Do we excrete waste in same order of food intake? Obviously I do not expect we poop out molecule by molecule in exact order. All I am asking is, is it possible for us to pass stool's of today's food before we poop yesterday's food. Does our GI tract act like a First-In-First-Out pipeline always? Otherway to frame the questions is, If we consume hard to digest food 'A' first, easy to digest food 'B' next, is there a chance of B to come out before A?

It depends on the food you have eaten. Some foods are more difficult to digest than others, and will take longer to pass through your digestive tract. Once your waste has accumulated in your colon and formed a semi-solid/solid stool, then it's likely no material can make it past that, or combine with it, but until that point, it makes sense that material from items could intermingle and get jumbled up.

The following provides rough guidelines for how long it can take to digest food: <