While packing in the greens is always a good idea, drowning the good stuff in dressing - even when it's fat-free - isn't so much. An Iowa State University study discovered that when there is no dietary fat, disease-fighting nutrients in veggies aren't even absorbed by the human body. In the study, people who slathered their salads in fat-free dressing consistently did not absorb carotenoids, which are antioxidants that have been proven invaluable to the immune system. This is because fat is actually necessary for carotenoids to absorb the intestinal cells and thus get sucked into the rest of your system. So if you want to choose a dressing smartly, think of ones with healthy fats from olive or nut oil.