I can't cite the science behind all of it, but if you are eating salads with no dressing (and no fat at all if they look like the salad you described), then your body is probably having a hard time processing all of that roughage. I forgot my salad dressing last week, and got heartburn after eating my salad with veggies and lean chicken breast dry. Plus it didn't taste good.
A lot of the nutrients in veggies are fat-soluble, meaning our systems NEED to have them with fat in order to be able to utilize what's there. If anyone else has a more scientific way to say it I'd love the help
Get some good olive oil and balsamic vinegar, shake them together (heck, even add a little dijon and some spices) and you'll have a healthy salad dressing that should help your body digest the veggies more efficiently.