The basis behind psychoneuroimmunology is the fact that human behaviour is never really just human behaviour.
This is because the creature we call a human is really actually an ecosystem of tiny creatures who ideally function together in a complex dance involving sugars, proteins, and minerals. You are not really “you”, you are these little microscopic creatures utilizing energy. And your behaviours are really just a manifestation of the overall health of this system.
A recent example of this is the case of a woman in Florida who was brought into hospital to deliver her baby. She was found wandering the streets homeless and very pregnant; after delivery she bit her infant’s wrist for the blood. When the staff discovered this, they pried her off and diagnosed her with mental health problems; she will likely never see her baby again. The most likely cause for her behaviour is not actually psychiatric but molecular. Remember, up until two days prior she had been homeless, and pregnancy takes everything out of you. Chances are that her behaviour was motivated by extreme medical distress caused by iron and folate defunct causing a condition similar to pernicious anemia. Centuries ago we would have declared that she was demonically possessed. Today we declare her mentally ill. Hopefully tomorrow, we would recognize that she was in dire need of vitamins and minerals and that her cellular distress prevented her brain from engaging.
All her body likely knew was that it was absolutely desparate for the elements found in blood. I’m not suggesting that her behaviour was in any way appropriate or ok, but I am suggesting that it was motivated by biology and not merely “mental illness”. The whole case makes me incredibly sad actually, but it does illustrate how a nutritional deficiency can lead to extremes in human behaviour.
Not all mental illness is caused by deficiencies. Sometimes, it’s a manifestation of organ failure, like liver disease and neurotoxicity such as you see in Wilson’s disease or hematochromatosis, or mefloquine toxicity (though mefloquine toxicity is a lot more complicated than simple cirrhosis). Sometimes it’s indicating environmental health hazards like molds or bad water quality from aging infrastructure. Sometimes it indicates diabetes of celiac disease. But no matter the actual source of the problem, it indicates the distress of a collection of organisms desperately trying to function under increasingly difficult circumstances… A fact that is often overlooked due to the general stigma attached to mental health problems and one which acts as an impediment to people receiving real medical care.