Citric acid (citrate) is a weak acid that is formed in the tricarboxylic acid cycle or that may be introduced with diet. The evaluation of plasma citric acid is scarcely used in the diagnosis of human diseases. On the contrary urinary citrate excretion is a common tool in the differential diagnosis of kidney stones, renal tubular acidosis and it plays also a role in bone diseases. The importance of hypocitraturia should be considered with regard to bone mass, urine crystallization and urolithiasis. The secretory epithelial cells of the prostate gland of humans and other animals posses a unique citrate-related metabolic pathway regulated by testosterone and prolactin. This specialized hormone-regulated metabolic activity is responsible for the major prostate function of the production and secretion of extraordinarily high levels of citrate. The key regulatory enzymes directly associated with citrate production in the prostate cells are mitochondrial aspartate aminotransferase, pyruvate dehydrogenase, and mitochondrial aconitase. testosterone and prolactin are involved in the regulation of the corresponding genes associated with these enzymes. The regulatory regions of these genes contain the necessary response elements that confer the ability of both hormones to control gene transcription. Protein kinase C (PKC) is the signaling pathway for the prolactin regulation of the metabolic genes in prostate cells. testosterone and prolactin regulation of these metabolic genes (which are constitutively expressed in all mammalian cells is specific for these citrate-producing cells. Citric acid is found in citrus fruits, most concentrated in lemons and limes, where it can comprise as much as 8% of the dry weight of the fruit. Citric acid is a natural preservative and is also used to add an acidic (sour) taste to foods and soft drinks. The salts of citric acid (citrates) can be used as anticoagulants due to their calcium chelating ability. Intolerance to citric acid in the diet is known to exist. Little information is available as the condition appears to be rare, but like other types of food intolerance it is often described as a "pseudo-allergic" reaction. Moreover, citric acid is found to be associated with maple syrup urine disease, primary hypomagnesemia, propionic acidemia, and tyrosinemia I, which are inborn errors of metabolism.