Every food item has a chance for a positive and a negative effect. The specific effect is tied to the food item, so the chocolate bar effect will be different than a fudge, and will be different than a candy corn, for example. But I really don't remember the specific effects of each of them, the wiki also doesn't seem to be of much help.
On food
Three sources: vending machines, containers, people
1) Food from machines is different depending on its state, indicated by its glow: green, red, golden. Green food has positive effects, red food almost doesn't. The machine is green, if it's your first machine of the game (in which case it will give three pieces of food) or if it's a different machine from the previous one you used. The machine will stay red until you use another. The machine turns golden, if you used every other before. So, if you activated Dungeon, Forest, Hotel machines, the Mansion one will turn golden. Golden glow resets upon the machine being activated. Vending machines completely deactivate, when you have three of their food items in inventory - they re-activate, when you eat one of the food items... obviously.
'Green' food:
Toffee/Fudge/Chocolate bar - all restore points of body soreness. Toffee restores, I think, 2 (maybe 3), but there's a chance it will cause a temporary intelligence penalty. Fudge restores 5, maybe 6, but with a chance of a temporary agility penalty. Chocolate bar restores 8-9, but always with a side-effect: on a good luck roll it increases either your butt or bust size, on a bad luck roll it fills stomach with semen (if you already have semen there, the chance is either guaranteed or severely increased, not sure)
Pink nugget/Creme egg/Pink lace - temporary stat increases to strength/agility/intelligence. However, with a bad luck roll you will not receive the bonus and also suffer some punishment: a large increase in arousal/creme egg bursts into semen and you are forced to eat it/lace becomes animated and hurts you for quite some damage.
Lollipop - decreases sex addiction by 1 down to the minimum and massively reduces arousal. If you have natural horny submissive, the minimum is at 8 out of 20, not sure. Be aware, that, while the lollipop will always decrease the true value of sex addiction, if you have high bimbo score (aggregate of oral/anal/tit-fuck/urine/cum/bukkake addictions + heel skill divided by some bullshit), your final sex addiction will still be quite high. With a bad luck roll, it will still apply positive effect, but will additionally increase lip size or oral sex addiction, if lips are already at maximum value (with artificial changes fetish your maximum lip size is increased to bloated sex doll lips)
Chewing gum or something - increases magic by 1 and restores a lot of magic, if your mana pool is not full. Negative effect is a permanent decrease of intelligence by 1 - no idea about the effect, if it's already at the minimum.
'Red' food:
Digestive biscuit - no positive, but will completely tire you on a bad luck roll.
Candy corn - there are many possibilities. Increases make-up, cursed wisp appears and follows you, and some others I can't remember. But there is 1 positive effect - heals you by a very minor amount.
Black lace - no positive, but will temporarily decrease all stats by minor amount on a bad luck roll.
'Golden' food:
Golden creme egg - temporarily massively improves agility or turns into semen upon eating (basically, better creme egg).
2) From containers:
An apple, a bag of nuts, a pack of crisps, a banana, a lemon. Apple - increases intelligence by 1, crisps - strength by 1, banana - agility by 1, nuts - don't remember... I think, it increases your love for domination? Lemon - "handle sour tastes better" (no idea). They all have no negative effects.
Container food can be given to robo-chef for improvement. Robo-chef will only agree, if you give him jewelry worth at least 2 (so anything other than sapphire ring). Don't give anything valuable: he will completely reset after 1 use no matter the value of jewelry given, and it doesn't seem to improve the effect of the final product. Upon cooking the food it will transform into gelato, cookie or some other food items. Can't remember the effects, but they will often improve all the stats, heal you and also increase some body part's size. There's a chance of it doing nothing or being semen, I think.
Container food can only be found in special chests - these chests look the same as regular chests, but are always found in the same locations. These chests can also provide rare equipment, alchemical recipes or spells. Dungeon: altar room; room with a chest, a crate, AND a sack; alchemy room; maid bottle room, and others. Forest: witch altar, statue ladder room, and others. Mansion/hotel: no idea.
3) Guard, shopkeeper give random candy, I think.
Do remember that all vending machine food items have their own alchemical reactions (randomised each playthrough). If you discover a recipe for one of the 'red' foods, you can pretty much farm the schoolgirl class by constantly spamming the recipe. Since there are only three 'red' food items, you can guarantee the required food item by simply spam-activating the same machine.
The former would be ridiculous with something like the minotaur.
