Complex sentence: The lower gastrointestinal tract lacks the alcohol dehydrogenase enzyme present in the stomach and liver that breaks down ethanol into acetylaldehyde, which is actually more toxic than ethanol (drinking alcohol) and is responsible for most chronic effects of ethanol.
Split into simpler sentences: The lower gastrointestinal tract lacks the alcohol dehydrogenase enzyme present in the stomach and liver that breaks down ethanol into acetylaledhyde. Acetylaldehyde is actually more toxic than ethanol (drinking alcohol) and is responsible for most chronic effects of ethanol.