Watercress tops the list of powerhouse fruits and vegetables, according to a study by William Paterson University. The high nutrient packed powerhouse vegetable is followed by Chinese cabbage, chard, beet greens, spinach, chicory, leaf lettuce, parsley, romaine lettuce and collard greens.

The consumption of powerhouse fruits and vegetables is associated with reduced risk of chronic diseases.

Researchers say that until now the nutritional value of veggies hasn't been calculated to determine the most nutrient-dense powerhouse foods

For the study, the researchers created a list based on the nutritional density of fruits and vegetables, using data from the U.S. Department of Agriculture. Of 47 foods studied, all but 6 (raspberry, tangerine, cranberry, garlic, onion, and blueberry) met the powerhouse standard.

"Higher-ranking foods provide more nutrients per calories," Jennifer Di Noia, an associate professor of sociology said. "The scores may help focus consumers on their daily energy needs, and how best to get the most nutrients from their foods. The rankings provide clarity on the nutrient quality of the different foods and may aid in the selection of more nutrient-dense items within the powerhouse group," ivillage reports.

The top 10 list is dominated by cruciferous and dark green leafy vegetables that contain high levels of vitamin B, vitamin C, vitamin K, iron, riboflavin, niacin and folate. These nutrients help prevent cancer and heart disease.

Lauri Wright, a spokeswoman for the Academy of Nutrition and Dietetics, said that these green leafy vegetables have minerals and fiber in higher quantities and few calories.

Wright, an assistant professor at the University of South Florida College of Public Health in Tampa, said that when people chop off the leafy part of vegetables such as celery, carrots or beets, they are actually letting go some good nutrients.

Nutrition from the powerhouse veggies can be obtained either by consuming it raw or by cooking them.

"Fresh, you have 100 percent of the vitamins and minerals," Wright said. "When you cook it, you might lose a small percentage, but it's not significant."

Di Noia said that after boiling spinach or collard greens, the nutrient-rich water should either be served along with the vegetable or be reused in sauces or soups.

The finding is published in the journal Preventing Chronic Disease.

Top 10 "Powerhouse" Fruits and Vegetables (Credit CDC)