There is a rumor that eating bananas with milk causes diarrhea, and it offers the following explanations: one theory suggests that bananas are "cool" in nature while milk is "hot," and consuming them together leads to gastrointestinal incompatibility and likely causes diarrhea. Another theory analyzes the components of both: the citric acid in bananas causes the protein in milk to denature and precipitate, making it difficult to digest and absorb, thus leading to diarrhea.
Let's first address the first theory. To begin with, there is no scientific basis for classifying foods as "cool" or "hot." Even if we were to accept this theory, how would we explain the common practice of eating "cool" crabs with "hot" ginger sauce? Is it a "cool" and "cool" pairing, or a "cool" and "hot" one?
As for the second theory, the idea that the citric acid in bananas causes milk protein to precipitate and become difficult to digest is baseless. While milk proteins do denature and precipitate in an acidic environment, this does not affect their nutritional value. For instance, yogurt has denatured and precipitated proteins, but this does not diminish its rich nutritional value in any way.
For some people, milk can indeed cause diarrhea, which is likely due to lactose intolerance. Lactose intolerance refers to a condition where a person's intestines lack the digestive enzyme lactase, which is needed to effectively break down and absorb lactose from milk or other dairy products. The undigested lactose is fermented by bacteria in the gut, producing gas and causing bloating; in severe cases, it can lead to diarrhea. Lactose intolerance, which occurs after weaning when lactase production decreases, is very common in populations outside of those of white European descent, and most Chinese people have some degree of lactose intolerance. However, these individuals do not necessarily experience symptoms with every intake of lactose; symptoms usually appear only after a certain amount has been consumed. This "certain amount" varies from person to person.
In conclusion, eating bananas and milk together does not cause diarrhea. On the contrary, having milk and bananas for breakfast is a very healthy combination.