It is believed that the dessert interrupts the appetite, that is, as it was said, gives a feeling of satiety. It is also believed that "normal" food that you no longer want to eat is much healthier and more preferable to get enough of it than dessert. I suppose that this has nothing to do with the peculiarities of digestion as such. What enters the stomach and, then, into the intestine is somehow crushed, mixed, divided into proteins, fats, carbohydrates and macro-micro elements, being absorbed or moving into waste. In general, the order of the eaten is not so important - that 1 and 2, that 2 and 1, it will all mix up anyway. What may matter is the feeling of satiety that comes after eating some sugar. So, perhaps, my first thesis really does take place. On the other hand, this "quick" satiety passes just as quickly - and then you pay attention to the "normal" food and generally eat it too. So what is the difference, before or after? By the way, I disagree with the previous answer about the fact that fat and sugars (supposedly contained in excess in desserts, although even fruit can serve as a dessert, lol) are quickly deposited in excess fat. Pf, excess fat is deposited on the sides ONLY in calories. Weight gains only if you use less energy than you consume. If you feel so comfortable, you can eat 50% sweets if you manage to burn this energy.
In terms of digestion, there is some sense in this tradition: desserts tend to be high in simple carbohydrates and fats. When simple carbohydrates are consumed, the blood sugar level rises sharply and insulin is released, as a result of which the blood sugar also drops sharply and the person feels hungry. In addition, excess carbohydrates and fats are sent to the "reserve", namely to adipose tissue.
The main food eaten before dessert can somewhat "restrain" the absorption of these components, since, among other things, it contains dietary fiber , capable of reducing the rate of absorption of carbohydrates and fats and even contributes to the elimination of their excess.
In addition, I think that desserts are consumed at the end of the meal, as they kill the appetite. Their use creates a feeling of satiety, sometimes even excessive, due to the one-time intake of a large amount of glucose into the bloodstream. However, this feeling is short-lived.