That is because the fats found in rich dressings help the body absorb some of the most important nutrients in fruits and vegetables.

"If you want to utilize more from your fruits and vegetables, you have to pair them correctly with fat-based dressings," said Mario Ferruzzi, the study's lead author and a food science professor at Purdue University. "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."

Fat helps the body absorb carotenoids, chemicals found in plant foods that help counter the negative effects of free radicals. Free radicals are harmful compounds produced during metabolism and when people have infections. They can cause a plethora of health problems, including cancer, cardiovascular disease and immune system breakdowns.

Carotenoids, however, have antioxidants that stop free radicals from doing damage.

The Purdue researchers fed 29 people salads topped with dressing made from three different types of fat: Butter, a saturated fat; canola oil, a monounsaturated fat; and corn oil, a polyunsaturated fat. They also doled out different amounts of each.

They then tested the fat-soluble carotenoids in the subjects' blood.

The dressings made from monounsaturated fat were the least dose dependent -- meaning only a small amount was needed to absorb the carotenoids. So, olive oil or canola oil-based dressings are best.

Butter-based dressings were dose-dependent, but only a little bit, so those are also a good choice.

Corn oil dressings, however, required more than a hearty dollop to garner the intended effects.

The study was published in the journal Molecular Nutrition & Food Research.

 From the July 6, 2012, Prepared Foods Daily News