How does that work? Where does it go? Can the human body really break down pieces of glass or metal?
etc…
It comes out the other end, usually, in more or less the same state it went in. The human digestive system is incapable of breaking down metal or glass, except to the extent that stomach acids can etch some metals. I don’t imagine it’sany too good for their insides, either, and I certainly wouldn’t recommend anyone try it. There’s easier ways to get your RDA of iron and trace metals. 
Glass, if ground fine enough, is harmless.
The Dictionary Of Misinformation cites books going back about 200 years where doctors and scientist restate this. In one instance a doctor ate a spoonful of ground glass in open court.
The trick with glass eating is crushing the tumbler, or lightbulb between your teeth without cutting your mouth(I can’t recall where I read the interview with a sideshow performer who said this. At first, he’d eat 3 light bulbs a day for practice). Once you’ve crushed the stuff into a fine enough powder, it’s no more dangerous than swallowing gum.
Though used often as a poison in film (One of the Italians on the HBO series OZ died this way) or books, it just isn’t true. Glass crushed fine enough so that a victim wouldn’t detect it in their food, is crushed fine enough to be harmless.
Re Metal
I don’t know what the trick with metal eating is. However, some metals (zinc fer instance) are broken down by digestive juices. I can’t recall the cite, but I’ve read that kids can’t be harmed by swallowing a solid copper penny, but nickels are dangerous. If the outside layers seperate, the zinc core can end up pitted or jagged due to stomach acids and may cut the digestive system.
<similar> Stomach acid can’t touch the glass that you make out of sand and soda ash (technically, you can make a non-crystalline substance out of any substance. Quickly-frozen water is amorphous, as is quickly-frozen pure sugar glaze), but I would not suggest eating any metal in pieces big enough to see. Your body needs trace amounts, i.e., the amounts that are collected by the organisms you eat as food.
I don’t honestly know whether or not that guy who’s in the Guinness should be alive. He’s eating steel, aluminum, and copper all the time, including a Cessna airplane and shopping carts. I know for a fact that heavy metals like lead, cadmium, chromium, and mercury are dangerous to ingest.
Just a guess, but isn’t it likely that he simply vomits these metal pieces up once he’s off-camera?
Unlikely, as there are many, many witnesses and many people like you and me who would go and bug his apartment (well, probably a trailer) to make sure he didn’t. Also, there’s the question of professionalism: why do something as incredibly dumb as eat a Cessna and not go all the way through it? If it’s about attracting attention, then you have to make it real.
I remember that I once heard that you can reduce the risk of injuries by eating large amounts of kleenex strips and curd.
So far I have not found a cite and can’t remember the exact source and that is a really bad sign.
So if one of you bleeds to death trying that, please don’t blame me.
Which brings us to an excellent point.
DO NOT TRY THIS AT HOME!
The sideshow performer I cited earlier learned from a proffessional glass eater. This is not a skill you can safely acquire through websites or books. Yes, it is safe when done properly. So is chainsaw juggling. Neither should be attempted without the presence of a teacher who can guide you and call paramedics if necessary.