My nutritionist and surgeon recommends that we all start eating clean as soon as we begin the consult process. We are making enough lifestyle changes that it just makes sense to adopt a diet where we avoid processed foods as much as possible.
To each their own.
There was also a time that doctors used to encourage smoking to treat ailments.
Soda, diet or otherwise, is incredibly unhealthy for you. Look at the ingredients. I don't even know what half of those chemicals are. I have seen soda take paint off of cars so, while I used to love it, I stopped drinking it as part of getting ready for surgery.
I am just a newbie pre-op but I have been using GNC Iso-define powder in Swiss chocolate. I can get 30 grams of protein for 225 calories and it tastes pretty good.