. >> colleen thompson and ellen shanley are dieticians -- experts in nutrition, especially when it comes to fast food. they even wrote a book about it, "fueling the teen machine." >> we thought that teens were at a point where they're really inquisitive about things, and food and nutrition happens to be one of them. they're asking lots of different questions. we thought it was important to write a book geared to the teen, addressed to them, telling them the facts. >> just what is fast food? >> fast food is really anything we grab and go, right? you're running on the go, you grab something that's convenient, that's quick, and usually affordable -- could be a burger and fries, could be a tall latte and a muffin, could be a humongo-gulp and a hot dog. >> so what are the top four things we need to beware of? >> we think of number four being sugar, specifically added sugar. when teens go to fast-food restaurants, most frequently they buy soda, and soda is loaded with sugar -- lots of empty calories. so, not really a good choice. >> what's number three? >> the next one is fat. we all know that