system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does eating "junk" food after a long time of eating "clean" wholesome food result in discomfort in the body.

Like if you have been eating wholesome healthy meals for a long time without consuming "junk food" (e.g. fried/oily food, fast food), your body will feel discomfort when such "junk food" are consumed compared to people who have been eating "junk food" regularly in conjunction with normal wholesome meals.
gpt
Actually, going from junk food to healthy food also makes you feel discomfort in the early stages.

It's not so much the type of food that makes you feel discomfort, than the sudden change of diet. Your body is simply not used to breaking down these ingredients and needs time to adapt. In the meantime, you feel discomfort because your body is struggling to take care of these long-forgotten type of food.