Malnourished children eating a fruit (lychee) without any previous meal could get poisoned and have a intense decrease in blood sugar levels. That make me wond

Malnourished children eating a fruit (lychee) without any previous meal could get poisoned and have a intense decrease in blood sugar levels. That make me wonders on how many religious practices, superstitions, etc., still exists in modern cultures, transmitted mouth-to-mouth, due to some misunderstanding of the true mechanisms of some occurrence in the past? I can easily see that a "children can't eat fruits without eating some cereal" could have come as religious belief, from a long past. And it would be effective for hundreds of children.