Characters Unique Habits are usually Hints about their Nature or DFs
So Ryokugyu fasting for Years is our Big Clue
This suggests that his DF allows him to be Self-Sustained, meaning his Body is Modified, indicating he ate either Zoan or Body Paramecia or Logia, and i doubt it's Logia because logically all those Natural Elements don't need Food but we've never seen any Logia User fasting or claiming such thing, so it must be something else
If we Google for Animals that can survive longest without Food, Crocodile is pretty much only one who can last like Ryokugyu, so maybe he ate Ancient Zoan Crocodile DF. But if he ate a Paramecia, then maybe he ate Plant/Tree DF & he relies on Sunlight to survive
However it should be noted that Admirals should represent their Animal Theme & Color
It would look weird if Ryo ate a Crocodile DF but he represents a Bull
Imo Ryokugyu have Plant/Tree DF which allows him to survive without Food, be a Natural Disaster just like all other Admirals, he could have an Attack shaped after a Bull (Similar to Wolf Plant that Usopp have), his DF is related to Green which is his Color Theme, and also he could create Flowers, fitting for his Personality as someone who loves Beautiful Girls