Lactic acid, molecular model. Lactic acid is a carboxylic acid that is produced constantly in the body as part of normal metabolism. Excess lactic acid and its equivalent (lactate) builds up in the muscles during exercise. Lactic acid is also produced by some bacteria used in food production and fermentation, and when present in the mouth these bacteria can contribute to tooth decay. Atoms are represented as spheres and are colour-coded: carbon (black), hydrogen (white) and oxygen (red).