Originally Posted by kingwood-kid
If your friends can't tell the difference between light-colored and light in calories, I'm not sure any experiment would help. I also don't advise doing any brewing experiments you can't see yourself drinking. Guinness without roast is a slightly-sour blonde with a whole lot of flaked barley.
Sour? Why? It's not Foreign Extra Stout...
Other than that, I agree. A blonde Guinness will be totally unrecognizable, and thus won't prove anything. "This is a different beer!" they'll say. The roasted barley is pretty much the entire source of Guinness' character, and by extension its identity. So your friends will be right. Guinness isn't Guinness without the roasted barley. There is NO SUCH THING as "Guinness without the roasted barley". You'll just end up with a very boring beer.
I have a far better suggestion, which is much more likely to work. If you really want to convince them that Guinness isn't heavy, then show them something that is!
Give them two or three examples of the heaviest milk stouts, the chewiest oatmeal stouts, and/or the most monstrous Russian imperial stouts you can find. And then follow them up with a Guinness. If they don't then notice that it's, if anything, actually a *watery* beer, then nothing is ever going to work. Certainly not a "Not Guinness", anyways.