Simple carbohydrates from refined grains are absorbed early in the GI tract, while complex carbohydrates from fiber reach the colon for microbial fermentation.

“The more simple carbohydrates, and like Justin was saying, these are the ones that are absorbed early in our GI tract so these are things from highly refined grains or packaged food types of simple sugars that are so prevalent in much of our Western diet, but then there's these complex carbohydrates. The type of carbohydrates that are found in dietary fiber and these are things that you find in whole grains, fruits, and vegetables, and these are the carbohydrates that the human genome is not very good at degrading.”