Tomatoes are a good source of the antioxidants lycopene and beta-carotene. But if you eat a tomato without adding a little fat, your body is unlikely to absorb all these nutrients.
Scientists recruited graduate students to eat bowls of salad greens with tomatoes and various types of salad dressings. The researchers put IV lines into the participants’ veins and drew blood samples before and after they’d eaten the salads in order to get precise measurements of the absorption of nutrients.
When researchers went back and analyzed the blood samples, they realized that people who had eaten fat-free or low-fat dressings didn’t absorb the beneficial carotenoids from the salad. Only when they had eaten the oil-based dressing did they get the nutrients.
Sources: NPR July 27, 2009