It's not just that they can't eat as much though - there's very specific things that the drug discourages them from eating because it takes so long to digest and leaves them feeling sick to their stomach - primarily fatty meat. The ones I've seen on it seem to be able to handle starch just fine. They can also handle beans, vegetables and fruit. The starch component is probably the easiest thing to eat though because it's still digested faster than the other foods. (But still much more slowly than without the drug - and they can only eat about half as much as usual)

The most difficult thing for them to digest is fat, followed by meat, especially fatty meat, which means that 1/4 lb burger patty is not going to sit well in their stomach.

As far as I can tell, the main thing these drugs do is to enforce a low fat, low protein diet, and cuts calories even more by prolonging the amount of time it takes to digest even the most easily digestible foods (starch).

How that will translate into the fast food world, I really don't know, unless they decide to offer even smaller meat portions, with lower fat turkey/chicken burgers instead of hamburgers.

More likely through would be through the magic of fast food science, come up with easily digestible burgers that taste like beef, with the "mouth feel" of full fat burgers (while being low fat), offering the same visual volume of the Quarter pounder or Big Mac, but is lower fat, lower calorie, and digests more easily.

They could continue to offer starches, but smaller portions of them - although that probably won't go over well now that everyone is used to getting huge portions (even if the diet drug users can't really eat that much).

So that problem would go back to the fast food mad scientists - maybe they can come up with some kind of new fry-making process that provides as much volume as the current fries, while reducing the actual fat content so they're easy enough to digest, and providing the same taste and similar texture as the current fries... I dunno, maybe some kind of air-puffed potato inside with a low-fat crispy outside.

Same sort of thing with the burger buns. I don't see them back-tracking on portion size on any of this. They can already offer super-sized diet sodas, so that's not likely to change.

I don't see it ending in real food that's not being mucked about with to keep the diet drug users eating it. They'll do whatever is necessary in order to keep profits up.