Question: My sister-in-law said that she won’t drink diet coke because it turns into formaldehyde in the body. I want to know if this is true, and if so how this occurs.

Answer: The artificial sweetener used in diet coke is aspartame. Aspartame has the following chemical structure:


When aspartame is metabolized, it gets broken down into phenylalanine, aspartate, and methanol (1). Aspartate and phenylalanine are both amino acids that the human body uses to make proteins. Methanol is a toxic alcohol, which gets oxidized in the human body to formaldehyde (2). Formaldehyde is then further oxidized to formate in approximately 1-2 minutes (3). Formate can be reduced to S-adenosylmethione in the folate pathway or oxidized to CO2 and tetrahydrofolate (3). As humans tend to have relatively low levels of the enzymes that oxidize and reduce formate, formate can build up causing acidosis if high levels of methanol are consumed (3). One study found that although most formaldehyde is oxidized to formate, approximately 2% of formaldehyde formed adducts with nucleic acids and proteins of rats who consumed high levels of aspartame (4). Formaldehyde adducts accumulate over time and may lead to DNA mutation (4).

  • Tiffany E.


  1. Walters, E. (2001, January 25). Aspartame, a sweet-tasting dipeptide. Retrieved June 16, 2016, from
  1. Methanol health effects. (2011, July). Retrieved June 16, 2016, from
  2.  T.R. Tephly. Comments on the purported generation of formaldehyde and adduct formation from the sweetener aspartame. (1999). Life Sciences. Vol.65, No.13, pp. 157-160
  3. Trocho, C., Pardo, R., Rafecas, I., Virgili, J, Remesar, X., Fernandez-Lopez, J.A., Alemana, M. Formaldehyde Derived from Dietary Aspartame Binds to Tissue Components in Vivo. (1998). Life Sciences. Vol.63, No.5, pp. 337-349.