Lettuce belongs to the daisy or sunflower family (compositae). Lettuce exudes a small amount of a white, milky liquid when their leaves are broken.
Since it is primarily water, lettuce provides little health benefit beyond its nutrient content. In general, the darker the lettuce, the greater its nutrient content. All varieties of lettuce are a good source of chlorophyll and vitamin K. Romaine lettuce is generally the most nutrient-dense lettuce and is an excellent source of vitamin A, folic acid, and vitamins C, B1, and B2. It is also an excellent source of the minerals manganese and chromium.
Green leaf lettuce contains more than 100% of the daily value for bone-healthy vitamin K. The beta-carotene in green leaf lettuce helps with vision, while lutein and zeaxanthin protect the eye from the sun’s damaging rays, making this leafy vegetable super eye-healthy.
Lettuce has an extremely low calorie content (15 calories per 100 g) serving, and high water volume.
Sourced from: http://www.doctoroz.com/question?query=lettuce
Preparation and Storage
Dry the leaves with a salad spinner to prepare them for use in salads or for storage. Rinse and dry before storing in the refrigerator to remove excess moisture.
Crispheaded lettuce (eg. romaine) will keep for 5-7 days.
Butterhead and loose-leaf lettuce for 2-3 days.