What could be healthier than a salad? The raw vegetables and fruits that comprise the foundation of any healthful bowl are chock-full of vitamins, nutrients, fiber and water -- all things your body needs to function optimally. And while you might think you're doing yourself a favor by adding a low- or no-fat dressing atop your pile of leafy greens, the truth is that these diet options may be preventing you from getting the full benefit of many of the nutrients that make salad such a good choice.

That's because an important class of nutrients called carotenoids, which include lutein, lycopene, beta-carotene and zeaxanthin, are fat-soluble. In other words, carotenoid-rich foods must be eaten with some measure of fat to make the nutrients available to humans.

Carotenoids, found in high numbers in colorful vegetables like carrots, tomatoes and onions, are associated with a reduction in cancer risk, protection of eyesight and a lower risk of cardiovascular-related death. One study even found that they can make you appear more beautiful.

"If you want to utilize more from your fruits and vegetables, you have to pair them correctly with fat-based dressings," Mario Ferruzzi, the lead author of a study and associate professor of food science at Purdue said in a statement. "If you have a salad with a fat-free dressing, there is a reduction in calories, but you lose some of the benefits of the vegetables."

The study, published in the journal Molecular Nutrition & Food Research confirmed this principle with a clinical study involving 29 people. Over the course of several days, researchers gave the volunteers three salads, each dressed with a discrete type of fat: butter (saturated fat), canola oil (monounsaturated fat), and corn oil (polyunsaturated fat). They also varied the fat content of each salad at either three grams, eight grams or 20 grams. After each salad, the researchers tested the volunteers' blood samples to determine carotenoid absorption.

When it came to polyunsaturated and saturated fat, the more a person consumed, the more carotenoids they absorbed. By contrast, people who ate salads dressed with monounsaturated fats absorbed about the same amount of carotenoids whether they had three grams or 20 of the fat-based dressing. Thanks to that difference, researchers believe that monounsaturated fat-based dressings might be a good option for those who are looking to restrict their fat intake. Dressings with monounsaturated fats include those based with olive oil or avocado, in addition to canola oil.