when i was a new mommy to my son i learned a couple of interesting things...
1. when you crave a certain food, e.g. orange juice, tomatoes, stuff that's high in vitamin c, it's your body telling you that you need that particular nutrient. if you don't feel like eating it, you tend to not desire that type of food.
2. kids will eat what they want, when they want and in the portions they want. his pediatrician told me that was because they have kindof an inbuilt nutrition calculator. when their body tells them that's enough calcium/vita c/starch/calories/etc then they'll stop eating (or stop eating that particular food) for that sitting.
animals are the same way. a cat will flat out refuse to eat particular kinds of food if they don't like it. they'll take their food in portions, instead of scarfing it down until they feel full (which is actually NOT what you're supposed to do and does contribute to obesity). turtles will stop eating food if they get bored with the same portions (e.g. feeding them certain foods every single day) and will leave it alone for awhile. but when they need that nutrient they will go for it.
i'm quite verbose and i'm not sure i made my thoughts clear. if there's any misunderstanding i'll try to clarify it for you. or something. sorry

muddled thoughts.