The type of oil you use in your salad dressing might make a big difference in how well your body utilizes the nutrients in those leafy greens and other salad fixings. Monounsaturated fats, like those found in higher amounts in olive and canola oil, are most effective at liberating the fat-soluble nutrients in salad veggies, according to new Purdue University research published in Molecular Nutrition & Food Research. The study compared dressings made with higher percentages of saturated fat (butter), polyunsaturated fat (corn oil) and monounsaturated fat (canola) at three different levels of fats. Salads were fed to 29 volunteers, whose blood was subsequently tested for fat-soluble carotenoids such as vitamin A. Salads dressed with 3 grams of monounsaturated fat promoted as much carotenoid absorption as those with 20 grams of the other types of fat. The findings also raise a caution about low-fat salad dressings, scientists noted, suggesting that you may be losing out on nutrients as well as cutting fat.