This is an interesting issue. I've been battling with it ever since I started taking a serious interest in the effect of the food I eat on my health. More recently, I've become increasingly aware of the importance of other factors such as controlling stress (and now the nocebo effect), on health and so have made an effort to reduce unnecessary stress as much as possible.

The result is that I live in a delicate balance of worry/perfectionism vs. a more lax and accepting attitude. In other words, I tend to oscillate between periods where the negative (i.e. worry) predominates, and others where I decide I've been fretting too much and I should relax and enjoy. So there are times where I'm more careful with the things I eat and buy (no factory chicken, remove fat from non-grassfed meat, no nuts, no fruit whatsoever, no chocolate, etc.), followed by times where I let myself drift towards whatever feels good or whatever I can afford, with minimal guilt. After a few weeks the guilt increases and I tighten up again, and so on. Always within a paleo framework, though.

The placebo effect is widely accepted as being a real phenomenon, so I don't see why the nocebo effect would be any less significant.

I am pretty convinced that our mental attitudes and expectations can directly affect our physical wellbeing. Stress can definitely make you ill, whether the source is your negative bank balance or the fact that you just ate a piece of farmed salmon laced with sugary soy sauce. If the physical effect is strong enough, it could cancel any positive effects you might otherwise be getting from an even moderately healthy diet. Most likely this effect varies hugely between people and depending on circumstances, making it impossible to predict.

In summary, my philosophy is: avoid danger to a reasonable extent, but once it's done don't fret about it. Just learn from it, make the best of it and move on. Damage control. It's all about the balance. Maximise the positive (make reasonable efforts to eat the healthiest diet you can), minimise the negative (don't worry yourself sick about the bits that you can't control or the occasional mistakes you make).

It sounds like the argument here is: "don't worry so much about what you eat." Worry and stress is one thing, but one can eat in a principled way without worrying or stress.

For example, one of my principles is that I don't eat anything with sugar in it. I don't worry about it; I just don't do it. If I did accidentally eat some, I wouldn't stress out about it; I would just try to learn from the experience and make sure it didn't happen again.

For some people, small amounts of unhealthy foods cause real symptoms that are unpleasant: fatigue, brain fog, etc. There are good biological reasons for this, and very often the foods are ingested with the expectation that it will be a pleasant experience, not a bad one (no fear). For example, I had a glass of wine once, after not having had any alcohol for a couple of years, and had night sweats afterward. Along similar lines, if you've been on Paleo for some time, in or near ketosis, and then suddenly have a big bolus of carbs, you're likely not going to feel so hot.

BTW, if you're sensitive to nightshades, occasional cheats can completely prevent you from escaping their effects, since it can take several months for immune system effects to completely wear off, and even small amounts can cause symptoms to recur.

DISCLAIMER: Food allergies and intolerances are real and many people become paleo for these very real biological effects. My girlfriend is celiac and even a minute amounts of gluten unknowingly eaten causes severe stomach aches, so I understand this.

However I think the nocebo effect is little known and probably common when it comes to food. Unfortunatly it does not require a strong belief to cause it, orthorexia is not necessary. Simply being told something is bad for you and can cause some particular effects is enough to triggert those effects in some people. The nocebo effect is real and the physiological effects than resulting symptoms are comepletely real, not imagined, they can be measured in the body. One way to be sure an effect is actually related to a food is if you eat it unknowingly and feel ill afterwards.

A large percentage of the population now report having food sensitivities in surveys. In scientific studies on food hypersensitivities most of these people, when they are tested blindly (they do not know they are eating the food) do not react to their allergic food.

Vegetarians may get unpleasent reactions after eating meat.

Vegans after eating meat and dairy.

Raw foodists after eating cooked food.

Paleo people after eating grains.

Zero carb people after eating anything but meat.

In my opinion some of these result from the food and some do not, it is hard to untangle the two. Beware of symptoms after eating foods you think "should" be bad for you.

Yes. I would imagine that anyone who becomes orthorexic about a certain way of eating would be prone to this.