Why can't some intestinal system tolerate apples or oranges?

Answer 1

It is common for many people to have intestinal issues with citrus fruit. Citrus fruit, including Oranges and Orange Juice, are high in citric acid. Acidic fruits can cause a stomach ache, or even go as far as to cause some cramping and diarrhea in some people. This can be avoided for some people by not consuming citrus fruits on an empty stomach (especially in the morning), and taking Tums or other antacids along with the fruit.

As far as apples, Im not sure why most people would have a problem tolerating them. They are not particularly high in acid, and in fact, applesauce is recommended for people with a weak stomach, or after a fast or being sick.