People taking part in commercial treks are exposed to high altitude (HA) while eating ad libitum, a situation that has been related to under eating and body weight loss. We investigated this phenomenon in westerners taking part in a typical Himalayan trek. Body weight, energy intake (EI), macronutrient composition of the diet, and acute mountain sickness symptoms (AMS) were measured in seven healthy individuals (aged 45 +/- 12 yr and BMI 24.6 +/- 4.4 kg/m(2)) during a 19-day trek to Makalu Base Camp. Comparisons were made between three periods of the trek: a 5-day climb below 2500 m (low altitude 1: LA1), an 8-day period between 3500 m and 5200 m (high altitude: HA), and a 4-day descent below 2500 m (low altitude 2: LA2). Body weight was significantly reduced at the end of the trek (p < 0.01). A decrease in total EI was observed at HA (p < 0.05). This was followed by an increase in EI upon return to LA2 (p < 0.001). Neither EI nor macronutrient composition of the diet was associated with the presence of AMS. In conclusion, a decrease in EI was observed in HA, which does not seem to be macronutrient specific.