Protein for healthy muscle and bone
As we saw earlier this week, proteins in the diet contribute significantly to the production of energy for the body.
Dietary proteins provide amino acids as building blocks for growth, repair and maintenance of proteins in the body. The body does not have a mechanism for storing amino acids or protein, unlike carbohydrates which can be stored as glycogen and fats that can be stored as triacylglycerides.
When we have sufficient protein and an adequate supply of energy in our diet, the excess amino acids that are not used for growth or repair are either converted to glucose in the liver or are used to provide energy. Without enough supply of dietary energy from carbohydrates and fats, the body preferentially uses amino acids for energy production at the expense of growth, maintenance and repair. This means that it is particularly important as we get older to provide enough energy in the diet from carbohydrates and fats so that dietary protein can be used for repair and maintenance.
Dietary protein is a key nutrient for muscle and bone health. Adequate dietary protein during childhood and adolescent years is essential in order to accumulate an adequate bone and muscle mass during growth. For muscles to grow, they need both protein and exercise, and the same is true also for bone. Dietary protein can also help slow down the rate of bone and muscle loss that occurs as we get older. Low and high protein diets can affect bone metabolism in different ways.
Skeletal muscle represents a pool of protein that can be called upon as a source of energy when food is scarce or when the body is stressed such as after injury. A lack of protein in the diet will lead to poor muscle strength which can increase the risk of falling, especially in the elderly and contributes to poor recovery in patients who have had fractures. Low protein diets are also associated with decreased intestinal calcium absorption and an increase in the levels of the key bone regulatory hormone parathyroid hormone.
High protein diets can increase calcium absorption and lead to an increased excretion of calcium in urine. High protein diets are also linked to increased production of an important bone growth factor - insulin-like growth factor 1. High protein diets are associated with an increased bone mineral content, a decrease in the risk of fractures, and improved fracture repair and recovery after injury.
Muscle contains about 30% protein by weight. You might be surprised to learn that bone contains a higher percentage of protein than muscle weight for weight. This is because, in muscle, the proteins are mainly found inside the muscle cells, which contain a lot of water, while in bone, much of the protein is found outside of cells in densely packed fibres that contain little water.
Muscle and bone have evolved to use specialised proteins for their main functions. In muscle, the most important and abundant functional proteins are actin and myosin that make up the bulk of the muscle tissue protein. There are many other proteins such as troponin that are also needed for muscles to function, which we’ll look at in more detail in the next step. In bones, the most important and abundant functional protein is the fibrous collagen into which the calcium phosphate mineral is embedded.
We all need some protein in our diet to replace proteins that are regularly turned over, and any excess dietary protein is normally used for energy production. If we don’t obtain any protein from our diet, even if our energy needs are met from carbohydrates and fats, then we will lose proteins from our body when they are degraded for turn-over. Later in the course, we’ll discuss how much protein we need to stay healthy.
© University of Liverpool/The University of Sheffield/Newcastle University