Lettuce - eat it fresh

As you would expect storage of vegetables post harvest reduces their nutrient content. Still on the lettuce theme shredding lettuce then exposing the lettuce to light results in massive losses of nutrients. For example 

Shredding of lettuce leaf followed by exposure to light produced significant losses of the flavonoid moiety in the green oak leaf (94%), red oak leaf (43%), iceberg (36%), green batavia (25%), lollo biondo (24%), and lollo rosso (6%) samples, whereas cos and green salad bowl samples did not show an overall loss

 So depending on level of lettuce you can destroy all the nutrients in some varieties. Why is this important? Well in modern food processing they often shred the lettuce well in advance of serving it. For example a burger that has shredded lettuce - it isn't shredded when you ordered it - no it was pre shredded. Who knows the time that it has been processed for. I suspect the 12 - 36 hour space.....

They also looked at storage of lettuce. They found over a week in low temperature storage (household fridges are normally at 3 -4 degrees) they found extensive reduction in antioxidants.

Storage of whole heads of both lettuce and endive in the dark at 1 °C and 98% humidity for 7 days resulted in losses of total flavonol glycosides in the range of 7−46%.

So prep your lettuce just before consumption - and try to grow your own so it has optimal nutrient levels.

Reference: M. Susan DuPont, Zofia Mondin, Gary Williamson, and Keith R. Price Effect of Variety, Processing, and Storage on the Flavonoid Glycoside Content and Composition of Lettuce and Endive J. Agric. Food Chem., 2000, 48 (9), pp 3957–3964 DOI: 10.1021/jf0002387