> I don't know of toxic substances in nature. Any substance can be toxic if
> you eat too much of it or your body doesn't want it now. The question for
> me is not which toxic substances could be in the skin of a fruit but if
> I am adapted to digest them. And this is judged by my instinct which tells
> me, whether the skin tastes well or not. Antioxidants are needed in your
> body to react with free radicals (e.g. from cosmic rays) and reduce some
> oxidised substances.

I basically agree with you: a substance is not intrinsically toxic, it
only becomes so when the body doesn't need it anymore and has to
eliminate it. The point is that the skin generally contains more toxins
than the flesh, that's why it tastes worse.

I guess that our tastebuds act as chemical receptors which make a
nutritional analysis of the ingesta, so that the degree of pleasure
is proportional to the usefulness.

Suppose X is beneficial (pleasure +4) and Y is harmful (pleasure -6).
Suppose also that X is in the flesh, and Y in the skin. So, why not
eat X alone, instead of burdening the body with substances it doesn't want?

Another question would be: what is the resulting pleasure when we eat X
and Y altogether? +4-6=-2? Not so obvious... Maybe the brain can think:
"well, if I eat X and Y, it would still be beneficial to my body, the
job of eliminating Y is worth it since I will get the nutrients that are
in X". So, maybe some enzymes will be secreted in the saliva to make
our tastebuds insensitive to Y, and so the pleasure will still be +4.
Of course, things are more complicated than that, but I believe it is
basically what happens with pineapples: X could be, say, a sugar, and Y
is bromelain (an enzyme that digests proteins). While X is still beneficial,
we do not notice the acidity, but when our needs are fulfilled, X becomes
tasteless, and since anti-bromelains are no longer secreted, the taste
change is rather abrupt.

I guess that one of the effects of varietal selection was to eliminate
substances like Y, so, when X becomes useless, the there is no abrupt taste
change, the taste just varies smoothly, and we can eat many apples
without much pleasure (and without much displeasure either).

I also think that, in order to keep the balance between predator and
prey, the more useful the prey, the more efficient its natural defences
and/or fertility. That applies to animals, vegetables and fruits. The
defenses can be physical (coconut husks, durian hulls,...) or chemical
(anti-nutritional substances in grains and so on). Coconuts and almonds
do not need to synthetize toxins, but avocados and mangosteens do, and
I can see no advantage in eating them, except for the reasons I mentioned
(more abrupt taste change, digestibility,...). My hypothetic fruit,
whose flesh is very useful to the body, should have very efficient
defences: either have a very hard hull, diffucult to open without
tools, or have a very toxic skin, otherwise too many fruits would be
eaten and the species wouldn't survive...

Best wishes (hope I didn't bore too many people),

Jean-Louis