I'm afraid all lettuces don't have a lot of nutrients in them even the romaine and green leaf lettuce. Here is a good food chart for you:
http://www.greenigsociety.org/foodchart.htm It tells you which greens and veggies to feed iguanas on occasion or as staple, but you can use it for bearded dragon. They pretty much eat the same thing in captivity. The occasional food is OK to feed like maybe 2-3 times a week to add variety. I usually mix the occasional greens and veggies in with the good ones, so this way my dragon and iguana don't get the full dose of it just like how they eat in the wild.... A little bit of this and a little bit of that so it won't harm them if they happened to eat something that could be harmful. But, yeah, the occasional food may be bad to feed as the main staple, but they do have a lot of other nutrients that you bearded and iguanas need, too. That's why I feed them...to add variety.