Optimal nutrition serves to maintain normal organ function and to preserve body energy stores to guarantee survival during times of shortage of food. Especially total body protein content is an important determinant of survival. However, recommendations about nutrition refer mostly to total energy intake with either no emphasis on total protein content or protein intake only considered as a fixed percentage of caloric intake. This paper focuses on the role of total body protein mass or lean body mass (= mass of organs and muscle) (LBM) on survival of healthy humans and critically ill patients. Recommendations on the amount of protein per kg bodyweight are made based on the scarce evidence available in humans.