Romaine is the most nutritious of all lettuces.  But like all lettuces romaine has a vary high water content and has very few calories (about 10 calories per cup). Generally, the darker the green, the more vitamins and minerals (beta-carotene, calcium and iron, etc.) So it should be no surprise that . Romaine is an excellent source of vitamin C, (more than 5 times that of iceberg lettuce).