It's called eating a balanced diet OTL.
Actually that's how you are supposed to reach a body that you prefer in Mabi. Eat the appropriate combination of food O_O. Eat too many berries and you turn into a stick. Eat too much meat and you grow fat.
...and you would think exercise would be involved in this. OTL
...although there are some people who like making their characters ultra-skinny. It's just personal preference.
Find me on Mabinogi:
-IGN: Riyokku (total level 149), Pyriah (total level 55)
-Regular Channel: Ch1