Whether it's natural or not is irrelevant [0] (though I get your larger point) - but explanations would have been helpful.
The argument that, say, "walking for six hours a day is unlikely to be dangerous, since early humans did so" may or may not be factually correct, but it's a reasoned argument.
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.