It’s not yet clear how diet soda would cause us to pack on the pounds, if it does. It could be that artificial sweeteners serve to train our flavor preferences, meaning that the additives dull our taste for healthy foods and prime our mouths for sugar. (Animal studies show that a sweet tooth can be induced, even in the womb.) If sweet taste is addictive in this way, then artificial flavors might worsen our dependence.
Another theory holds that artificial sweeteners fool our bodies from within. Sugar substitutes may lock onto receptors in the gut—receptors that have only lately been discovered—and gum up the body’s metabolic mechanisms. That way the additives could produce a hormone spike affecting hunger and digestion, even though they offer few if any calories of their own.