I'd recently heard of fusel alcohol in a reference that it causes headaches/hangovers. First what is fusel alcohol, and what creates it (can I avoid it)? Secondly, is fusel alcohol really the main culprit behind hangovers? I know I stopped making/drinking mead as it was a guaranteed hangover for me the next day.
Fusel alcohols refer to alcohols that contain more than two carbons. Ethanol (the good stuff) is a two carbon atom molecule. They happen naturally during all fermentations, with higher concentrations occurring when the yeast are under stress (including high temps). Keep in mind stress is a relative term biochemically speaking.
Fusels contribute to the severity of a hangover for certain. However, hangovers are mostly still a function of dehydrated after consuming ethanol alone. The fusels just amplify the effect. Consuming too much pure ethanol will still give you a hangover, if ones hydration isn't taken care of too.
I think it's generally accepted that fusels are produced when fermentation temperature is not properly controlled, or nutrients are not at the proper concentration. Essentially, stressed yeast produce fusel alcohols. Apparently, most people believe that fusels promote headaches, but it's not scientifically proven.
In general, fermentation conditions that promote cell growth, such as temperature, aeration, and nitrogen, result in higher levels of fusel alcohols.
--Yeast, The Practical Guide to Beer Fermentation: White & Zainasheff, pp36-37
Yeast produce higher alcohol during fermentation as side product of metabolic pathway from certain amino acid i.e. Leucine, valine, serine etc. The fusel oil produced in fermented wash depends on the number of factors such as yeast strain used, nitrogen content and temperature. Fusel oil gives the pungent taste. It is less vita tile than the ethanol.
Ingested methanol (wood alcohol) is metabolized to formaldehyde which is a pretty nasty toxin to the human body. Too much methanol/formaldehyde can cause blindness because it messes with the retina...among other things.
Ethanol on the other hand metabolizes to acetaldehyde and then to acids which the human body can handle relatively well compared to formaldehyde.
Keep in mind that all alcohols are "poisonous" to the human body but some are more "poisonous" then others. Much of a hangover is your body's reaction to these "poisons".