The creature's diet is determined of what specific mouth part the creature possesses that is either carnivorous herbivorous, or omnivorous. For example, if a creature has a carnivorous mouth like D'orca, it will turn it into to a carnivore, placing a herbivorous mouth that will turn it to the herbivore, and an omnivorous mouth will turn it to an omnivore. Likewise, placing both carnivorous and herbivorous mouths on the creature's body will convert the creature's diet to omnivore.
Lacking a mouth will make them "mouthless" and disallows to save that creature without using hacks or modifications.
Possessing specific mouth part will restrict the cell from consuming a food that doesn't correspond with the diet. For instance, swimming towards the meat chunks with Filter Mouth or Proboscis cannot digest it, unless it possess the Jaw mouth to chomp it down when in contact.
Consuming specific food will affect the meter of traits in the evolution timeline, with an arrow pointing the trait. Consuming meat/prey or drinking cells will raise the meter up to the carnivorous side, while on the contrary, consuming plants will drop to the herbivorous side. Eating a balanced amount of food simultaneously will balance the meter to the omnivorous side. Advancing to the next stage with that correlating trait will influence the availability of mouths and restricts mouths with distinct diets that doesn't correlate with the trait in the Creature Creator. That trait also rewards the consequence abilities and benefits in following stages.
If the creature started as that specific diet or evolved from a cell with that associated diet trait--as previously mentioned--all of the mouths with distinct diet will be forbidden to obtain throughout the stage except for those that correlate with the Cell Stage's trait or specific diet is chosen from the beginning of the Creature Stage.
Depending on the creature's diet, the diet of the creature alters the effects of eating a certain food. Eating the right food that coordinates with the creature's diet will replenish the hunger bar and some health points. As an opposite, eating the wrong food will cause the creature to vomit and give no nutrition value to fill the hunger bar.
The diet of a tribal species will affect what sources of food and abilities are available in the Tribal Stage. For example, if a tribe is herbivorous, they will gather seaweed in fishing hot spots and restricted from collecting meat, while carnivorous and omnivorous tribes exclusively gather fishes.
The traits from the Cell Stage also make them get unique consequence abilities to efficiently obtain food for each diet respectively. These include Refreshing Storm, which makes fruit for herbivores re-grow and fall from fruitless plants. Carnivores get Traps, which attract wild creatures and then kill them and any creatures nearby, allowing the tribe to get meat. Omnivores get Flying Fish, which summons the Sea Monster to force fish onto shore.
Another type of diet, not usually seen in-game, is the "starvivore" diet. The term "starvivore" is derived from Spore's game files referring to the starvivore: "cl_starvivore.prop.xml", "starvivore.prop.xml", "starvivoretemplate.prop.xml", "#DD41AEA5.prop.xml", and "#A007B1DB.prop.xml", and it is for creatures that lack the diet from the mouth and it is basically an alternative term for "Mouthless" which can be seen in-game.