But there is also a theoretical reason not to believe things are so linear. The body is designed to react to stress, and a minimal amount of stress can even be beneficial, because of the way the body reacts to it. At least in some cases this is unequivocal.
- Muscles must be stressed and sustain damage ("microtrauma") before muscle growth occurs.
- DNA must be damaged by UVB radiation in order for the body to create melanin.
- Allergen immunotherapy works by exposing the body to larger and larger amounts of a substance which causes an allergic reaction. This process is clearly nonlinear and involves hysteresis!
I'm not arguing that ethanol is beneficial to neuronal health, but rather that the body can maintain homeostasis under stress, to an extent, as it does in so many other systems. For alcohol, we know that drinkers develop a tolerance. Heavier drinkers are able to produce enzymes more quickly and in higher numbers, and greater amounts of alcohol can be eliminated at a time. (Sufficiently heavy drinkers are still sustaining higher levels of all concerned chemicals, of course).
Ethanol exists naturally in the body, in small amounts (~3g/day), as a byproduct of fermentation in the digestive system. We're always producing a small amount of the enzymes which convert alcohol to acetaldehyde, and then acetaldehyde to acetic acid and acetyl-CoA, and so on. Is it such a stretch to imagine that the body has a feedback mechanism to deal with variations in these levels, and that that mechanism is effective at preventing neurotoxicity for small enough variation in these levels? At some point I think to argue there is no theoretic way the body regulates these low levels of ethanol, you'd just be arguing that living, itself, is neurotoxic.