Terms for these diets arose in response to growing numbers of people (particularly in the United States ) who have systematically restricted diets that do not meet the restrictions of more stringent diets such as vegetarianism or veganism. The logic of the terms is fairly simple: pollo is Latin for chicken, pesce is Latin for fish. These prefixes are then appended to the root word vegetarian. Since a vegetarian is one who eats plant-based foods but restricts or excludes animal products, a pesco-vegetarian eats fish and plants but may restrict or exclude other meats or animal products, and a pollo-vegetarian allows chicken.
Pesco/pollo vegetarianism, "pescetarianism" or "semi-vegetarianism" are neologisms coined to describe certain lifestyles of restricted diet. Most commonly, these include the practice of not eating certain types of meat (most commonly red meat such as beef, pork, lamb) while allowing others, such as seafood. There are usually no restrictions on non-flesh animal products such as dairy, eggs. Those observing such a diet often do so for health reasons although many do practice for ethical or religious reasons.
Semi-vegetarian is even more general, but is also fairly ambiguous. Arguably any normal diet could be called semi-vegetarian. In Britain during the early 1990s, some people used the term demi-vegetarian.
Pescetarian is a variant of pesco-vegetarian that dates back in print to at least 1993 . As of August 2004, "pescatarian", "pescotarian", and "piscatarian" could also be found on the internet, but "pescetarian" was perhaps the most popular. (The word is usually pronounced as English, not Italian, so this particular spelling awkwardly contravenes the usual rule in English that c before e is pronounced as s.) "Pescavore" is also quite common, formed by analogy with "carnivore" (though the more regular word piscivore already existed). "Fishetarian" was also used in print as early as 1992, but is no longer very common.
The word "pollotarian" can also be found in internet sources.
Note that these are ad hoc coinages using Latinate (not genuine Latin) stems to form new words. The Latin stem meaning "fish" is pisci- and the stem meaning "chicken" is pulli-.
Most objections to these new terms come from those who feel that pesco/pollo-vegetarians are misrepresenting themselves. Some fear that this may lead to the public understanding of "vegetarian" becoming skewed, and claim that already diners asking for vegetarian meals at many restaurants are offered dishes with "not very much meat" or "only seafood or chicken".
On the other hand, pesco- and pollo-vegetarians have a practical reason to ask for vegetarian food as a simple way to avoid red meats.
There are many possible rationales for maintaining a pesco/pollo-vegetarian diet. One is health, based on the perception that "red" meat is detrimental, perhaps due to hypercholesterolemia. Many pesco-vegetarians claim that eating certain kinds of fish raises HDL levels that protect against this condition, and also point out that some fish are a convenient source of omega-3 fatty acids.
While vegetarians and vegans sometimes claim environmental concerns as their motivation, this is less clear-cut among pesco/pollo-vegetarians. In particular, the pesco-vegetarian diet can be environmentally unfriendly if further precautions are not taken, due to the problems of overfishing, habitat damage, and by-catch. For this reason, environmentally conscious pescetarians commonly focus on eating the species that are most sustainably fished and avoid many farmed fish (e.g. salmon) as well.
For some the rationale is ethics: believing that either the treatment, or simply the killing and eating, of mass market meat animals is unethical. The justification for eating chicken or fish in this case is usually either "I have to eat some kind of meat" (see also complete protein), "chicken and/or fish are less intelligent than other animals", or in the case of pescetarians "fish are not mistreated in the same way that factory farmed animals are" or "hooked/netted fish do not suffer as much as land animals that are shot in the wild".
Another ethical consideration of many pescetarians has to do with the inefficiency of red meat as a food source. Most cattle, pork and chickens that supply our meat market are not free range. Instead, they are fed grains that are grown for the sole purpose of animal feed. Far more grain is needed to feed a cow, pig, and chicken (to a lesser extent) than the meat they provide. Were this grain to be grown for human consumption instead, far more food could be provided. Considerations of overpopulation and restricted amount of arable land usually play a role in this pescetarian rationale.
Yet another reason for maintaining a diet which can be defined as semi-vegetarian is if a person is physically unable to eat most kinds of meat.