Energy balance maintenance is crucial to preserve astronaut’s wellness and quality of life during long-duration spaceflight and nutritional intervention can effectively counteract the detrimental effect of microgravity on skeletal muscle mass and metabolism. NutrISS (Nutrition monitoring for the International Space Station) is an Italian Space Agency (ASI) biomedical experiment investigating the changes in body composition during long-term spaceflight. The goal of this proof-of-concept study is to monitor the body composition of astronauts and, if needed, to provide nutritional advice during the mission. It has been executed on the ISS, from July 2019 till January 2020 and sponsored by ESA. The payloads selected for the mission “BEYOND” result from a public-call funded and coordinated by ASI, in the frame of its national mission of promoting and fostering the culture of space across the Country and providing access to the ISS as a laboratory in space to the Italian research community. The utilization support services is ensured by Argotec/Telespazio (UTISS Team). An astronaut underwent to baseline data collections, i.e. body composition, anthropometric evaluation and energy and metabolic assessment carried out by the scientific team. Body composition, estimated through resistance/reactance to a low-intensity current has been assessed with a Bio-Impedance Analyser (BIA) device manufactured by Akern and modified by Kayser Italia. To maintain the participant in near-neutral energy balance, the science team monitored monthly the astronaut body mass during the entire space flight period. Advice on energy intake was given to the astronaut when needed.