Lettuce is a leafy green vegetable that grows in bunches and is popular worldwide. There are many varieties of lettuce grown today including romaine, butterhead, cos, endive, leaf, oak leaf, and others. Lettuce contains some nutrients that help to prevent certain types of cancer and heart disease. These include vitamin A, vitamin B, vitamin C, fiber, folate, iron, manganese, niacin, pantothenic acid, protein, riboflavin, thiamine, vitamin K, zinc, and copper.