Yes there are opioids receptors in the gut for endogenous physiological role. So exogenous opioids disturb the normal physiological function of endogenous receptors. There are also opioid receptors in our brains too. Exactly why having too much codeine and heroin are not good for you, and we also know that having codeine gives you constipation to say the least.
Nature has intended that the mammals drink their own milk. So mice will drink murine milk, human drink human milk, goat drink caprine milk, cows drink bovine milk, horse drink equine milk, in all these phylogenetically diverse mammals all of them have preserved the A2 protein in their breast milk, not A1. Thus nature has intended mammals to consume A2 protein from their respective maternal milk.
A few thousands years are short in relation to evolution. The modern human then and the modern human now were essentially the same. There are not enough time to "evolve" tolerance. The adverse effects, if any, were probably not enough to exhibit themselves when human life span was short, but with longer life span the adverse effects may manifest.
I agree that money is better spent to look into motility symptoms because that is the lowest hanging fruit.
A2M Price at posting:
$1.05 Sentiment: Buy Disclosure: Held
(20min delay)