Causes
Ethanol has a dehydrating effect by causing increased urine production (diuresis), which causes headaches, dry mouth, and lethargy. Dehydration also causes fluids in the brain to be less plentiful. This can be mitigated by drinking water after consumption of alcohol. Alcohol's effect on the stomach lining can account for nausea.
Another factor contributing to a hangover is the presence of products from the breakdown of ethanol by liver enzymes. Ethanol is converted to acetaldehyde by the enzyme alcohol dehydrogenase, and then from acetaldehyde to acetic acid by the enzyme acetaldehyde dehydrogenase. Acetaldehyde (ethanal) is between 10 and 30 times more toxic than alcohol itself,[5] as well as being cocarcinogenic (not carcinogenic solely by itself) and mutagenic.[6]
These two reactions also require the conversion of NAD+ to NADH. With an excess of NADH, the lactate dehydrogenase reaction is driven to produce lactate from pyruvate (the end product of glycolysis) in order to regenerate NAD+ and sustain life. This diverts pyruvate from other pathways such as gluconeogenesis, thereby impairing the ability of the liver to supply glucose to tissues, especially the brain. Because glucose is the primary energy source of the brain, this lack of glucose contributes to hangover symptoms such as fatigue, weakness, mood disturbances, and decreased attention and concentration.
Alcohol consumption can result in depletion of the liver's supply of glutathione[7] and other reductive detoxification agents,[6] reducing its ability to effectively remove acetaldehyde and other toxins from the bloodstream. Additionally, alcohol induces the CYP2E1 enzyme, which itself can produce additional toxins and free radicals.[8]
There are various nervous system effects: the removal of the depressive effects of alcohol in the brain probably account for the light and noise sensitivity.
In addition, it is thought that the presence of other alcohols (such as fusel oils), by-products of the alcoholic fermentation also called congeners, exaggerate many of the symptoms (congeners may also be zinc or other metals added primarily to sweet liqueurs to enhance their flavor); this probably accounts for the mitigation of the effects when distilled alcohol, particularly vodka, is consumed instead.[9]
Red wines have more congeners than white wines, and some people note less of a hangover with white wine. Some individuals have a strong negative reaction to red wine, distinct from hangover, called
red wine headache that can affect them within 15 minutes after drinking a single glass of red wine. The headache is usually accompanied by nausea and flushing[citation needed].
In alcohol metabolism, one molecule of ethanol (the primary active ingredient in alcoholic beverages) produces 2 molecules of NADH, using vitamin B12 as a coenzyme. Over-consumption of ethanol may cause vitamin B12 deficiency as well.