[header] How to choose the healthiest salad greens [title] Look for low-calorie greens. [step] If your idea of healthy is synonymous with low in calories, you might choose red leaf lettuce. This lettuce, like all varieties of lettuce, is mostly water. 
But it has half the calories of an equal serving of romaine lettuce, and about 40% of the calories you'd get in an equal serving of iceberg lettuce. It also has vitamin a and vitamin k in ample amounts.