Roly polys eat decomposing vegetation. This includes rotting vegetables, fruits, plants, grasses and weeds. When roly polys are raised in captivity, they can be fed fish flakes, apples, carrots, potatoes, lettuce and wild leaf litter. Any leftover cooked vegetables can also be added to their diet, although they prefer fruits and vegetables that have been left out for a few days and are free of mold.
Roly polys also eat their own feces. This is because it contains copper that the creatures need to survive. Every time the creatures relieve themselves, copper is lost from their bodies, so they ingest their waste to recycle the resource. These crustaceans also eat decaying animals, fungi and wood, although their diet also includes live plants and seedlings. Live plants are a very small portion of their diet.
In addition to drinking with their mouths, roly polys can also drink liquids through their anus using tube-shaped structures called uropods. Despite this extra water-ingesting ability, roly polys don't urinate. Unlike other creatures that must release the ammonia their bodies produce through urination, roly polys can tolerate the ammonia gas their bodies produce easily. Instead of urinating, the crustaceans release the excess ammonia through their exoskeletons.