Well if I wish to be hardcore scientific about it I would so, but I didn't say ALL vermins/insects are. Even though I'm being a generalist grammatically and so. The reason behind why I writes it like this "vermin/insects" are because DF doesn't make any distinctions between them two, as it seems. I'm well aware of the fact that not all insects and vermins, in RL, are nutritious. I'm not that dumb to believe that eating rats and whatnots which is known to carry diseases are good foodstuff. But they can be if breed properly.
Peoples doesn't just "say so", as you put it, but they're actually proving it everyday throughout our world by actually eating insects and being healthy even though insects in general might spread diseases. Take the common cockroach as an example of a very common and sometimes dangerous vermin, because of diseases, that can be eaten and is nutritious. Again if breed properly in a safe and toxin free environment they're a viable food source.
So if I should carry on and post under this thread should I grammatically cover all my flanks and carry a tower shield of correct scientific terminologies relating to this subject? Since I usually believe in common sense and that peoples, in general, has one which means that I don't have to explain everything in detail.
We should, instead of being a hassle to each others, spend time and energy on this subject since it's quite kewl!