Dieters often say, “Hold the dressing,” but hold on: That might make those virtuous vegetables less nutritious, a study shows.


Researchers at Purdue University found that some fat matters – and that the kind of fat matters too.


For the study, 29 people ate salads topped with dressings that had varying fats: saturated, monounsaturated and polyunsaturated. Each salad was served with 3 grams, 8 grams or 20 grams of fat from dressing. The researchers tested the diners’ blood to learn how well they absorbed compounds such as lycopene and beta-carotene, which are associated with a reduced risk of problems such as cancer and heart disease.


Monounsaturated fats are found in canola and olive oils. Saturated fats are found in butter and cream. Polyunsaturated fats are found in corn and soybean oils.


Fat-phobes, take note: The researchers found that people needed the least of the dressings made with monounsatured fat to get the most absorption.


"If you want to utilize more from your fruits and vegetables, you have to pair them correctly with fat-based dressings," Mario Ferruzzi, the study's lead author and a Purdue associate professor of food science, 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 was published online Wednesday in the journal Molecular Nutrition & Food Research.
Researchers found that soybean oil was the most dependent on how much was eaten. The more fat, the more carotenoids the subjects absorbed. Saturated fat — butter, in this case butter — also depended on volume, but less so.


Olive oil lovers are in luck: Monounsaturated fat-rich dressings promoted the equivalent carotenoid absorption at 3 grams of fat as it did 20 grams.


“You can absorb significant amounts of carotenoids with saturated or polyunsaturated fats at low levels, but you would see more carotenoid absorption as you increase the amounts of those fats on a salad," Ferruzzi said.


The findings build on a 2004 Iowa State University study that showed better carotenoid absorption from full-fat dressings, when compared with low-fat or fat-free dressings.