High-altitude exposure leads to many physiological challenges, such as weight loss and dehydration. However, little attention has been posed to the role of nutrition and ethnic differences. Aiming to fulfill this gap, five Italian trekkers and seven Nepalese porters, all males, recorded their diet in diaries during a Himalayan expedition (19 days), and the average daily intake of micro and macro-nutrients were calculated. Bioimpedance analysis was performed five times during the trek; muscle ultrasound was performed before and after the expedition, only for the Italians. The Nepalese group consumed a lot of rice and only Italians consumed cheese. Water intake was slightly over 3000 g/d for both groups. Nepalese diet had a higher density of dietary fibre and lower density of riboflavin, vitamins A, K, and B12. Intake of calcium was lower than recommended levels. Body mass index, waist circumference, fat-free mass, and total body water decreased in both groups, whereas resistance (Rz) increased. Italians reactance (Xc) increased at day 9, whereas that of Nepalese occurred at days 5, 9, and 16. The cross-sectional area of the Vastus lateralis was reduced after the expedition. Specific nutritional and food-related risk factors guidance is needed for diverse expedition groups. Loss of muscle mass and balance of fluids both deserve a particular focus as concerns altitude expeditions.