When it comes to packing a nutritional punch, a colorful salad can't be beat. But the dressing you choose may be just as important as the vegetables for maximizing the body's intake of important nutrients, a new study shows.

When researchers at Purdue University fed study participants vegetable salads topped with different types of oil, they found that dressings containing canola oil best promoted the absorption of fat-soluble carotenoids.

Carotenoids such as lutein, lycopene, beta-carotene, and zeaxanthin are widely believed to promote good health. Specifically, studies suggest that they lower the risk for certain cancers, heart disease, and the age-related eye condition macular degeneration.

In the study, published online in the journal Molecular Nutrition & Food Research, salads dressed with canola oil, which is a monounsaturated fat, required the least fat intake to get a substantial level of carotenoid absorption.

People who ate dressings made with polyunsaturated soybean oil and butter, which is a saturated fat, had to eat far more fat to get the same nutritional benefit.
All Oils Aren't Created Equal

The study was small, with just 29 people.

But the findings suggest that when it comes to the absorption of fat-soluble carotenoids, all dietary oils are not created equal, says researcher Mario Ferruzzi, PhD, who is an associate professor of food science at Purdue University in West Lafayette, Ind.

In earlier research, Ferruzzi and colleagues found that people who ate salads topped with fat-free dressing took in almost none of the available carotenoids, suggesting that dietary fat is necessary to absorb fat-soluble nutrients.

In the new study, the researchers examined whether the amount and type of fat plays a role in nutrient intake.

To do this, study participants ate salads dressed with either 3 grams, 8 grams, or 20 grams of canola oil, soybean oil, or butter.

The typical commercial full-fat salad dressing has 10 to 20 grams of fat per serving, while low-fat versions have about 3 grams.

Nutrient intake was measured in the study subjects through blood samples.

Surprisingly, salads made with polyunsaturated soybean oil were the most dose dependent. The more soybean oil the subjects consumed the more carotenoids they absorbed.

Saturated fat dressings were also dose dependent, but to a lesser extent.

Monounsaturated canola oil dressings, on the other hand, promoted the same level of carotenoid absorption with a 3 gram serving as a 20 gram serving.
Olive Oil Also Monounsaturated

Olive oil and oils labeled "Hi-oleic," including some commercial soybean, safflower, and sunflower oils, are also good sources of monounsaturated fat.

The message is that monounsaturated oils may be the best choices for low-fat salad dressings if the goal is to maximize the absorption of fat-soluble nutrients, Ferruzzi tells WebMD.

Nutritionist Marion Nestle, PhD, of New York University, says the study adds to the now overwhelming evidence that eating a balanced diet is best.

She is the author of several best-selling books on nutrition, including the book What to Eat, published in 2006.

"Carotenoids and other fat-soluble nutrients are poorly absorbed from fat-free diets," she tells WebMD. "So this study provides confirming evidence that it's best to eat a diet balanced in fats as well as proteins and carbohydrates."