We aimed to examine the effects of different energy intakes on a range of responses that are relevant to the safety of hill walkers. In a balanced design, 16 males completed a strenuous selfpaced mountainous hill walk over 21 km, under either a low-energy (2.6 MJ; 616 kcal) intake (LEI) or high-energy (12.7 MJ; 3019 kcal) intake (HEI) condition. During the hill walk, rectal temperatures were measured continuously and blood samples for the analysis of metabolites and hormones were drawn before breakfast and immediately after the walk. Subjects also completed a battery of performance tests which included muscular strength, reaction times, flexibility, balance and kinaesthetic differentiation tests. During the LEI, mean blood glucose concentratio ns levelled off at the low-mid range of normoglycemia whereas, on the HEI, they were significantly elevated compared with the LEI. The maintained blood glucose concentrations, during the LEI, were probably mediated via the marked fat mobilisation, reflected by a 2- to 5-fold increase in NEFA, 3-OHB and glycerol concentrations. The LEI group showed significantly slower 1- and 2-finger reaction time, an impaired ability to balance and were compromised in their ability to maintain body temperature, when compared with the HEI group. The modestly impaired performance (particularly with respect to balance) and thermoregulation during the LEI condition may increase susceptibly to both fatigue and injury whilst pursuing recreational activity outdoors.