My capstone project was actually about how sugar affects metabolism. There is a lot of interesting research and unexpected interactions in this area.
One of the things that is most interesting is that they have found the same proteins that form our taste buds in the walls of our small intestine. It appears that the gut "tastes" food and "tells" the body what kind of food is coming and how to prepare for it. One of these pathways ends up producing insulin.
The interesting thing when it comes to sugar is that sweetness is not directly correlated with caloric content (or glycemic index, which is the measure of the sugar insulin actually works on). An obvious example are artificial sugars, which taste incredibly sweet and have zero calories. Natural sugars vary in sweetness as well though - using sucrose/table sugar as a baseline, fructose is 1.73 times sweeter for its caloric content. Furthermore, fructose a glycemic index of 19, compared to 55 for table sugar.
This is really interesting because body regulates hormone levels with feedback loops. So, if the taste buds in your gut taste sweetness and initially trigger a release of insulin that would be appropriate for table sugar, you could have 5 times more insulin floating around in your blood stream than your body was expecting. This might trigger some of these sugar taste buds to stop sending the signal to release insulin when they taste sweetness.
Worst case scenario is when someone drinks a diet pop on an empty stomach. All of the sudden there is the HUGE burst of sweetness, with no sugar in the bloodstream for the insulin to bind to.
This might seem very speculative, but there is some physical evidence that the body actually behaves this way. When a gastric bypass is preformed, a length of the small intestine closest to the stomach is removed. This section would have the most deactivated "taste buds", since it would be receiving the full load of unabsorbed sugars from the stomach. "Taste buds" further down the small intestine would not be deactivated because they haven't been triggering falsely.
Indeed, after the surgery, people appear to lose weight much faster than expected based on their previous metabolic rate, and their diabetic symptoms decrease far more than expected as well.
The question I get asked the most after talking about this is whether it's better to drink diet pop or normal pop. Unfortunately, it appears the best thing to do is to not drink pop at all. Limiting your sugar intake is a good idea for a lot of reasons, and soda is just far too sweet and/or too sugary to be healthy as a regular part of your diet.