The aim of this case study was to examine energy expenditure (EE) in one cyclist during an extreme endurance cycling race - the “XXAlps 2004” (2,272 km distance and 55,455 m altitude) which was completed in 5 days and 7 hours - and whether the energy deficit derives primarily from the degradation of subcutaneous adipose tissue or loss of muscle mass. Energy intake (EI) was continuously recorded. EE was estimated using two different methods: 1) Continuous heart rate recording using a portable heart rate monitor (POLAR^® S710) and 2) using the individual relationship between heart rate and oxygen uptake (VO₂) determined under laboratory conditions. Body composition was assessed by measuring body mass, skinfold thickness and extremity circumferences. The cyclist lost 2.0 kg body mass, corresponding to 11,950 kcal (50 MJ). Fat mass was reduced by 790 g (7,110 kcal; 30 MJ) and fat free mass by 1.21 kg (4,840 kcal; 20 MJ). Circumferences of the lower extremities were reduced, in contrast skinfold thickness at the lower limbs increased. Energy deficit (ED) was calculated as the difference between EI and EE. Energy deficit using continuous heart rate monitoring was 29,554 kcal (124 MJ), and using the individual relationship between heart rate and VO₂ was 7,111 kcal (30 MJ). The results show that the difference between ED due to decreased body mass and ED estimated from continuous heart rate monitoring was 74 MJ (124 MJ - 50 MJ). In contrast the difference between ED due to decreased body mass and ED estimated from laboratory data was 20 MJ (30 - 50 MJ). This difference between methodologies cannot properly be explained. Body mass and skinfold thickness may be overestimated due to hypoproteinemic oedemas during endurance exercise. Data from the present study suggests the individual relationship between heart rate and VO₂ may provide a closer estimation of EE during extreme endurance exercise compared with corresponding data derived from continuous heart rate monitoring using the POLAR^® S710.