Prolonged endurance events provide a unique opportunity to study the physiological limits of human performance. When nutrition is adequate, maintaining weight is directly related to energy expenditure. In extreme environments however, such as ocean rowing, the investigators have shown that rowers lost 10% weight, had a high energy expenditure (5,000kcal/d or 3.1 x basal metabolic rate) which was not met by energy intake (EI). Even in the presence of over \~5000kcal food per day, rowers chose to eat only \~3800kcal/d (2.4 x RMR) and lost weight, rather than eat more. It is also rare, outside of elite performance, that EI is above 2.5 x RMR and is suggestive of some sort of regulatory mechanism. Previous work has shown that high energy expenditures (EE) are not sustainable over long periods and that EE decreases with duration, plateauing at around 2.5 x RMR. This is in attempt to balance energy requirements in order to minimise body mass loss and increase chances of survival. Using data from the previous PhD study in ocean rowers (Holsgrove-West., 1st PhD study), and extrapolating data from the studies used to model TEE and duration, and overfeeding studies, voluntary energy intake calculated from body mass loss were consistently around 2.4 x RMR, suggesting a possible numerical value to the limit of EI. This has not been measured experimentally. If the theory of maximal EI is true, any intake above the maximum \[2.4 x RMR\] cannot be used for energy and would be excreted. Any BM loss would therefore reflect difference between EE and the maximal EI, not the actual EI. To investigate the EI limit therefore requires simultaneous measurements of EE, EI and body mass changes whilst ensuring EI above the theoretical maximum.In the presence of energy deficit and body mass loss, ocean rowers demonstrated an adaptive response by selectively maintaining or losing muscle. Loaded muscles were preserved whilst unloaded muscles atrophied. An obvious countermeasure could be to load the muscles that wasted, but to what expense? With finite energy intake, and greater demand from the muscle mass, other components of EE such as RMR, NEAT or exercise component may compensate, or endure a greater body mass loss. This interaction warrants further investigation. Logistically it is difficult to ask rowers to exercise mid-race, and compliance would be unknown. An expedition that involves greater muscle involvement and high EE is needed. Trekking across Greenland provides the high EE environment without the lower limb disuse. In addition, due to logistical reasons of sample storage in ocean rowing, energy expenditure and intake were collected only in the final week of the row, and energy balance did not match with body mass loss. This would indicate a different energy deficit and different energy expenditure or intake earlier on in the row. These initial acute changes are unknown and possibly responsible for the large weight loss seen by the end of the row. This project will attempt to fill that gap and measure these initial changes by measuring daily EI and EE.EI is rarely measured alongside EE and body mass loss and instead inferred from EE. This study aims to establish whether this maximal EI exists, by directly measuring both components of energy balance and how the body compensates with changes in the components of EE.