As a general rule, people born north of the Río Bravo (or Río Grande as some call it) have zero tolerance for any food which is mildly spicy. They even have a name for the effect it causes: “The revenge of Moctezuma.”
People born south of the Río Bravo on the other hand, have trouble taking down processed, deep-fried junk food. We don’t have a name for it but we can certainly smell it after about an hour of eating.
Conclusion: there’s something magical about the water in that river that requires further study with regards to its adverse effects on stomachs’ capacity to process food.