Kale may not be king when it comes to the nutritional powers of leafy greens, a new study reports.

Researchers at William Patterson University in New Jersey analyzed 47 types of produce for 17 vital nutrients-potassium, fiber, protein, calcium, iron, thiamin, riboflavin, niacin, folate, zinc, and vitamins A, B6, B12, C, D, E, and K-then ranked them based on their "Nutrition Density Scores."

While the entire list is interesting, what surprised us is how the various leafy greens' scores compared.

  • Watercress: 100.00
  • Chinese cabbage: 91.99
  • Chard: 89.27
  • Beet green: 87.08
  • Spinach: 86.43
  • Leaf lettuce: 70.73
  • Romaine lettuce: 63.48
  • Collard green: 62.49
  • Turnip green: 62.12
  • Mustard green: 61.39
  • Endive: 60.44
  • Kale: 49.07
  • Dandelion green: 46.34
  • Arugula: 37.65
  • Iceberg lettuce: 18.28

How in the world does romaine outrank kale? Heather Mangieri, R.D., a nutritionist in Pittsburgh and spokesperson for the Academy of Nutrition and Dietetics, says this type of ranking doesn't tell the whole story.

The list was calculated based on nutrients per calorie, so a Nutrient Density Score of 49 means that you can get roughly 49 percent of your daily value for those 17 nutrients in 100 calories worth of food, she explains. And some vegetables are lower in calories than others, she adds.

For example, watercress has only 4 calories a cup, while kale has 33. "You'd have to eat a lot more watercress to get the same amount of calories-and therefore that same amount of nutrients-as in a smaller serving of kale," says Mangieri.

Looking at nutrients by serving size gives a slightly better idea of what you might actually be consuming. Case in point: One cup of chopped watercress contains 0.2g fiber, 41mg calcium, and 112mg potassium. One cup of chopped kale, on the other hand, has 2.4g fiber, 100mg calcium, and 239mg potassium. Winner? Good ol' kale.

As for the calorie difference between kale and watercress, it shouldn't matter, not even to people watching their weight, Mangieri says. "Pretty much all vegetables are low in calories compared to the other foods we're eating, and the majority of us need more of them, not less."

Overall Mangieri says that variety is still the best way to go when choosing your daily greens, and that we should pick greens (and other fruits and veggies) that we actually enjoy eating. "Dark leafy greens are still great and packed with nutrients," she says. "But instead of sticking with just one, try to incorporate a mix of new ones. The best part is, you really can't go wrong with any of them."