BACKGROUND: The upper rates of energy expenditure (EE) and the corresponding regulation of energy intake (EI), as described in younger trained subjects, are not well elucidated in older subjects. OBJECTIVES: The aim was to investigate EE in older men during prolonged cycling and determine whether it is sufficiently matched by EI to maintain energy balance. In addition, we investigated appetite ratings and concentrations of appetite-regulating hormones. DESIGN: Six men (mean +/- SE age: 61 +/- 3 y) completed 2706 km of cycling, from Copenhagen to Nordkapp, in 14 d. EE was measured by using doubly labeled water, and food and drink intake was recorded by the accompanying scientific staff. Energy balance was calculated as the discrepancy between EI and EE and from changes in body energy stores as derived from deuterium dilution. Fasting hormones were measured before and after cycling, and appetite ratings were recorded twice daily. RESULTS: EE (+/-SE) increased from 17 +/- 1 MJ/d before to 30 +/- 2 MJ/d during the cycling trip (P < 0.001), which is equivalent to 4.0 +/- 0.1 times the basal metabolic rate. Although body weight remained stable during the 14 d of cycling, body fat decreased (-2.2 +/- 0.7 kg; P = 0.02) and fat-free mass increased (2.5 +/- 0.6 kg; P = 0.01). EI was 25 +/- 1 MJ/d during cycling, resulting in a negative energy balance calculated by the EE - EI gap (-5.2 +/- 1.2 MJ/d). Calculated from changes in body energy stores, energy balance was also negative (-4.8 +/- 2.0 MJ/d) during the first week. In the morning and evening, hunger ratings increased (both P = 0.02), whereas ratings of fullness decreased in the evening (P = 0.04). Fasting plasma concentrations of insulin increased by 120% +/- 15% (P = 0.02), glucagon-like peptide 1 (GLP-1) by 60% +/- 20% (P < 0.01), and Polypeptide YY3-36 by 80% +/- 30% (P < 0.02) after cycling. CONCLUSIONS: Older male cyclists sustained near-maximal rates of EE during prolonged cycling but were unable to upregulate EI to maintain energy balance. Despite the presence of increased motivation to eat, a more profound counteracting physiologic stimulus inhibiting increases in EI was present. This trial was registered at clinicaltrials.gov as NCT02353624.