Every food in this world is bad for somebody somewhere, at some point in their life. A food that is nutritious for you may be toxic to someone else, and vice versa. The one you can't get along without--the basis of your cultural diet--may make someone else sicker than sick. Might even kill them.
Even a food you've eaten happily all your life can turn on you, both with the changing biochemistry of age and with external events. For the former type of change...I used to do OK with cabbage and its relatives. Wasn't particularly fond of them (though young raw cabbage was pretty tasty!) but they caused me no real trouble, and I included cabbage in most of the vegetable soups I made. Then, in middle age, I made my usual soup, and with the first bowl of it I was on the fast track to the small tiled room and its plumbing. OK, maybe I was coming down with something. But no. It soon became obvious that cabbage and its close relatives were going to cause a huge inconvenience any time I ate them. Something had changed in my internal biochemistry, and cabbage was out (in more ways than one) so I changed how I made vegetable soup. And I wince when diet gurus extol the foods that are "superfoods" and supposedly good for me--and that I know will make me sick for a minimum of 24 hours.
For the second kind of change, take the Lone Star tick, Amblyomma americana, a common tick where I live. These ticks happily suck blood from humans as well as other mammals, and their saliva contains a sugar that is also found in red meat. The saliva also induces a typical immune response (redness, itching, swelling) and therein lies the way that a single tick bite can create an allergy to red meat. The body says "that sugar's bad!" and...there you are. Staring at your favorite steak or burger but unable to eat it. Bad enough the ticks can give you tick borne diseases, but forcing a dietary change in addition to sucking your blood seems...unfair. (Nobody said life was fair. I know.) Other sudden changes? People can become violently allergic to something unexpectedly. I knew someone once who had been eating shellfish without incident for years...until the day he had a life-threatening reaction.
The take home lesson here is that no general statements about foods are universally true. Cabbage makes me sick; it may not make you sick and may in fact make you feel better. What makes me feel good may make you sick. Some thrive on more protein than others. Some thrive on less.
When our son was growing up, there was a lot of hoorah in the media about special diets for autistic kids. Eliminate that, leave out this, add these (expensive!) supplements, on and on and on. Some parents opted for the extremely restrictive diets that had been recommended; some tried less strict diets, and some (like me) were not convinced that a reasonable diet for most kids wouldn't be reasonable for our kids. (My young autist had food likes and dislikes, of course, but most people do. And he turned against cheese and milk (previously among his favorites) suddenly one year and I...shrugged and made sure he had other calcium sources. He was going to be tall; he needed strong bones.) Aside from restricting sweets about as much as I would have for any kid, if he would eat it, he could have it. As I observed the lack of success most people had with restrictive diets and listened to the increasing hype online, on TV, in the news about food allergies and restrictions that were now supposed to be the cure for everything under the sun, I became convinced that a lot of what we hear about foods in the media is...driven by someone's desire for power or profit or both.
Obviously the food industry wants to profit from us. They produce the food; they want the market to support them. I am not a fan of most segments of the food industry--I don't think the fast food joints belong in schools, and we got along fine in elementary and junior high without soda machines or candy machines in the schools at all. I'm opposed to the routine use of antibiotics in healthy animals (increases antibiotic-resistant bacteria) and strongly prefer management practices that are "natural" (cattle and hogs on open ground, chickens running around eating greens and bugs.) Range beef tastes better, in my opinion; less of it is more satisfying than stockyard beef, and when we've been able to have a garden, we gardened organically. It's fairly easy to evade a lot of the advertising pressure if you cook your own meals most of the time, from scratch. Harder, if you're really short of time, or can't find a nearby market with good fresh vegetables and fruits and meat (which basically means, if you're poor and live in a big city) or can't have at least a small garden of your own. But still possible to eat a reasonably good, if imperfect, diet that no, really, is not about to kill you. When I was a kid, the only bread available was white bread; the only potatoes available were russet potatoes; every kid I knew ate white bread, white potatoes, meat, often canned vegetables...and we weren't fat or sick.
Increasingly, the diet industry and fitness industry are filled with "experts" who spout absolute nonsense about food, each with the aim of getting the consumer to buy their book and/or special supplements, and who try to scare people into their particular corner of the food industry--usually more expensive than whatever you were eating. The guru who tells you humans are genetically incapable of handling gluten...is dead wrong. Historically wrong. Ancient peoples ate wild grains that contain gluten for many thousands of years before humans started farming the wheat ancestors. Sure, some people get really sick if they eat anything with gluten in it. Others have no problem with it. The same is true of every other food that's been part of a traditional diet. Some people can't eat tomatoes or strawberries or cabbage or squash or melons or peaches or pineapple or papaya....the list goes on. Some people can eat tomatoes or strawberries or cabbage, etc. every day without any trouble at all. What a person can eat without a problem can change--slowly or suddenly, slightly or completely, for internal or external reasons. But fad after fad sweeps over the diet and fitness industries, and--just as with Big Agriculture--those fads are tied much more to someone's desire for power over others and/or profit from others than to the reality of food and any individual person.
I have friends who are vegetarian and friends who are not. Both are healthy. I have friends who are going gluten-free, and friends who are not. And both are healthy. Some don't do dairy; some do. Both are healthy. What we (the friends involved) do that works is let one another eat what each one wants without trying to change the other person. (Some of us weren't like this when we were younger, but we learned tolerance.) Given the reality of individual reactions to individual foods and diets, and the ephemeral nature of fads proposed by "experts", and the fact that in a large enough population you can find healthy people whose diets are completely different...this seems a sensible and humane approach. I believe there is no perfect diet for everyone. I believe there is no "superfood" and no totally evil food. The foods that make us--individually, one by one, and differently--healthy are the right foods for us. And they are not the same.