One version is that we shouldn’t eat foods high in carbohydrates in the same meal as foods high in proteins or fats. One frequent explanation is that protein, carbs and fat require different digestive enzymes.
Second version is that Carbohydrates, fat and protein are digested the same way whether you eat them alone or not. What may vary is how fast your stomach empties. It’s true that if you eat a high-carb, low-fat, low-protein meal, your stomach will empty faster — but you’ll also get hungry sooner. To avoid spikes and crashes in your blood sugar — and your energy levels — mixed meals are the way to go. This does slow down digestion, in a good way, because it helps you stay satisfied longer after eating.
The text is taken from the page: http://www.seattletimes.com/life/wellness/carbs-and-proteins-dont-mix-and-other-nutrition-urban-legends/
I’m not a professional so I do not know what’s right but I know if I do not mix carbohydrates and proteins I do not feel weigh in the stomach, and I much faster losing weight.