The 'overactive immune system' is the dominant theory and for a pretty good reason. The bacteria in our gut interact with our immune system and can go on to rev it up or calm it down. It's not just that our immune system is reacting to our gut bacteria, it's that the population is the wrong mix of bacteria and thus are creating metabolites that aggravate the immune system.
Often when we discuss intestinal health, it's the waste products of our bacteria that matter. They somewhat pre-process some of our food and they create vitamins that we cannot make.
Rabbits eat their poop as a source of protein. They eat grasses and leaves, and then they partially digest them for calories. Their gut then seeds the stool with nitrogen-fixing bacteria, which can make protein by pulling nitrogen out of the air. This occurs in the later stages of their digestive tracts, and so (for rabbits) the poop is nutritious. It's their way of getting complete protein.
Cows are similar, but backwards. Their giant stomachs contain bacteria that can create protein out of the air (and by digesting the cellulose in the plant material). This protein (in the form of bacterial organisms) is then digested by the later stomachs.
Often when we discuss intestinal health, it's the waste products of our bacteria that matter. They somewhat pre-process some of our food and they create vitamins that we cannot make.
Coprophagia is quite common. In the animal kingdom at large, that is. Somewhat less common among human beings for some reason.
Rabbits for instance, I believe, would rather eat their own droppings than anything else.
(Though I'm prone to exaggeration from time to time, I should point out.)
Rabbits eat their poop as a source of protein. They eat grasses and leaves, and then they partially digest them for calories. Their gut then seeds the stool with nitrogen-fixing bacteria, which can make protein by pulling nitrogen out of the air. This occurs in the later stages of their digestive tracts, and so (for rabbits) the poop is nutritious. It's their way of getting complete protein.
Cows are similar, but backwards. Their giant stomachs contain bacteria that can create protein out of the air (and by digesting the cellulose in the plant material). This protein (in the form of bacterial organisms) is then digested by the later stomachs.