Are body proteins broken down daily?
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During a single day roughly 1/4 to 1 lb 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. |
Protein is either broken down or manufactured to allow us to adapt to the most current metabolic situation within cells, and also in our body. 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.
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 couple 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.).
Are amino acids used for energy?
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In addition to the amino acids used to make important body chemicals such as certain hormones and neurotransmitters as well as key metabolic factors (e.g. carnitine, creatine), amino acids are used for energy. Typically 20 to 40 g of body protein in the form of free amino acids is utilized to make each day as energy. If our diet failed to include protein we would lose a significant amount of body protein over time. The RDA level for protein factors this in and has provides some padding as well. Some situations can increase the reliance on amino acids as a fuel source such is the case of weight loss and higher levels of exercise. |
What happens if we do not eat enough protein?
Our diet needs to at least replace a quantity of protein equivalent to what is lost to energy pathways and processes that produce amino acid-derived molecules such as neurotransmitters, nucleic acids, some hormones, niacin, etc. If one or more amino acids are in limited quantity in our cells, then protein synthesis is limited to that level as discussed above. If this continues over time, there will be a decrease in total body protein content. This would be visually obvious as skeletal muscle mass is reduced. If the deficiency continues, the level of various proteins in blood would decrease and our immune system could become compromised, leaving us more prone to infections.
What happens to body protein when we don’t eat enough calories?
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Situations can occur that increase the use of body protein for energy. Eating too few calories or fasting increases the reliance on body protein as an energy source. In these situations the level of circulating glucagon and cortisol increase. Cortisol, the stress hormone, will promote the breakdown of our body proteins to amino acids. Meanwhile, both of these hormones promote the conversion of amino acids to glucose in our liver which is released to serve as fuel. The amount of amino acids used to make glucose is related to the length and degree of caloric restriction and the intensity and duration of exercise. Simply stated as glycogen stores in the liver and muscle become depleted as in prolonged fasting and aerobic exercise the reliance upon amino acids to make glucose increases. |
During a longer period of fasting (e.g. more than a week) the reliance on amino acids lessens as our brain adapts to utilize more ketone bodies. This is one way that our body attempts to slow the loss of protein, however the use of amino acids for energy is still greater than during more normal times. If the loss of body protein continues for months a person can reach a critical level of body protein whereby normal function is compromised and illness can occur and over more protracted periods death is possible. Even if the cause of death is due to an infection, the true cause is probably a failure to maintain an optimal immune defense because of poor protein status.
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