I've noticed especially in the past few years that my body doesn't digest certain combinations of food very well- meat and white potatoes, hamburger and bun, barbeque chicken and potato salad. But if I eat the protein and veggies/salad or the carb and veggies/salad, I feel better.
My experience corresponds to the recommedations of food combining experts who say that our bodies weren't designed to mix all of these foods together at one time. They believe poor digestion can lead to other physical ailments.
I feel more energetic and healthier when I stick to proper food combining. You can do an online search for "food combining" to find resources that explain in more detail.