Extreme energy expenditure during a 200 km three-day open-water swim: A doubly labeled water case study.

The upper limits of total energy expenditure (TEE) in humans have primarily been described during single-day ultra-endurance events, or during multistage land-based endurance events across several days. However, little is known about the physiological limits of sustained energy turnover during shorter multi-day, near-continuous exercise events, particularly when performed in an aquatic environment. Prolonged immersion in water imposes substantial thermoregulatory demands due to increased conductive heat loss, which may further elevate metabolic requirements beyond those observed in (cold) land-based events. The 200 km Elfstedentocht swim, completed over approximately three days with minimal sleep and near-continuous swimming, provides a unique opportunity to investigate the upper limits of human energy expenditure during prolonged multi-day exercise in water. In this case study, we therefore assessed TEE in a former Olympic athlete (mass 103 kg, height 2.04 m, age 38 years) during a 200-km swim completed over 3 days using doubly labeled water. TEE was 195 Megajoule (MJ) (46,700 kcals), corresponding to 6.17x basal metabolic rate over the three-day period. This TEE is among the highest TEE reported thus far over similar periods, suggesting that the combined near-continuous exercise and imposed sustained thermoregulatory demands due to conductive heat loss lead to extreme energy expenditure. Notably, the absolute energy expenditure of ∼14,700 kcal per day exceeds previous limits.