Diet Soda is man made chemicals to "trick" your body in to believing you are consuming calories (and namely sucrose or everyone's favorite ... HFCS)

Trouble is, the mucous membranes (the things you "taste" with) ... AKA "taste buds ... They tell the pancreas to start producing insulin (ie. The hormone in your body that tells you cells to "wakeup and eat").

But, just like in "The Boy Who Cried Wolf" ... It doesn't take long for the cells in your body to start *ignoring" that alarm / warning that "food" (ie. glucose) is "on its way." They "learn" that the signal isn't really reliable ... So, they stop listening to it as much ... And start to really only wakeup when they're hungry.

This is called Insulin Resistance ... or ... Type II Diabetes.

Essentially, at that time, we need more and more garbage collectors at that time, as well as more garbage dumps. This manifests itself as cholesterol (garbage collectors) and fat (garbage dumps).

Eating better foods (nuts is a good example) helps increase the amount of stealthy or "thin" assassinators (aka good cholesterol) that is able to escape to the lower intestine via the liver ... The "garbage cops" (AKA bad or large cholesterol) still kinda help clear out the unwanteds in the blood, but they're too big to exit via the liver ... And eventually fall down and accumulate in strange places ... Otherwise known as heart attacks or stroke.

Water, it's just chill... It quietly slips in the front door without "alerting the staff" that they're walking in... If slides on down to the central gathering area, and is immediately allowed to come right in ... No problems.

^(And yeah, I could probably continue or expand and further "clarify" the analogy...)