Every time she gets a hit of the sugar in the soda, she'd get an insulin reaction... if she's always drinking it, her body is releasing more and more insulin and never getting a chance to normalize itself.

When's the last time she's been to the doctor, and has she been HONEST with him or her about how much she's consuming? I don't know ANY doctor who would prescribe chugging sugar all day as a tool to keep her blood sugar up. I suspect, being a nurse, she's diagnosing herself and self-treating... it's a common thing. Doctors and nurses can make the WORST patients!