By: Darrell Miller
L-citrulline is an alpha-amino acid, first isolated from the watermelon in 1930: hence the name, citrullis being Latin for that fruit, the skin of which is rich in the substance. It is used to enhance performance in sports, particularly through aiding recovery after exercise, and also helps the liver to detoxify the blood.
It is not an essential amino acid in that it is produced by the body and need not be part of your diet. It is a precursor to arginine, which involves the sustained release of nitric oxide in the endothelium that promotes increased flow of blood and the blood vessels as described further below. One of the biochemical pathways for its biosynthesis involves the urea cycle, whereby the toxic ammonia is detoxified into an easily excreted form through its conversion to urea.
The urea cycle consists of five reactions, and citrulline is formed in the second of these. In the first of these, ammonia reacts with bicarbonate to form carbamoyl phosphate, the phosphate coming from the two molecule of ATP used to energize the reaction. These are converted to adenosine diphosphate (ADP), and the carbamoyl phosphate then reacts with ornithine to form citrulline, which takes part in step three that eventually leads to the formation of urea.
The second way in which L-citrulline can be biosynthesized is from the oxidation of arginine, a natural amino acid. Arginine is oxidized into N-hydroxyarginine, and then into L-citrulline with the release of nitric oxide.
So that’s how it is produced in the body, but how does it help sportsmen, and what part does it play in detoxification? Its effect on recovery after exercise is connected with blood flow. Energy is created in the mitochondria that are contained in every cell off the body. Among the raw materials needed for the production of energy are glucose and oxygen: glucose obtained from the carbohydrates in your diet, and oxygen transported by the hemoglobin or red blood cells.
Both of these rely on blood flow: greater the volume of blood transported to the cells then the greater the ability of these cells to produce energy. During periods of exercise, a good supply of blood is required to provide the raw materials needed for the energy demand of the muscles involved in the exercise. Not only that however, but recovery after exercise involves the replacement of electrolytes, the glycogen used in extensive aerobic and anaerobic exercise and protein replacement, particularly where catabolism has occurred.
In order to supply these raw materials at an adequate rate, it is necessary for the flow of blood to the appropriate muscles to be sufficient. A major restriction to increase blood flow is elasticity of the blood vessels and cells. Although a healthy heart is capable of providing the necessary quantities of blood, and hence of nutrients, any restriction to the flow could cause dangerously high blood pressure.
Nitric oxide plays a signaling role in enlarging blood vessels to allow an increased blood flow when it is needed by the body. It can provide more blood to the stomach during digestion and to the muscles during exercise and recovery.
During hard exercise, nitric oxide can act as a pump that provides blood during exercise and also during recovery. It can therefore provide more rapid gains in lean mass, increased endurance and faster muscle recovery. The way it does this is to send a signal to the smooth muscles to relax; smooth muscles such as those found in veins and arteries, so resulting in vasodilation therefore allowing increased blood flow.
It is the endothelium, the inner lining of blood vessels, which uses the nitric oxide to provide the relaxation signal to the smooth muscles surrounding it. In fact, it is the effect of nitric oxide that enables those living at high altitudes to develop increased stamina and speed over those at lower altitudes, and find more world records seem to be broken during athletics meetings, such as Olympic Games, held at high altitudes. This is because the production of nitric acid is increased at higher altitudes with slightly lower oxygen levels. This is the same effect that is used by vasodilators such as amyl nitrite and Viagra that work by increasing nitric oxide levels in the smooth muscle wall of the blood vessels.
It should not be surprising therefore, that L-citrulline should work in a similar way, since it is intimately involved in the production of nitric oxide. Although this is now generally understood, what part does supplemental citrulline play in the body if it is a non-essential amino acid?
Supplemental L-citrulline is useful in supporting the detoxification of ammonia in the liver when supplies of ornithine carbamoyl transferase is naturally in short supply. This is the enzyme that catalyzes the reaction between ornithine and carbamoyl phosphate to form citrulline. Supplements can then help in the removal of ammonia from the blood, and also provide material for the continued production of nitric oxide support muscular activity and its recovery after exercise.
Ammonia itself is a by-product of intensive exercise, and without the urea cycle the body would rapidly become polluted. It is a very toxic product, and causes the death of thousands of people each year. This is generally in people who suffer liver and kidney disease, and the ammonia can be broken down and excreted.
Your brain cells are particularly sensitive to ammonia, and as levels increase the effect progressively ranges from drowsiness thru tremors to coma and eventual death. Any condition, therefore, that reduces the body’s capability of metabolizing ammonia is potentially very serious, and any supplement that can help prevent this is valuable.
L-citrulline is believed to help in such situations, although any condition affecting the efficiency of the kidneys or liver and that can cause toxicity due to ammonia or any other toxic substance, should be referred to your physician. It is for its effect in increasing blood flow to provide sufficient raw material, for both the energy needed for high levels of exercise and for muscle recovery, that citrulline is predominantly used as a supplement.