I've been a veggie for years, but recently I've been told that if I ever accidentally or intentionally eat meat again I may become violently ill, because my body isn't used to it. I've eaten small mouthfuls of meat a few times when there have been order mix-ups and I've never experienced this, but maybe it requires larger quantities.
Does this statement have any basis in fact?
Short answer:
Most likely not.
Long answer:
A sudden transition from a vegetarian diet to an omnivorous one will clearly have some effects, as any sudden change in diet will have. According to this article, these problems may appear:
A person might have a little problem if he or she started right out on a huge steak. Their body might not have sufficient levels of the right enzymes. Along the same lines, it can be hard to digest any large meal after a long fast or period of starvation.
However, there is a little (< 1%) chance of having meat allergy (independent of the fact one is vegetarian or not):
A very few – less than one percent of the population – are allergic to meats from common livestock, such as pork and beef. If a vegetarian has this kind of allergy, she could have a reaction from just a speck of these meats.
Yes, it's possible. I wouldn't say it's a guarantee though.
I was raised vegetarian and haven't had more than two or three meals with meat in my life (tried it, didn't like it).
I can always tell when I've accidentally consumed meat. The last time was a potato dish that had bacon in it (why??). It makes me sick, but not usually for more than the rest of the day, and I've never vomited.
The specific type of sickness is quite recognizable. Some of my shorter-term vegetarian friends report similar symptoms, but not all.
I assume I'm one of the least meat-adapted people around, so yes, consuming meat can make vegetarians ill but not likely violently.
It can happen but it depends on your medical situation. I wouldn't describe it as an allergy but some people are intolerant to meat.
I literally haven't been able to stomach meat for a very long time. Initially I thought this may be psychosomatic, having caught rotavirus while having surgery as a child (and outright refusing to eat meat for some time afterwards). If I'm not aware of a few traces of meat (e.g., fat residues) it won't set me off so I think it can still be digested but I have trouble with the texture and find it nauseating to swallow or sometimes even to smell (particularly mince and red meat). Thus I've been de facto vegetarian for a long time.
I'll admit this is very rare but I have met several other people who as vegetarian for such "medical reasons". Long-term vegetarians may indeed develop an intolerance but it may not be permanent. Some Indians raised in vegetarian families have been able to reintroduce meat into their diet. Guest from other cultures fasting from Ramadan (and abstaining from eating meat) are advised to gradually reintroduce it into their diets (fish, white meat, and then red meat). I found that I can tolerate fish products better and practice Pescetarianism (particularly with Asian cuisine where it difficult to avoid).
Thus I think it does occur but it is possible to reintroduce meat into your diet gradually, even if you do experience intolerance symptoms. Becoming vegetarian is not a permanent transition, it is reversible.
I know of no factual basis for these statements whatsoever. I have heard them perpetuated many times at many places and consider them to be some sort of urban legend.
As indicated in Alexei's answer, there are people with meat allergies, but for these people, meat would be the "illness", not the transition to it.
The only reason for feeling unwell after eating meat for someone following vegetarian diet that I can see is a psychological one. I have experienced slight nausea and stomach pains after discovering that in the foods I ate, there were parts of animals, like their fat or bones/tendons (these are used in my country to give a certain "taste" to soups). It still does not match the violently part of "violently ill" though.
EDIT: As Azor-Ahai pointed out, in his case (vegetarian since he was born), the effects of eating meat can be physical as well, due to the body not being used to break down meat at all. I suspect this would be the case with long-term (10+ years) vegans/vegetarians as well. Do not take my word on it though.
