Lettuce leaves turned brown--------- UNSAFE??

               No, its not a health issue and does not indicate a loss of nutrients. Still, most people find brown lettuce unappealing and will want to cut off the discoloured parts.

Browning usually results from oxidation caused by exposure to air when the leaves have been bruised or cut.Bacteria and fungi can also cause browning, but these are not a health hazard. 

• Select lettuce that looks fresh (not wilted), since it will taste better and last longer.
• To reduce browning at home, keep lettuce in the coolest part of the refrigerator, away from high- ethylene- producing fruits (such as apples, pears, peaches, bananas , tomatoes and melon), the chemical which is a ripening agent and aids browning .Keep it dry.