1
$\begingroup$

I know that digestion of cellulose is difficult in mammals and requires a lot of steps. But I am fascinated by the idea of one day achieving human digestion of cellulose.

Which got me thinking about how insects digest cellulose much faster than mammals, how is this possible?

$\endgroup$
  • $\begingroup$ What precisely do you think you "know" about human digestion of cellulose. The reference you quote says that the limited and variable ability of humans to digest cellulose depends on gut bacteria and not their own enzymes. It also says nothing about the number of steps. You cite no sources for your statement about insects but it appears that, apart from termites and the like, most insects depend on bacteria for their ability to digest cellulose too. So are you asking about the differences in human and insect microbiota? I won't speculate on the answer in a comment, but what do you think? $\endgroup$ – David Aug 17 at 22:26
  • $\begingroup$ So from your comment and the answer below, its seems like there is little difference between the ways cellulose is metabolized. This has helped me update my understanding. Thank you. $\endgroup$ – Gabriel Fair Aug 18 at 18:27
  • $\begingroup$ …except the bacteria might well be different, especially as they are symbiotic. The intestinal system of insects have some similarities to higher animals, but… $\endgroup$ – David Aug 18 at 20:23
  • $\begingroup$ What attempts to answer this question have you already taken? We ask that all question posters here attempt to search for an answer to their own question and explicitly indicate what research they've already done, what they learned, and what is still confusing or unknown to them. Our goal is not to simply be an answer site, but rather a site that promotes self-learning with some expert help along the way :). Please take a moment to edit your post with this additional detail, and it will likely be received more positively by our community. Thanks! $\endgroup$ – theforestecologist Aug 20 at 22:04
4
$\begingroup$

Update 2 (interpreting "faster" in terms of efficiency because it seems easier to measure)

The question has been updated but I still don't see a reference for insects being more efficient than mammals. Therefore I investigated this and did not find convincing evidence that it is the case.

Granted, the means of measuring efficiency are not necessarily comparable, but nonetheless it appears that ruminant cellulose digestion efficiency is comparable to overall distribution of cellulose-digesting bacteria.

(Overall, in both clades, the vast majority of species do not appreciably digest cellulose, so I focus on those that do.)

This paper suggests that while termites have remarkable ("nearly 99%") efficiencies they are a dramatic outlier among insects. In different groupings efficiencies range from 12-68%, 40-90%, and 11-50%.

This paper suggests that the "digestibility" of cellulose by cattle is in the range 0.7-0.8 (that is, 70%-80%). It is observed that the rate is higher in sheep (and goats?), though comparable figures are not provided. Other figures in the paper are in terms of animal body weight gain, which is unfortunately a poor measure due to the high energy cost of just living for warm-blooded animals.

If I interpret these numbers correctly, this means that ruminants in fact compare favorably to most celullolytic insects in their cellulose digestion efficiency. The very high efficiency of termites specifically is likely a facet of their being little microbial factories with legs; they are definitely an outlier among insects.

In conclusion, I don't think that there is any particular intrinsic advantage for insects in cellulose digestion.

Update

I went back and read the linked post and realized that much of my answer was covered there and thus somewhat duplicative.

What is not covered is the insect/animal comparison. I do not see the source material for the assertion that insects are more efficient at cellulose digestion, it might be true or be somewhat trivially true but I would need to see where you got this information to assess it.

I would suggest doing a little initial research to maybe get closer to an answer. For example, here is a review on cellulose digestion in insects, and here is a resource for mammals, here is a comparison between the two.

Original

Neither insects nor mammals (as far as I know) have the appropriate digestive enzymes to digest cellulose efficiently on their own. However, some special animals like termites and ruminants have assemblages of microbes in their gut that can break down cellulose. These enzymes are called cellulases.

However using these assemblages efficiently requires an extremely specialized gut system such as the rumen.

So you might get humans to digest cellulose by making transgenic humans with cellulases in their digestive system or completely redesigning the human digestive system. Possibly there would be a way to supplement with cellulase the way some people do with lactase.

A different concern might be that human physiology isn't set up to support that style of nutrition. For example I imagine that diabetes incidence would increase. But it's hard to predict.

If the idea is to feed a growing population of earth in a less carbon-intensive way, there are likely less complicated ways to do that by altering diet to e.g. less meat, popularization of sea vegetables, etc. But that's just my opinion, maybe you had something else in mind.

Hope that helps.

$\endgroup$
  • $\begingroup$ wow, you have worked so hard to answer my question. I know very little about human digestion of cellulose, but when I mentioned that cellulose being difficult in ruminant mammals, I was considering how much time it takes for grass to make its way though a cow's 'four stomachs', and the need for additional mechanical action, 'cud' chewing in order to make use of the cellulose. Where as locust, and snails seem (to my neophyte understanding) to have much less complicated digestion system. The locust digestive system having only 3 parts like humans. $\endgroup$ – Gabriel Fair Aug 17 at 23:16
  • $\begingroup$ ah, ok. Sorry to have gone down some unrelated rabbit holes. I agree that the stuff going through stomachs is probably faster purely in terms of having less tubes to go through in insects. However, that seems difficult to measure, particularly with regard to how it effects the animal's ability to actually digest cellulose, so I wasn't sure how to make a good investigation of it and did what I could. Hope it had some benefit! $\endgroup$ – Maximilian Press Aug 18 at 2:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.