Body Protein is Turned Over Daily
What is body protein turnover or balance?
Every day and every minute, proteins are being made and broken down in cells throughout our body. Protein is either broken down or manufactured to allow us to adapt to the most current metabolic situation within cells and tissue on an immediate and short-term basis as well as to adapt over time. This process also allows us to maintain the integrity of proteins subjected to daily wear and tear. These activities allow cells to make or break down enzymes, which are either involved or not involved in different metabolic states such as fasting, feeding, and exercise. These processes also allow us to remodel tissue such as muscle and bone and to make and break down hormones and neurotransmitters. It is important to remember that our cells are constantly active. This allows us to grow, heal, remodel, and internally defend ourselves on a continual basis. During a single day roughly ¼ to 1lb of our body protein is broken down to amino acids. The lower end of the range would apply more to a smaller woman while the higher end of the range would be more applicable to a larger, more muscular man. Much of the breakdown occurs in the liver and muscle and during the same day an equivalent amount of protein is made (synthesis). Protein breakdown and production considered together is called “protein turnover” and even though there is this significant quantity of protein turnover we are mostly the same from one day to the next if there are no unusual circumstances or influences.
How long do body proteins last?
All proteins in our body have a certain life expectancy. For instance, when insulin and glucagon are released into our blood an individual molecule of either will circulate for about five to ten minutes before they are removed and broken down. Meanwhile, some enzymes within cells may exist only for a few minutes or so before they are replaced or not remade. This can allow cells to shift metabolic gears, so to speak, when going from a fasting to a fed state, resting to exercise state, and so on. Contractile proteins in muscle (e.g., myosin and actin) may last only a couple of days, while connective tissue proteins, such as collagen, may last weeks to months before they are broken down and replaced. The rate of turnover or remodeling of skeletal muscle contractile proteins and connective tissue proteins helps us understand why the human body seems to get bigger and stronger in just a few of weeks or so when lifting weights regularly. Meanwhile, it seems to take months and years for scar tissue, which is largely connective tissue, to change.
Are there “free” amino acids in the body?
Free amino acids are found in the body as a result of digestion of food protein and the absorption of amino acids as well as a product of protein breakdown in cells. Free amino acids account for about 1% of the amino acids in our body, the rest of course would be part of peptides and proteins. Most cells in the body have a small assortment of free amino acids, meaning they are independent and not linked to other amino acids as part of peptides and proteins. In addition there is a small amount of amino acids circulating in the blood which, although this increases after a protein containing meal. Circulation provides a deliver system for diet derived amino acids to get to all tissue as well as a means for amino acids to be exchanged between tissue such as during fasting and exercise. Free amino acids in cells and in the blood are collectively referred to as the “amino acid pool” and these amino acids are available to make new body protein or amino acid-derived substances (e.g. neurotransmitters, hormones, metabolic factors, etc.).