Low-fat foods are considered smart, healthy choices—usually. New research published in Molecular Nutrition & Food Research found that higher-fat salad dressings poured over fresh green salads unleashed more vitamins and nutrients into the digestive system than low-fat dressings.

Scientists in Indiana and Ohio prepared salad dressings based on canola oil, soybean oil and butter at three different fat levels. The dressings, which contained 3 grams, 8 grams or 20 grams of fat per serving, were poured over fresh-vegetable salads...