Following a protein-containing meal, amino acids produced by digestion go from the stomach to the liver via the hepatic portal vein. Alanine and other amino acids are transported to the liver, where carbons are transformed to glucose and ketone bodies and nitrogen is turned to urea, which is eliminated by the kidneys. The remaining amino acids are processed into proteins for reuse or catabolism.
The body can only use about 5% of the available nitrogen, so the rest is removed in urine. The main uses for nitrogen are building muscles, repairing tissue, making enzymes, and producing antibodies. Muscle growth requires nitrogen to be taken in with food to produce muscle fiber cells and connective tissue. Tissue repair involves the production of collagen and mucopolysaccharides by fibroblasts that divide and multiply. Enzymes are complex proteins that act as catalysts in biochemical reactions; they are needed in very small amounts (nanograms). Antibodies are proteins made by special cells, called B lymphocytes, that fight off infection by binding to harmful particles or organisms and tagging them for destruction by other cells. Anti-nutrients in foods can prevent the body from using some of the nutrients inside the proteins and carbohydrates they contain.
For example, phytic acid binds to minerals such as zinc and calcium and prevents them from being absorbed by the body. Once bound, the minerals cannot be used for anything else and must be eliminated in your stool or urine.
Protein. During digestion, amino acids are delivered to the liver, where the majority of the body's protein is created. If there is an excess of protein, amino acids can be turned into fat and stored in fat depots, or they can be transformed into glucose for energy through gluconeogenesis, as previously described. Protein that isn't used for building new tissue is degraded into ammonia and carbon dioxide by enzymes in the liver and kidneys. The ammonia could be toxic if not removed from the body so it is converted into urea by enzymes in the liver and then dumped into the urine.
Of the total amount of protein consumed, about 6% is typically absorbed by the human body. The rest is eliminated in the feces. Therefore, if you eat more protein than you need, most of it will be excreted rather than being used for growth or repair of tissues. The only exception is if you have a disease or medical condition that limits your ability to digest protein properly, in which case you would need to limit your intake of these foods.
The main concern with consuming too much protein is when it is derived from animal products. As mentioned, excess protein leads to its conversion into fat and then to glucose - especially if you aren't physically active. This can lead to obesity and other health problems such as high blood pressure, high cholesterol, and increased risk of developing diabetes mellitus. Eating too much protein also causes acidity in the body due to the formation of ammonia during digestion.
When too much protein is consumed, the extra amino acids created by protein digestion are transferred from the small intestine to the liver. Because excess amino acids must be eliminated safely, the liver regulates the amino acid content in the body. It does this by breaking down some of the amino acids or storing them away for future use.
The two main ways the liver eliminates excess amino acids are conversion to ammonia and creation of toxic compounds. Ammonia is a chemical that is used as a neurotransmitter in the brain and has many other functions within the body. Too much ammonia, however, can cause serious problems with cognition and muscle control. The liver also creates harmless substances called ketones when it detects there's an excessive amount of free amino acids in the blood. Ketones are used as energy sources by the body's cells. When the liver produces more ketones than can be used as energy, another pathway is activated which converts most of the excess into carbon dioxide and water.
If you have excess amino acids in your bloodstream because you consume too much protein, try reducing your daily dose of protein to below what is considered healthy. As long as you don't consume more than 100 percent of your body weight in calories, you should be able to eliminate excess amino acids through normal metabolism processes without harming yourself.