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.