I actually agree with the OP's "general" point, just not the specific one. Yes, most people who are LC especially VLC, if you suddenly eat a ton of carbs, there's going to be 'adaptation' issues starting with the fact that your body isn't carrying the water weight necessary for proper processing and is likely to dehydrate important areas to come up with it pronto.

The larger issue is usually more that the 'cheat foods' contain gluten or lactose or other things people are usually mostly weaned off until then.

I also think you have to consider though -- and this does support the adaptation argument, but it highlights that there is a good reason for the shock to the system -- that sometimes, it isn't even just an issue of a far more SEVERE reaction than one used to have because you aren't used to it now -- though that is part of it; not merely about eating something different than you're used to; but actually about the change in our mental evaluation of the results, due to our new experience.

For example some people eat poorly for many years and they never know how bad they feel.They feel that way all the time, so like the reference to how a frog will boil in water if the heat turns up gradually enough, people just don't realize it. They go LC and as a side-effect usually take grains out of the diet, and they're shocked, once the detox period passes, at how good they feel, comparatively. Eventually they get used to that, and that is normal. Then they eat the stuff they used to, that they miss, and they're overwhelmed by how BAD they feel -- and sometimes realize that they always felt that bad and just hadn't realized it, and only changing their eating got them out of that cycle.

Sometimes it is as much an emotional realization of the situation, as a physical one.

PJ