The finish line and what led me there

• At kilometer 38 of the 35°C Sydney Marathon, Alex McDonald passed a collapsed runner, ambulances, and stretchers; 26 runners were taken to hospital. He continued toward the finish.
• The Sydney race was his fifth marathon in five days, after 110 hours without food, with only water and salt. In September 2023, at age 42, he became the first person on record to complete 211 km in five fasted marathons, having never previously run one.
• McDonald was a Sydney father, not a doctor, dietitian, or established athlete. In 2010, Art De Vany inspired him through fitness into his seventies, activity with his grandchildren, and eating foods available before modern agriculture.
• McDonald moved from paleo to keto to carnivore, avoiding grains and processed food and increasing fat and protein. He experienced progressively better well-being, encountered objections, and spent about ten years studying the topic.

The fuel-access problem

• Dietary carbohydrates are not essential nutrients. The available glycogen reservoir contains about 2,000 calories, less than a marathon requires; fat adaptation permits use of the much larger body-fat reservoir.
• His DEXA scan showed 14% body fat, estimated at 100,000 calories: roughly 50 times his carbohydrate reservoir and the equivalent of approximately 25 marathons, despite his lean body composition.
• An average Australian man carries an estimated 227,000-plus calories in body fat, equivalent to roughly 55 marathons. The distinction is between stored energy and access to that energy.
• Stephen Phinney used the analogy of a fuel tanker running out of petrol. Carbohydrates raise insulin, which promotes fat storage and inhibits fat burning; lower insulin permits greater access to stored fat.
• Carbohydrate loading can support performance while requiring planned refuelling; insufficient intake can lead to hitting the wall, and excess intake can cause gastrointestinal problems. McDonald sought to test endurance without this refuelling.

Designing and training for the five-marathon fast

• The attempt consisted of five 42.2-km marathons in five days without food. Lead doctor Paul Mason and a medical team supported it, with continuous glucose monitoring, blood ketones three times daily, blood testing, and pre/post DEXA scans.
• Preparation involved seven months, 77 runs, and 700 km, often starting at 4 a.m.; every run was fasted, followed by food. His diet comprised meat, seafood, eggs, and some dairy, with almost no carbohydrate.
• Fat adaptation took weeks, while the seven-month preparation conditioned his legs. The pacing plan aimed for a heart rate below 140 bpm, approximately zone two, and completion over speed.
• On day one he spent nearly five hours in zone three or higher. Days two to four followed the lower-intensity plan, while day five included three and a half hours in zone three or higher, with one hour above 164 bpm.
• The five days included about eight and a half hours of moderate-to-high-intensity running without food. Speed was not the measured question; he was both the fastest and slowest recorded finisher of this particular challenge.

Glucose, ketones, and sustained energy

• Across the 121-hour fast with water and salt but no food, continuous glucose measurements were generally 4–6, including during running. Glucose was produced internally through gluconeogenesis.
• Glucose dipped below 4 several times from day three, reaching 3.5 overnight. There was no experienced energy collapse or hitting the wall.
• The highest glucose reading, 7.7, occurred after the final finish, attributed to heat and stress. Blood ketones increased from 0.1 on day-one morning to 4.3 on day-five afternoon.
• Glucose remained broadly steady as ketones rose. The five fasted marathons were completed without exhaustion of available energy.

Heat, injuries, fluid balance, and finishing

• Day four combined 38°C heat, nearly 170 km already run, pain in ligaments and tendons, and loops around a barbecue-filled food festival. Fat adaptation supported fuelling but did not prevent physical wear.
• Following excess water intake in the heat, he had two sprained ankles, pitting oedema, and a 1.6-kg increase in one day; after four marathons and four food-free days, his weight equalled his starting weight.
• Paul Mason identified further overhydration as a greater danger than dehydration for the fifth marathon. McDonald elevated his feet, slept poorly, and received a 47/100 recovery score, while morning glucose was 4.3.
• In the 35°C Sydney Marathon, he finished on two sprained ankles with his second-fastest time of the week, while other runners required ambulances. Heat, fluid balance, and injuries were the limiting problems, not an energy shortfall.

Body composition and comparison with fasting research

• After five days and 211 km, body weight fell by 3 kg. DEXA indicated a loss of 1 kg fat and 2 kg lean mass; lean mass includes water and glycogen as well as muscle.
• For a fasting-only comparison, a 2025 Norwegian study followed 13 people during a seven-day water-only fast without marathons. [1]
• The Norwegian group lost about 8% of fat mass compared with his 9.5%, and about 8% of lean mass compared with his 3%, despite his 211 km of running. [1]
• The comparison differed in participant age, fat adaptation, seven versus five fasting days, and scan timing: the Norwegian group was still fasting, while McDonald had eaten for two days. His lean-mass change was smaller. [1]

Muscle damage, laboratory results, and gut tolerance

• His blood tests showed exertional stress, including an approximately twentyfold increase in muscle-damage markers and high inflammation, after 211 km as a first-time marathon runner.
• In a study of 32 trained ultrarunners who ate during a 200-km race, muscle-damage markers rose nearly one hundredfold and inflammation more than twentyfold. His measurements were within that exercise-related range. [2]
• His kidney markers and electrolytes were normal, fasting insulin declined from 5 to 2, and no gastrointestinal problems occurred during the event.
• Carbohydrate-fuelled runners plan gels, drinks, and feeding schedules, sometimes with digestive issues. Water and salt eliminated food and gel scheduling for McDonald, and he experienced no digestive disruption.

A second fasted endurance test in 2024

• In October 2024, he entered a backyard ultra with a 6.7-km loop every hour until elimination. Nearly 300 started; he completed 15 hours and 100 km, outlasted about two-thirds, and finished in the top quarter.
• The 100 km exceeded twice his previous longest single-day distance and was completed fasted with water and salt, while other runners carried substantial race fuel.
• Continuous glucose and continuous ketone monitors showed stable glucose and ketones rising from zero at the start to above 4 at the finish, paralleling the five-marathon pattern.

The wider point

• The fast eliminated food intake during the event as an energy source. The central distinction is stored energy versus access to it: body-fat stores supported prolonged exertion without dietary carbohydrates.
• Marathons served as a demonstration. The intended everyday outcome is sufficient energy for activity, including playing with grandchildren into older age, as exemplified by Art De Vany.
• McDonald serves on the board of the Australasian Metabolic Health Society to support practitioner education and broader access to metabolic-health knowledge.

References

1. [14:48] Effects of seven days’ fasting on physical performance and metabolic adaptation during exercise in humans — https://doi.org/10.1038/s41467-024-55418-0
2. [16:13] Creatine kinase isoenzyme activity during and after an ultra-distance (200 km) run — https://doi.org/10.5604/20831862.1163384