Will they keep you fat? Absolutely not, as long as they are low-calorie or you budget for the calories (if you drank the full sugar ones and didn't count them in your calorie budget, that's a different story).
There are thousands of ways to encounter less-than-beneficial chemicals. Personally, I believe that environmental chemicals are far more dangerous than those approved by the FDA. Ultimately though, this is a personal choice. I highly recommend reading a book or two on food, cosmetic, and household chemicals (just type food additive guide in the amazon.com search button and a lot of good books will pop up. Read the reviews to find the ones you're most interested in and then check them out or order them through your library.
Once you know what the ingredients are, and the issues around the source and safety, you'll be less and less stumped by an ingredient label.
"Don't eat it, if you can't pronounce it," isn't terrible advice, but once you learn that some of those "mystery ingredients" are actually natural additives or even vitamins, you can make even better and less limiting choices.