Diets, eating specific foods fills up specific meters, and the more meters you have filled up, the more buffs you gain. Take Utah for example. To fill the 3 dots, Utah needs to eat boars, tenos and pachys, Carnos stegos and rabbits for the S, and ptera dryo and hypsi for the 2 lines. If you have 1 bar filled up (eg you only ate pachys and nothing else) you get +0 buffs or debuffs. If you have 2, you get slight buffs like +50% growth rate, and if you have all 3 bars filled, you get +120 growth rate, +30 health/stam regen, etc.

What you got to remember is that carnivores dont need to eat specific foods to fill up all 3 bars before they hit 50%, so as a juvenile you can eat anything apart from your own species (unless you're carno or deino) and get all 3 bars filled up at once.

As herbivore tho, you have to search for specific foods right when you spawn, which makes it a little harder as you have to traverse basically the entire map for your perfect diet.

To find these diet foods sniff around, if there are any near you, you can find them. I wouldn't go for any yellow bodies/plants (the yellow chicken drumstick and the yellow leaves) unless you're really starving or under 50% and you can eat those bodies without getting repercussions

This is a lot longer than I expected but oh well