Scientists have discovered that quercetin, which is found in grapes, is the reason why you are getting headaches after drinking red wine. Despite its health benefits, it is shown to contain an enzyme called ALDH2, which the body uses to break down alcohol. However, this causes a buildup of toxic chemicals as the quercetin prevents the enzyme from working effectively. As a result, levels of a toxic chemical made from the processing of alcohol called acetaldehyde are left to build up, causing headaches. "When quercetin gets in your bloodstream, your body converts it to a different form called quercetin glucuronide. In that form, it blocks the metabolism of alcohol," said Andrew Waterhouse, professor emeritus at the University of California, Davis. Studies of the chemical and the enzyme show that a 150ml glass of wine can result in 37% of the activity of the enzyme being inhibited. Quercetin is produced in grapes due to sun exposure and acts as a protective layer for the grapes, like sunscreen for humans.