Body Protein Can Be Used For Energy
Are amino acids used for energy?
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 grams 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 is largely based on this, however some situations can increase the reliance on amino acids as a fuel source or building incremental protein over time. This reinforces that the RDA is designed to prevent marked protein deficiency over time under usual conditions. However, it is not appropriate for dietary planning for optimal health and to address conditions and influences like exercise training, injury, pregnancy and lactation.
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?
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, especially muscle 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.
What happens to body protein when we exercise?
During prolonged aerobic (cardiovascular) exercise, muscle protein is broken down and amino acids, mostly alanine and glutamine are released into the blood. Alanine is one of the principle amino acids used to make glucose in the liver and the new glucose can help maintain blood glucose levels and fuel muscle during long aerobic exercise bouts. This process is driven by primarily by cortisol as well as epinephrine, both of which are elevated in circulation during exercise. Cortisol promotes muscle protein breakdown during the exercise while epinephrine promotes the conversion of amino acids to glucose in the liver. Since cortisol is a stress related hormone, the degree to which this happen depends on how hard you are exercising and for how long. Thus for shorter, less intense exercise sessions (e.g. walking, casual bicycling) this isn’t a consideration; however for endurance athletes and heavy weight trainers it is.
What happens to the nitrogen derived from amino acids when they are used for energy purposes?
The composition of elements in amino acids differs from carbohydrate and fat, as they contain nitrogen (N). This creates an additional consideration for the body if it wishes to use amino acids for energy or to make fat (in an overfed state) or glucose (in a fasting or exercise state). Thus an important step in using amino acids for any of these purposes is to remove the nitrogen-containing portion of the molecule. The removed nitrogen portion of amino acids will become ammonia (NH4+), which is potentially toxic to the brain. Thus it must be removed from the body before it builds up in the blood. The most prevalent way to rid the body of the nitrogen removed from amino acids is as urea. Urea is made by the liver and released into the blood, circulates to the kidneys and is subsequently lost from the body in urine. Each molecule of urea allows us to efficiently remove two nitrogen atoms from our body.