Just one cup of this dark, leafy green (30 calories) contains about 3 grams of protein, 4 grams of fiber, 106 percent of your daily vitamins A and C, 17 percent iron (so much energy!), over 500 milligrams of potassium (more than a banana), 10 percent of your vitamin B6, and 34 percent of your daily magnesium. Here are some reasons you should be adding this green into your repertoire.