My recent posting on diets started with a distinction (not original with me, but generally recognized in dictionaries) between two principal senses of the count noun diet in English. Restricting the definitions to people:
diet-1: the kinds of food that someone habitually eats [usually sg]
diet-2: a special course of food someone restricts themselves to, either to lose weight or for medical reasons [sg or pl]
Commenting on that posting, irrationalpoint noted a cultural specialization of diet-2, to
“a restricted set of foods, supposedly to lose weight, but with a dodgy scientific/medical basis, usually promoted by a celebrity or women’s magazine”. As in “I’m on a cheese diet” — which means “I’m eating only cheese”. This sense of “diet” got sufficiently popular when I was in my last two years of school that if anyone said they were “on a diet”, they would immediately [be] asked “what kind?” and if the answer was “generally healthy eating, you know, to lose weight. I’m not eating so much junk food”, then the reply was “oh, I though you meant a celebrity diet”.
There’s also a distinction that some people make between two subtypes of diets-1, between those, on the one hand, that are imposed by cultural authorities (as in religious prescriptions of and proscriptions against certain foods, in general or on certain occasions), are otherwise culturally normative (“what we eat and don’t eat around here”), or are necessitated by some people’s physiology (as allergies in the technical sense, inabilities to digest certain foods, or other inabilities to tolerate certain foods); and those, on the other hand, that are avoided or sought out as a matter of individual taste (perhaps a taste shared with others, or one arrived at on a principled basis, or simply a personal food attraction or food aversion). Call the two subtypes O and I (suggesting “obligation” and “individual”, though those glosses are just hints, not definitions).
A: Someone avoids shellfish because shellfish are proscribed by Jewish law or by the (vegetarian) Hinduism they practice;
B: Someone avoids shellfish because of an allergy to shellfish;
C: Someone avoids shellfish because they are committed to vegetarianism (outside of a framework of religious practice)
D: Someone avoids shellfish because they find shellfish disgusting in taste or texture
In the first two situations, we have O-type dietary choices, in the other two, I-type dietary choices.
Note that all are choices; you could choose to do otherwise. And all are choices that go against majority cultural practices.
But for O-type choices, there are consequences, sometimes quite severe ones. You can choose to disregard these consequences (many American Jews will happily eat dishes with shellfish or pork in them — definitely treyf — but only when these dishes are Chinese, in a restaurant or as take-out), and even in the case of physically-based food intolerances, people sometimes indulge in the forbidden foods, because they get pleasure from them, and then live with the short-term consequences — not really an option for people with life-threatening allergies, of course (as here).
Why make a distinction between O-type and I-type dietary choices? Because the two kinds of choices tend to be treated quite differently in our culture: I-type choices are likely to be dismissed as eccentric or frivolous and only grudgingly tolerated, or even derided. If I inquire, at a meal with acquaintances, whether the broth in some soup is a meat broth (chicken or beef), I might well be asked why I want to know (soup is soup, after all), and if I say that I’m a vegetarian, I might be further asked if I’m a Hindu or something like that; individually-chosen vegetarianism is likely to be treated as picky and frivolous and, frankly, annoying, while religion-based or culture-based vegetarianism calls for tolerance (perhaps grudging tolerance), at least in some places in our culture.
The urge to enforce general cultural norms is strong.
I come back now to diets-2. Diet-2 is a specialized development from diet-1, and diets-2 seem to be fact I-type diets: going on a diet is an individual choice. However, the distinction irrationalpoint saw at the beginning of this posting mirrors the O-type vs. I-type split: some diets-2 are undertaken simply for good reasons of health, while other choices of diet-2 strike many people as frivolous, faddish, undertaken for reasons of fashion, etc., and therefore open to ridicule.
People are quick to assign motives to other people’s behavior, even in the absence of evidence for these motives.