Protein intake requirements and recommendations are becoming increasingly segmented by age and their purpose. Since the traditional nitrogen balance method for measuring protein metabolism has been highlighted as an underestimate, new methods have been used in combination in recent years. Increased intake recommendations for the elderly are needed, especially for the prevention of sarcopenia and maintenance of lean body mass, while resistance training is also important. In addition, the quantity and quality of dietary protein are associated with the risk of many diseases, including obesity, type 2 diabetes, chronic kidney disease, cardiovascular disease, cancer, and osteoporosis. Animal and plant proteins differ not only in their nutritional value and health outcomes but also in their environmental effects from their production. Various protein sources, including novel ones, have been studied, and further investigation is expected in the future since they may have different nutritional physiological functions and availability depending on the characteristics of consumers and region.