What do adult snapping turtles eat?

They will eat anything that comes close enough to their mouth to snap at, including some things that might not be too good for them to eat.

In the wild they eat fish, birds, small mammals, insects, lizards, frogs/toads, plants, and just about anything else that gets too close.

In captivity, feed them turtle pellets from the pet store. This will give the turtle all the nutrients they need. For an occasional treat throw in crickets, meal worms, bloodworms, nightcrawlers, or small fish. I wouldn't feed them goldfish because they add a chemical at the fish farm to help the goldfish shine, and I doubt that it's good for the turtle to eat.