Fiber Removes the Intestinal Mucus
One interesting patent from 1934 says it all – fiber is important even for carnivorous animals because it cleans the intestines. 
The author of that patent is Herbert J. Smith who worked for Ralston Purina Company, an animal feed company founded in 1894. Since 2001 this company is a part of Nestlé as Nestlé Purina PetCare (merged with Friskies PetCare Company).
Fiber counteracts the laxative effect of food
The invention described in patent is about using the beet fiber in preparation of animal feed for omnivores and carnivores. The author also stated that adding the molasses and cod liver oil to animal feed is preferable since the animals fed with feed containing the molasses and cod liver oil show better quality fur and are much more fertile than those fed on meat alone.
Herbert J. Smith in patent application also described why animals should be fed also with cereals and vegetables as vitamin and carbohydrate sources. He writes that wild living animals obtain that kind of nutrients from the stomach and intestines of their prey.
On the other hand the poultry which is fed mostly with grains would benefit from adding the animal source of protein like dried meat or fish meal.
However the mixed feeds with carbohydrates for carnivores and animal protein for omnivores have the laxative effects upon those animals, according to data in patent. The foxes fed with mixed diet showed on autopsy the intestines lined with mucous material which clearly interferes with proper food assimilation.
Now we will ignore the raising question – why laxative effect and why mucus in the intestines?
Let’s check how this laxative effect and mucus can be counteracted.
Beet fiber acts as a laxative in herbivorous animals but the author of the patent discovered that if fed to carnivores or omnivores it has the opposite effect.
The addition of beet fiber to the feed makes a huge difference since the foxes fed with mixed diet which included the fiber had no slimy mucous coating in the intestines. Their intestines were clean similarly like in those foxes living in the wild.
The beet fiber also counteracted the laxative effect of mixed feed (which included cereals and vegetables). In 1934 the author did not know how the fiber counteracts the laxative effect of mixed feed. Now we know that bile acids absorption by beet fiber is probably responsible for that effect.
We can debate if the beet fiber won’t negatively affect the animals fed on this diet (due to possible bile acids depletion, fat-soluble vitamin deficiencies, fat deficiencies, etc…) but the positive effect of fiber on slimy coating of the intestinal walls is what we should be focused on.
The patent is about the commercial feed for animals so 34% of cereals, 15% of dried milk and 10% of fiber and molasses is clearly far away from what would dog eat in the wild – almost 100% animal protein.
And that’s why addition of fiber to that mixed feed is just a fix for the problem – the production of the effective feed for the animals easily and not expensively.
Fiber in our diet
I think that the same is going on in humans who eat mixed diet but without enough fiber. We have our intestines covered in mucus (we can consider this as biofilm) and even the best nutritious food in not properly digested and assimilated.
Eating one apple won’t correct the intestines because there is