Objective Typical diet plans are based on an individual's body mass; however, body composition may be important to consider when an individual is in a negative energy balance. This study examines if high initial body fat and dietary macronutrient content reduce muscle mass catabolism during excursions in temperate and cold high altitude environments. METHODS Subjects—53 healthy, un‐acclimated volunteers (37 males and 16 females)—took part in 12‐16 week‐long outdoor education courses in moderately high altitude temperate and cold climates in the western United States. Body mass, body fat percentage, fat mass, and muscle mass were measured before and after each excursion. Total energy expenditure and dietary intake were also measured. RESULTS In temperate and cold environments, both sexes lost significant amounts of body mass. In temperate climates both sexes lost a significant amount of fat mass, but not muscle mass. In cold climates, there was no significant change in fat mass for either sex; however, females gained muscle mass while males lost muscle mass. In both climates subjects with lower initial body fat percentages lost significantly more muscle mass than subjects with higher initial body fat percentages. There was no significant relationship between macronutrient intake and muscle mass loss for either sex. CONCLUSION These results suggests that during a negative energy balance dietary macronutrient content cannot abate the loss of muscle mass, but body fat may have a protective effect. This information should be used to improve individualized diets based on body composition, not body mass.