Biotransformation means chemical alteration of chemicals such as nutrients, amino acids, toxins, and drugs in the body. It is also needed to render non-polar compounds polar so that they are not reabsorbed in renal tubules and are excreted. However, metabolism is sometimes not specific for the transformation process but may include other phases of toxicokinetics. Biotransformation is the metabolic conversion of endogenous and xenobiotic chemicals to more water-soluble compounds.