A big bowl of crisp greens and colorful veggies may seem like the ideal vitamin- and mineral-packed meal. But in order to get the full nutrient punch from a salad, you've got to fatten it up – or else your body won't absorb the vegetables' carotenoids, which are plant pigments that can reduce the risk of cancer and heart disease.

You should still hold off on creamy dressings, bacon crumbles, and garlic-butter-soaked croutons, though – new research finds that when it comes to unlocking carotenoids' full potential, all fats are not created equal. Purdue University scientists discovered that heart-healthy monounsaturated fat, found in certain plant oils, provides the best carotenoid absorption per gram, while artery-clogging saturated fat (that's your blue cheese, bacon, and butter) offers by far the worst.

To determine this, researchers fed testers salads dressed with 3, 8, or 20 grams of canola oil (monounsaturated), soybean oil (polyunsaturated), or butter (saturated), each whisked with vinegar and water, and then checked their blood for absorption of key carotenoids such as lutein, lycopene, beta-carotene, and zeaxanthin.

Surprisingly, with both the butter and soybean oil, absorption capabilities increased as the dose sizes got larger, but the canola oil pulled in the same amount of carotenoids at 3, 8, and 20 grams. Translation for lunchtime: You'd have to use a whole lot more of a saturated-fat-based dressing – which would also tack on tons of extra calories – to get the same amount of carotenoids that just a drizzle of a monounsaturated fat–based salad topper could net you.

When shopping for salad dressing, check labels to see which types of fat are used, and go with monounsaturated canola, olive, or almond oil, says Jennifer McDaniel, MS, RD, spokesperson for the Academy of Nutrition and Dietetics."Typically, the shorter the ingredients list, the healthier a dressing will be," she says. "Simple vinaigrettes tend to have a nutritional advantage over cream-based options." And steer clear of nonfat dressings: They merely add calories while offering zero health benefits. McDaniel also suggests whipping up your own dressing from ingredients already in your kitchen, such as olive oil, vinegar, lemon juice, shallots, and garlic.

If you don't dig dressing, McDaniel says nuts, avocados, and olives are solid monounsaturated fat sources that can easily be tossed in with your greens to aid nutrient delivery, not to mention make them more filling. "Fat is satisfying, so if a salad is your main meal, adding fat will help it stick with you longer," she says.

McDaniel also points out that, along with carotenoids, fat aids absorption of other salad nutrients such as vitamins A, D, E, and K.