A healthy diet is the basis for optimal bone development in childhood and bone and muscle maintenance in adulthood. The bone is mechanically and biologically linked to the muscular system and in old age the loss of bone mass (osteoporosis) is closely related to the loss of muscle mass (sarcopenia). Broken bones due to osteoporosis have a significant negative impact on the health of those affected and their prevalence will increase in the coming years. Modifiable factors that contribute to muscle and bone health, such as an adequate supply of proteins, are therefore becoming increasingly important. Protein intake is particularly important for older and frail people, since the protein requirement increases with age, but at the same time the risk of protein malnutrition increases. The following review article summarizes how proteins have a physiological effect on bones and muscles and how protein intake affects muscle and bone health in the course of life. In addition, the protein requirement for different age groups is discussed.