What’s weird to me is that this is a fairly simple experiment. A high school student could reproduce it. So if there was anything to it, some could and should have by now.
Ethical constraints? Either imposed or personal to the person performing the experiment…?
Perhaps at the high school level, but someone beyond that ought to have managed to give it a try. And as I posted above, “Mythbusters” did it and one of the cardboard-limited rodents was so starved that it resorted to cannibalism.
I thought about the experiment some more. I think some (perhaps all) universities have a bioethics committee that approves all animal experimentation to see if the pain and/or death caused to the animals is justified by the nature of the experiment. If there are such committees, I can’t imagine that such an experiment could be justified. So that may be a reason that this experiment hasn’t been reproduced.
The source cited by emmablue said:
To test that, you don’t need cardboard-fed rats as a control group: you just need to feed one group of rats on cornflakes, and another on ordinary rat pellets.
And frankly I don’t think it’s plausible. There must be millions of wild rats that have eaten breakfast cereals that they found in kitchen cupboards and pantries. If eating those cereals causes such extreme results, why haven’t they been observed in the wild?
Presumably rats in the wild aren’t subsisting only on cereal from the cupboard, but are supplementing with multivitamins from the medicine cabinet.
Ruminants (cows, etc), termites, and some other animals have symbiotic microbes in their digestive tracts that digest cellulose and provide the nutrients to the animal host. Rats, however, are not one of them.
To a termite, cardboard would be like crepes suzettes. 
I can’t believe you’d take that nonsense seriously. Toxic protein fragments! Oh noes!
Isn’t the whole point of the rat that it lives long enough to reproduce on almost ANY diet? Longevity is only valuable to animals with long long gestation periods, and which also raise their young for years. Thus a bad diet may kill a man in his 50’s. But he could be a grandparent by then. A rat which is 3 years old has bred how many times?
Perhaps, but ‘any diet’ still has to be one that’s composed of digestible food. Cardboard is not a digestible food for rats.
That’s why cereal is “part of this nutritious breakfast” … not the entirety of this nutritious breakfast. 
Well, if hunger’s all your worried about, I guess eating the box might help you stay not hungry longer…
Don’t know whether that’ll be because you’re throwing up later, though.