A few recent studies suggest that consuming fake sugar actually trains your insulin response to store more fat, not less. Basically when you consume real sugar, your taste buds send an alert to your pancreas that says, “Hey, calories are on the way! Prepare to produce insulin!” The insulin then helps break down the sugars, which either provide immediate energy or go into fat cells for storage. If your body interprets something as sweet when there’s not really sugar on the table, though, it may end up producing that same insulin response. So that diet soda is still prompting your pancreas to store fat, even though you’re not getting to enjoy real sugar—your brain can tell the difference. Artificial sweeteners don’t trigger our reward circuits the same way, so you don’t get the satisfaction of ingesting sugar.
And on top of that, constantly pumping up your insulin response eventually leads it to malfunction. This is essentially what happens in type 2 diabetes, but can occur to a lesser—but still harmful—extent in otherwise healthy people. Eventually your pancreas starts producing too much insulin in response to all food, making you pack on the pounds.
This whole theory is still being tested, but it’s in line with what we observe in people who drink diet beverages: they tend to gain weight. In observational studies you can’t tell whether that’s causative or if it’s just that overweight people tend to drink more diet soda than people at a healthy weight. One clue is that scientists observe what’s known as a dose response. The more artificial sweeteners people consume, the more weight they seem to put on. That suggests it might be the fake sugars themselves that prompt the gain in poundage, not just an association. [Source: Popular Science | Sara Chodosh