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"In people who have been long-term vegetarians, studies have found that they actually lose the ability to digest meat". Is this true, at a biological level? (As opposed to any possible psychological effect.)

(Yes, the link is from the SciFi SE, but I'm sure there are plenty of vegetarian sites that would make the exact same claim.) I actually came to skeptics expecting to find a counter-response already, that I could link in to the SciFi post - I can't believe someone hasn't already asked this. :)

Another claim: Why do long-term vegetarians find it difficult to digest meat? - by Dr. Mehmet Oz, who is Professor of Surgery at Columbia University.

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John C
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    This isn't terribly notable. If you think other sites make the claim, that'd be a good thing to link in. Perhaps step 1 should be to ask this person for references? Or at least point them here so they can defend themselves! – Oddthinking Dec 16 '11 at 15:07
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    @Oddthinking, is a Professor at [Columbia University](http://en.wikipedia.org/wiki/Columbia_university), notable enough? :) – John C Dec 16 '11 at 15:18
  • I would expect this works similar to the way that antibiotics can impact digestion. So the [reintroduction of cultured bacteria](http://en.wikipedia.org/wiki/Fecal_bacteriotherapy) to the digestive tract could reverse it as well. – Chad Dec 16 '11 at 15:24
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    I'll accept [claims by him](http://skeptics.stackexchange.com/questions/6198/does-apple-juice-contain-dangerous-amounts-of-arsenic) as notable. Answers from him I'd want to double-check though. – Oddthinking Dec 16 '11 at 15:26
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    So is the claim "The first couple of meals of a novel diet will make you feel uncomfortable." which I find entirely plausible, or "You will never be able to digest meat again." which is how I read the original claim? – Oddthinking Dec 16 '11 at 15:29
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    @Oddthinking, that's a good question - neither of the claims specifically addresses a *time-factor*, they just say it exists. Rather than my changing the question, I think it's better to leave the issue for any answers. An example might be, "Yes there are enzymatic changes, but they are reversible after X days/weeks/ or with a specific diet plan". Of course, the answer *could* also be, "nothing changes, it's all psychological". :) – John C Dec 16 '11 at 15:46
  • It seems very unlikely: the ability to digest meat is not a muscle that can atrophy past the point of no return. I've been a vegetarian for more than ten years, and I'm still able to eat and digest a piece of meat when the circumstances require it. – Laurent Pireyn Dec 20 '11 at 13:28

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Becoming vegetarian does change your digestive and hormone production in the body. If you don't eat meat there is less demand upon the pancreas to produce the enzymes necessary to digest the meat. Given our digestive tract is suited to a omnivorous diet our body is likely to adapt to the decreased requirement for the digestive enzymes (i.e. stop producing them in significant quantities) and gut flora will change to the new dietry regime. For more see this study.

Changing back to a meat diet would then mean there is a shortfall in pancreatic secretions required to digest the meat in the short term. The changed flora in the gut would also be impacted. The "sick" feeling is likely linked to one or both of these factors and possibly something related to the hormonal changes the meat re/introduction could cause.

Tim Scanlon
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  • That first link is a good answer, it definitely shows enzyme changes - not sure the second link is relevant to digestive processes, as it involves changes in menstrual cycle length from meat versus soy. Also, neither link mentions changes in gut flora. I suspect you're right, as science has been discovering how important, gut biota are to health - but an actual reference would be nice. – John C Jan 17 '12 at 14:03
  • Sorry John, I did have more links but new posters are limited to two (didn't save the flora one). The reason I included the hormonal change was to emphasise the other body changes that can occur. Theoretically general feelings of ill health could actually be due to hormonal responses from the changed diet combined with poor digestion. – Tim Scanlon Jan 18 '12 at 01:32
  • Ah, forgot about the link limit. If you do find it, maybe put it in a comment, thanks. – John C Jan 18 '12 at 13:23
  • First link is not working anymore. – nullpotent Feb 05 '13 at 01:31
  • iccthedral: I've fixed the link and added another reference. JohnC, sorry about the flora reference, I'll try and remember it next time I'm at the work computer. – Tim Scanlon Feb 05 '13 at 14:39