A vegetarian is likely to experience gas, bloating and nausea after eating meat. The vegetarian's enzymes for animal protein digestion may have decreased, but if the vegetarian returns to a diet that includes meat, his body takes some time to adjust.
The Divine Caroline website quotes nutritionist Rania Batayneh as saying, "If you haven't consumed a type of food in a long time and you eat it, it just might take a longer time to digest, especially with meat." Because a vegetarian has a lower-fat diet, suddenly consuming a meat product that is high in fat also leads to uncomfortable digestion; eating leaner meats provides an easier transition to an omnivorous diet. "A person might have a little problem if he or she started right out on a huge steak," according to Professor Bjørn Skålhegg of the University of Oslo's Department of Nutrition. "Their body might not have sufficient levels of the right enzymes."
A small percentage of the population is allergic to certain meats; if a vegetarian has this type of allergy, he is likely to have an allergic reaction from ingesting a small amount of meat. Skålhegg indicates that a vegetarian diet does not necessarily lead to this type of food sensitivity, however.