The substance is still psychoactive and will probably take a toll on her dopamine capacity and receptors if this behavior remains consistent. sugar has a low potency due to it's high LD50 and low psychoactivity but it is still having an effect, especially if she consistently drinks liters of soda a day. try switching her to juice or a lower calorie low sugar substitute and try having her phase out the soda. Use sugar as a reward for maintaining healthy habits not a daily pick-me-up.