I really must defer to Taleb on this, I'll simply say he means natural in a more specific way than "all substances that exist in nature," and something more like, "the culture and patterns mankind has successfully practiced in relationship to nature for thousands of years with proven success."
That doesn't sound specific at all to me, but rather extremely vague and open to whatever interpretations might suit... What "culture and patterns" have we "successfully practised" "in relationship to nature for thousands of years"?
Diets vary crazily much, even within small geographical areas. Things like lactose intolerance is something we only "recently" started seeing on the wane, and it is wildly dependent on your heritage. I'm Scandiavian, and growing up I didn't even know there were people who had problems with dairy, as it was a total non-issue. I used to drink about a litre a day of milk on average growing up.... It was first as an adult I realised there are large parts of the world where people pretty much don't drink milk.
At the same time, I also consumed vast quantities of Scandinavian salted licquorice candy. Except most of if is ammonium chloride based. I doubt that would fit very well into the idea of something "natural", but it is an integral part of Scandinavian culture.
As is fish dissolved in caustic soda an integral part of Norwegian culture, and something we have "successfully practiced" for a very long time.
And processing such as hanging/drying, burying (until half rotted in some cases) and salting various types of food have been an essential part of human culture for a very long time.
And, yes, picking mushrooms and facing the risk of poison have been integral to our culture for a very long time as well, even though many of the most poisonous mushrooms keep claiming deaths up to this day.
Many of the traditional "natural" foods I grew up with are significantly less healthy than a modern heavily processed microwave meal, or the protein powder I use to supplement for my weight lifting.
