I think on most cruise lines they offer special health choices on the main dinner menu. I know Royal Caribbean does. If it's not on the menu, just ask your waiter for what ever you like. For lunch, the buffet will have enough of a variety for you to stuff yourself.
I've sailed HAL and NCL - both offered a wide variety of suger-free items. I've also found that it's also healthier to stick to dining room dining - both for portion control and dining choices.