If you know anyone who is dominantly or aggressively disagreeable - either constantly or randomly - then this one is for you. I mentioned it in the comments of this post on what happens with red dye and fructose malabsorption in my kids - kill off the serotonin level, and you kill off cooperative, calm agreeableness.
Tryptophan is essential for generating serotonin (and melatonin, and a bunch of other chemicals that make life go more smoothly). We don't make tryptophan, we have to absorb it from food (or supplements - I haven't tried 5-HTP, but I hear that's a useful one for people who don't absorb tryptophan well... a lot of people in the Fructose Malabsorption group I know have tried it and said it really helps in their cases). And if there's fermentation in the gut - anything from too much beans to too much fructose to IBS to whatever - then tryptophan absorption drops. And as a result, circulating serotonin drops. And then you get grouchy, stubborn, reactive, dominant-type disagreeable behavior. I won't. You can't make me. That's not fair. It has to be my way. Inflexible, emotionally rigid, un-sympathetic, hyper-individualistic (not 'us' first and certainly not 'YOU' first), crabby, yelly... yeah.
Keeping the fructose level down is our first line of defense - because the fructose and fructan levels in the US diet are so high in the first place (and have been since the 80's or so), AND we have this huge health kick thing (whole grain, lots of fruit, lots of veggies...) but still tend to indulge in the other stuff (lots of high fructose corn syrup or apple/pear sweeteners if you go organic/natural)... it's a big overload. And if we do that, then we might as well have just signed up for the study where they gave the participants tryptophan blocking medication and then tested their sense of fair play.
We tend to think we eat the same diet now as we did as kids - only we don't. When I was a kid, OJ was the juice, period. Apple juice was a treat, because it was expensive, and it wasn't considered healthful the way OJ was. Fruit was in season, because the out-of-season stuff was really kind of yucky. And it was expensive, too. Bread was sweetened with sugar (not HFCS), candy was sweetened with sugar, honey was a treat, and agave wasn't even on radar.
Now, most kid treats are sweetened with apple or pear plus HFCS. Apple and pear have a lot of fructose, plus sorbitol (naturally occuring), which reduces fructose absorption and creates fermentation. Gummy fruit and leather? Applesauce-based. We got fruit leather for Christmas as a kid - that wasn't a regular snack.
We did eat a lot of wheat, but most of it was refined. The most fructose is found in the wheat bran, which isn't in the white bread part. Go whole grain, and your fructose level jumps. It's part of what makes it work as a stool softener, too - osmotic diarrhea reaction to the fermentation ... that is, if you put a lot of fructose into the gut, your body will react to the fermentation by dumping fluids from bloodstream to gut (osmotic process - fluid going through a membrane). And then there's the fructans - long chains of fructose with a terminal glucose (think caboose on the train), which will also ferment. That's in the white part of the wheat. But again, we didn't only eat wheat as kids. We ate potatoes and rice and wheat and barley in soup and corn bread and corn meal mush. And we mostly ate big old idaho potatoes at that - the lowest in fructans. Meat and potatoes, very old style. Shepherd's pie with mashed potato on the top. Potatoes au gratin. Bad fake Chinese with rice. And yeah, a good bit of wheat, but not every meal. Lots of cheese and meat, in most houses I knew growing up. (Granted, I'm a little on the old side, if you grew up in the 70's and 80's, your experience will be different, already shifting.)
So, yeah, current US diet, way higher in fructose than we need. And that means the average US individual eating the average US diet will have less than their physiological normal level of serotonin in their body.
Issues with depression in the US, anyone? Issues with anxiety? Road rage? Going postal? Increasing social rudeness (okay, so that's the old grumpy me, GET OFF MY LAWN!, heh)? ME ME ME attitude? Non-negotiable stances? Stubborn 'bratty' kids (with equally inflexible tantrumy parents)? ADHD-like behavior is one of the hallmarks of my kids when fermentation strikes - especially loss of social inhibition (that is, they do things 'because' and despite the social cost down the line; those things are often mean or aggressive - like biting or hitting just because someone walked past too close).
Diet change would help. Now, the US diet is already starting to change in response to the backlash against HFCS. The cane sugar industry is hugely pleased, I'm sure. Cane juice is a little easier to digest, less likely to ferment (unless you exceed your personal absorption level, which isn't all that high, frankly).
I wonder also if some of the reason the Atkins diet works for people is that they are suddenly not fermenting anymore. Absorption of all nutrients likely goes up if the fluid is going OUT from the gut lining instead of pouring IN. Serotonin levels balance out, so mood-influenced eating is likely to go down. Bit of a hmmm there, for me.
Serotonin-related medication use is pretty high in the US. Depression rates are higher than many other countries. There are many reasons for that, but anything we can do to return our serotonin levels to normal is likely to help.