatal outcomes. Modern hydration protocols now emphasise drinking according to individual thirst sensations or calculating specific sweat rates, rather than adhering to rigid, high-volume drinking schedules.

Alongside fluid balance, the in-race ingestion of carbohydrates has undergone significant refinement. For many years, exercise scientists believed that the body could oxidise no more than approximately 60 grams of exogenous carbohydrates per hour during exercise. This apparent ceiling was determined by the saturation point of the sodium-dependent glucose transporter 1 (SGLT1), the primary protein responsible for moving glucose across the intestinal wall into the bloodstream. In the early 2000s, however, researchers made a pivotal breakthrough by combining glucose with fructose. Because fructose utilises a distinct transport mechanism known as GLUT5, it can be absorbed simultaneously without competing for SGLT1 binding sites. This multi-transportable formulation elevated potential carbohydrate absorption rates to 90 grams per hour or higher, substantially enhancing endurance performance.

Despite these biochemical advancements, consuming large quantities of concentrated sugars while running at high intensity poses substantial gastrointestinal challenges. High-osmolality solutions can draw water into the intestinal lumen, delaying gastric emptying and provoking nausea, cramping, and diarrhoea. To mitigate these adverse reactions, sports nutritionists recently developed hydrogel technology. By encapsulating carbohydrates within a pH-sensitive biopolymer matrix—typically composed of alginate and pectin—the nutrients remain shielded from digestion in the acidic environment of the stomach. Upon reaching the neutral pH of the small intestine, the hydrogel dissolves, releasing the sugars for rapid absorption while minimising gut irritation.

Pre-event nutritional preparation has likewise evolved beyond historical practices. In the late 1960s, Scandinavian researchers introduced the classic ‘glycogen supercompensation’ protocol, which required athletes to undergo several days of exhaustive exercise paired with an extremely low-carbohydrate diet, followed immediately by three days of rest and heavy carbohydrate loading. Although biochemically effective at maximising muscle glycogen, this initial depletion phase often caused severe fatigue, irritability, and gastrointestinal issues. By the 1980s, modified protocols demonstrated that equivalent glycogen saturation could be achieved simply by tapering exercise volume while consistently consuming a carbohydrate-dense diet over several days, rendering the punishing depletion phase unnecessary.

In contemporary training regimes, the focus has shifted toward periodised nutrition, where dietary intake is deliberately adjusted to match specific training objectives. Some distance runners incorporate ‘train-low’ sessions—exercising with deliberately reduced carbohydrate availability—to stimulate mitochondrial biogenesis and enhance lipid oxidation pathways. Nevertheless, sports physiologists caution that race day requires maximal carbohydrate availability. The current consensus holds that while training in a depleted state may stimulate cellular adaptations, successfully executing a marathon relies on a rigorously rehearsed nutritional plan tailored to the athlete’s distinct metabolic profile and gastrointestinal tolerance.