Humans have a special enzyme to digest animal faeces?

[CRSP]

While reading a blog this morning, I came across this bizarre claim in one of the comments:

My parents are both biochemists, and they occasionally have to teach nutrition classes for medical students. Here’s their response whenever someone brings up returning to a natural diet.

The human digestive system has a unique enzyme that is able to extract nutrients from the excrement of other animals. Assuming the necessary evolutionary pressure for the development of this enzyme, what does this tell you about what we used to eat?

The commenter hasn't posted back any evidence for this, and Googling doesn't seem to produce any results. The claim seems outlandish (animal faeces surely is pretty varied, depending on the species involved), but also may contain a kernel of truth: after all, people still eat weird things, like insects, especially those in harsh environments, and if you're starving, then you may turn to anything in order to survive.

Is there any truth in this? If so, what types of animals are we talking about (presumably not omni/carnivores?), and what nutrients can we extract?

[Alien Crouton]

Humans also have gills in the embryonic stage; I trust it's obvious that means "we" used to breathe liquid as well. Don't go reading too much into the ancestral "we". AFAIK biochemists aren't required to take any courses in evolution beyond what's covered in biology 101, so take their pronouncements on that subject with a large grain of salt.

That being said, I have never heard this claim so I can't say whether it is true or false. I have never heard of any such enzyme either. However, here is what I do know about animal fecal material (which from here forward I'll be calling just "fecal material", because, duh, plant fecal material?)

(a) Fecal material sometimes contains undigested or partially digested food like proteins, vitamins, mineral, and fiber
(b) Enzymes are for breaking down organic molecules. Fecal material is not an organic molecule, it is a mix of various other compounds. We have enzymes for breaking down proteins, but that doesn't imply this was evolved for the purpose of extracting protein from feces.
(c) The human disgust reflex has been shown to be undeveloped in children specifically for food molded to look like feces

So it does follow logically that children or early humans may have, in times of great need, supplemented their diets with feces from which they extracted protein, vitamins, and minerals. But I strongly doubt that the biochemical capacity to do so is any sort of adaptation to eating feces; rather it is just a demonstration of the ability to digest food that has already been digested once.

[Rube E. Tewesday]

I'm also not getting what they mean by a "unique" enzyme. Certainly, it cannot mean that "uniquely in the animal kingdom, humans can digest feces". Without knowing much biology at all, I know that pigs and rats routinely eat feces, and I'm sure there must be other species.

[Exy]

How many calories are present in feces?

I would just assume most animal digestive systems are efficient enough that if there was much of actual nutritional value, it would already be extracted. I know rodents (or maybe only some rodents?) have to eat their own poo in order to extract some sort of nutrient or something but I have a hard time believing that there's generally very much nutrition to be found in poo. And there would have to be a lot for it to make up for the obvious potential to contract diseases.

[Rube E. Tewesday]

FWIW, here's the wikipedia article on coprophagia. Nothing I can see there to support the blog claim.

http://en.wikipedia.org/wiki/Coprophagia

[featherlou]

I agree with Alien - we can digest fats, carbohydrates and proteins. If that's what's in the feces, we can digest them. The idea isn't actually all that farfetched; elephants routinely eat elephant feces because their diet is high in hard-to-digest fiber, and two times through the digestive system is a good idea. I don't know anything about any special enzyme for humans to digest animal feces, though, and from what I know about microbiology it isn't a good idea at all.

[EthelMercaptan]

I'm also not getting what they mean by a "unique" enzyme. Certainly, it cannot mean that "uniquely in the animal kingdom, humans can digest feces". Without knowing much biology at all, I know that pigs and rats routinely eat feces, and I'm sure there must be other species.

Dogs. Just ask any dual dog-and-cat owner. Mmm, litterbox snacks! With crunchy sprinkles!

I don't suppose by "unique enzyme," rather than "unique to humans" they mean "suited to digest some substance uniquely found in feces"? Even if that's the case, I don't know what component you might find in feces that you don't find anywhere else. I figure the child of biochemists got a little mixed up about what they were actually saying.

[Zuul]

There are animals who don't just eat feces, but need to. Rabbits are a good example of this. They have a particular type of dropping they excrete, which they then need to re-ingest to keep their intestines populated with bacteria. There are animals like the above mentioned elephants, for whom copraphagia is like cud chewing. There are scavengers who'll gladly eat feces and there are carnivores who go after them, possibly as some ingrained instinct to mask their own scent from prey.

Gorillas have been observed eating their own feces and it's theorized that this may hold a similar purpose to that of rabbits or elephants, in some way aiding their digestion, so it does exist in primates. Since we do not have a heavily plant based diet like rabbits, elephants, or gorillas, it's unlikely that this would do us much good. However, it's possible that a biochemist--who wouldn't be an expert on this topic anyway--might think that because other apes exhibit this behavior humans must retain the ability to engage in it, too. And then there came some confusion from their child about enzymes and such, as we saw.

[Stranger On A Train]

I would just assume most animal digestive systems are efficient enough that if there was much of actual nutritional value, it would already be extracted. I know rodents (or maybe only some rodents?) have to eat their own poo in order to extract some sort of nutrient or something but I have a hard time believing that there's generally very much nutrition to be found in poo. And there would have to be a lot for it to make up for the obvious potential to contract diseases.

And you would be wrong, especially at the lower energy density links of the food cycle. There is substantial nutritional and energy value in the scat of many animals; witness that cowchips will burn energetically, and (as Caerie points out) rabbits, among other species, will reconsume their droppings to extract more nutrients. And of course, bacteria (especially anaerobic bacteria) feed heavily on animal waste products. Diseases are primarily an issue for species who are unaccustomed to eating feces; pigs, cows, et cetera, routinely ingest material intimately contaminated with their own and other feces without problem.

As for enzymes, an enzyme is essentially a protein catalyst that helps to reconform other molecules to make them easier (i.e. requiring less energy) and faster to break down. There would be no single enzyme for breaking down ingested waste products; instead, there would have to be a whole host of enzymes (hundreds or thousands) to do this work. It sounds as if the person mentioned in the o.p. (who is obviously not himself a biochemist) either misunderstood or deliberately mangled the statements attributed to his parents. However, as previously mentioned, some primates do consume feces at least as supplementation to diet (probably largely for vitamins and other micronutrients that are not completely absorbed on the first pass rather than net caloric and protein value) and so it is possible that our simian ancestors were shit-eaters.

Stranger

[Exy]

And you would be wrong

Never contradict me again. I know where you live.

[Dangerously Unqualified]

And you would be wrong

Never contradict me again. I know where you live.

Well duh, he lives on a train.

[Muffin]

I have eaten in the dining car of a VIA Rail train on several occasions, which confirms the OP's query.