Proteins can stay in the stomach up to 6 hours before having completely moved on to the small intestines. Sweet foods which are high in simple sugars need to leave the stomach much sooner than the 6 hours. If they do not, they ferment really quickly.
If you chew both protein and sweet in the same bite, what you swallow is well mixed together. If you alternate one bite of sweet stuff, they end up stacked on top of one another and form a pile in the stomach. Either way, because there are no gastric mechanisms to first separate the different types of food, and then hold one off (protein) into the stomach while the other (sweet food) exits at its base and on to the small intestine, sweet foods will be help up with the protein. Under body core temperatures, sweet foods will ferment rapidly, bacterial decomposition will set in, and this will thwart protein digestion.
Many acids(as found in protein digestion) tend to destroy sugar molecules. As a result both protein and sweet food will go to waste, thus increasing the body’s toxic load and contributing to weight gain.
Examples of protein foods are eggs, fish, seafood, nuts, meat, seeds etc. Examples of refined sweets are cake, candies, cookies, ice cream, white sugar, brown sugar, muffins, sweeteners etc