You know you're supposed to eat plenty of fruits and vegetables, and a good rule of thumb is that those that are brightly colored or dark green and leafy tend to have more nutrients than other, paler, produce. But a new study's rankings of produce suggests that this is not always the case.

We’ve read and heard about powerhouse fruits and vegetables for some time. We know they are good for us. And they help prevent chronic disease. But we can be forgiven if we don't know which fruits and vegetables are best for us, since they have never been clearly rated.

The foods that didn't make the list might surprise you.

Now there is a nutritional report card on a wide variety of produce that can help you figure out how which have the most nutrients per calorie.

Using data from the US Department of Agriculture, Jennifer Di Noia, an associate professor of sociology at William Paterson University, looked at 17 nutrients the UN's Food and Agriculture Organization and the Institute of Medicine considered important to public health: potassium, fiber, protein, calcium, iron, thiamin, riboflavin, niacin, folate, zinc, and vitamins A, B6, B12, C, D, E, and K.

Di Noia ranked 47 powerhouse fruits and vegetables by calculating how many of these nutrients various fruits and veggies contained per calorie. Foods with the highest scores received the highest powerhouse rankings.

In other words, the most nutrient-dense foods made the list. Nutrient dense foods are those that provide the most nutrients in proportion to calories. The scoring system was not able to include all the phytochemicals present in foods because there aren't yet clear guidelines on their recommended intakes. So these scores don’t necessarily reflect all the components of a food that provide health benefits.

The top ten powerhouse foods on the list are all dark green leafy vegetables. Top scorer was watercress (with a score of 100), followed by Chinese cabbage, chard, beet greens, spinach, chicory, leaf lettuce, parsley, romaine lettuce, and collard greens.

The foods that didn't make the list might surprise you: raspberries, tangerines, cranberries, garlic, onions, and blueberries. Though they contain plenty of vitamins and minerals, they also have quite a few calories.

The study is published in Preventing Chronic Disease which also provides a Table with the entire list of Powerhouse Fruits and Vegetables by Ranking of Nutrient Density Scores. You may be surprised where some foods are ranked.

You can’t go wrong eating any fruits and vegetables. Just because your favorite fruit or vegetable didn’t make the list is no reason not to continue eating it. All fruits and vegetables are beneficial to our health, some just offer more nutrients — and fewer calories — than others.

It’s likely that future research will be able to take into account the phytochemicals fruits and vegetables offer. Such a list could look quite different. In the meantime, just keep eating those fruits and veggies.