[eivarv]

You're not really correct: There are, for instance, poisons and venoms in nature.

Additionally, the example argument you make isn't an appeal to nature; The argument is based upon what early humans did (not nature, or whether something is "natural"), and is a resonable conjecture.

What I had an issue with in the parent comment was "Fasting is kinda natural" - which doesn't say anything about fasting in and of itself.