Olestra was reformulated so that it was more hydrogenated. That pretty much removed the anal leakage side effect, except possibly in people who eat their own weight in potato chips at a sitting. Lay’s still sells it and people eat it without any problems.
They already sell gum that tastes like apple pie, strawberry shortcake, etc. I imagine they could make chewing gum that tastes like pizza if they thought people would buy it.
Many spices are essentially calorie-free in the amounts it takes to get the flavor from them. Not very filling, but if flavor is all you’re after, that should do the trick.
Is that the Vatican-approved diet plan?
“New ‘Pizz-zero’ – fortified with Bulimiamax!” 
That might count as “not ingesting food” but I don’t think it QUIIIIIIITE hits the mark of “zero calories,” since you’re going to have some trivial amount of the pizza go down your throat mixed into your saliva.
Did they get rid of the tongue-coating filthy sensation that Olestra debuted with? Because, if not, I’m still not buying.
The amount needed for flavoring is small enough that you can count it as zero calories; the same is true of aspartame (also made of amino acids) and some other artificial sweeteners, although some aren’t absorbed or metabolized.
There’s a product called Le Whif, which puffs out a little cloud of chocolate particles. Theoretically, you get the taste of a mouthful of chocolate while ingesting less than a calorie (and the manufacturer is careful to explain that you are meant to ingest the puff when it settles in your mouth, not inhale it). I think it also comes in a coffee flavor. Never tried it myself, though.
It seems that delivery method would work for other flavors. Not much help if you actually want to fill your stomach with something tasty yet low-cal, though…
Nice to see someone else who noshes on raw cabbage.
I like plain celery, plain cucumber, raw cabbage, plain iceberg romaine and butterhead lettuces as snacks. I really do not need any salad dressing or dips - but I am weird that way 
[I am a supertaster, and happen to like the flavors of what many people consider flavorless veggies.]
I just don’t understand stuff like this. Do you “pig out” on water, too? 
Seriously, what do these foods do for your appetite? Or is it just the “crunch” that you need? I personally don’t mind any of the foods listed above but the idea of eating them alone with no other food leaves me scratching my head.
Smell is a component of taste.
Also, texture is another component of taste. I like pizza not only because of its rich flavor but also because of its variety of texture - from its smooth cheese to its rough crust.
Can we replicate the texture and smell without calories? Smell - well, we have Le Whif. What about texture? I see that someone brought up lettuce as a good no calorie food. Well, I don’t find lettuce particularly appetizing party because of its uniform, and therefore, boring texture. Now, if it were gooey … like cheese …
I think you can make that happen already. Not particularly appetizing, IMHO, but gooey lettuce is certainly obtainable.