There is plenty of food included in the cruise price. On Adventure Of The Seas you have:
Included in the cruise:
Main dining room - breakfast, lunch, dinner (always free)
Windjammer buffet - breakfast, lunch, casual dinner (always free)
Cafe Promenade - desserts, sandwiches, snacks, coffee, tea - 24 hours (always free)
Room Service - sandwiches, salads, desserts - 24 hours (free except from midnight to 5am when they charge $3.95 service fee)
On Adventure the only places that charge to eat is Johnny Rockets ($4.95) and Portofino ($20). You can also get Ben & Jerry's ice cream and that costs extra.
Most cruise ship food is cooked to appeal to broad tastes, meaning it isn't highly seasoned. Gluten-free is available. So is vegetarian and lactose-free.
I did three cruises in the past 3 months. All on Liberty Of The Seas. I didn't spend one penny extra on meals and ate 'a lot' every day.