I would also like to see some (serious, medical) study, where it says that silicone lube is safe for internal use. I think some of it does get absorbed through the rectal tissue, which I think is not good. I don't think that the lube manufacturers do much testing, so I would not take it for granted that if it says on the label that it is safe, it is indeed so.
I guess a bit more testing goes into medical water based lubes, however, I haven't been able to find any in ordinary shops around my place. The ones found in online shops where of dubious origins.
I use olive and coconut oil as lubricants, although here in Europe you have to be careful what you pick up. If a manufacturer adds a single drop of olive oil into the regular vegetable oil, it can be declared as "olive" oil. Of course it is up to you, the customer, to find out what percentage is actual olive oil.
I guess (it is just a guess) that the most natural lube is your own saliva. But I think that it can be hazarduous, too, since it contain microbes and viruses that are harmless in your mouth, but can be otherwise in your bloodstream. I don't know the permeability of the rectal tissue to the microbes and viruses (should not be, since there is your own "natural" microbial world) , but in case of a tear or other injury to the tissue, they sure can get into the blood.
I guess the safest lube would be silicone (since it is most "pure" - the least different ingredients), but I really would like to see some medical research about it.
Happy (lubed?) play to everyone...