Tomatoes are certainly nutritious – a good source of the antioxidants lycopene and beta-carotene. But consider this: if you eat a tomato without adding a little fat – say a drizzle of olive oil – your body is unlikely to absorb all these nutrients.

Scientists at Iowa State University figured this out a while ago. They recruited graduate students to eat bowls of salad greens with tomatoes and various types of salad dressings – from fat-free to regular Italian. “Basically once a month for several months we’d show up first thing in the morning,” recalls participant Gregory Brown, now a professor of exercise science at the University of Nebraska. 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.

“The salads all tasted the same to me,” says Brown. But 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.

Carotenoids are the pigments responsible for red-, yellow- and orange-colored fruits and vegetables. And carotenoids are also found in dark green vegetables such as spinach. The compounds convert to Vitamin A in the body, and studies have found that carotenoids have anti-oxidant activity which may help protect cells from damage caused by free radicals. Human studies have linked high consumption of fruits and vegetables to reduced risk of cancer.