Personally, I feed my adult snails twice a week, and on one of those occasions, I give them some protein. Everything must ALWAYS be washed thoroughly and served raw, fresh, and without any seasoning.
Here is a list of edible vegetables:
Zucchini
Fennel (not very palatable)
Mixed greens (not very nutritious; best offered rarely)
Tomato (sour; best offered rarely)
Sweet bell pepper
Broccoli (contains oxalates; give sparingly)
Sweet potato
Asparagus
Artichoke (contains oxalates, give sparingly)
Cucumber (contains oxalates, give sparingly)
Herbs
Swiss chard
Turnip
Radish
Squash
Savoy cabbage
Sauerkraut
Strawberry
Blueberry
Banana
Apple
Pear
Ripe avocado
Ripe mango
Cherry
Melon
Watermelon
Plum
Apricot
Peach
Hibiscus flowers
Purslane
Mint
Basil
Common clover
Dandelion
Rose petals
Below is a list of edible protein sources:
Lentils
Beans
Chickpeas
Peas
Snow peas
Green beans
Fava beans
Soybeans
Corn
Hemp seeds
Flax seeds
Pumpkin seeds
Chia seeds
Sprouts from the seeds listed above
Edible mushrooms
Insect meal or edible insects (crickets, moths, cockroaches, silkworms) that have been dried or previously killed
Meat-based baby food without salt, preservatives, artificial colors, lemon juice, or flavor enhancers
As for supplements, the snail finds everything it needs in its substrate except for calcium. It is our responsibility to always keep calcium available in the terrarium so that the animal can eat it and, with it, build or repair its shell. There are several options:
Cuttlefish bone: contains approximately 85% calcium carbonate
Crushed mollusk shells: contain approximately 98% calcium carbonate
Crushed eggshells: contain 95% calcium carbonate
Pure calcium for reptiles, without D3
Chalk: contains calcium sulfate but is still edible and usable