I've spent the morning re-reading GAPs and SCD. Interesting stuff but the main thing I keep thinking is how much it fits with paleo philosophy. Since the idea if problems arise from starches and disaccharides not being fully broken down to monosaccharides, so they don't get absorbed and then they feed gut bacteria causing overgrowth etc etc. Since according to paleo it isn't natural for humans to eat the foods the diets include- those high in starches and disaccharides such as grains and dairy, it makes sense a lot of people might have these types of issues on a regular Western diet.
Though how that fits with serotonin issues, I don't know. the only idea I can come up with so far is that we have got used to a higher level of serotonin due to eating lots of carbs, so our receptors are downregulated, in which ase after switching to a lower carb diet it would take around 6 months for us to adjust to the lower level of serotonin. That's assuming serotonin receptors work similarly with endorphin ones ie like the 6 months "flat" period after cutting sugar on Radiant Recovery, while the BE receptos upregulate.