Brigid McKevith from the British Nutrition Foundation replies: Pre-packaged salads are usually stored in modified-atmosphere packaging that's designed to prevent browning, not to preserve the nutrients. The preparation, storage and processing involved in packaging foods can damage vitamins that are easily harmed by exposure to light and oxygen. But this doesn't mean a pre-packed salad will have fewer nutrients. In fact lettuce doesn't have that many nutrients anyway so try adding other vegetables like cherry tomatoes, grated carrot or diced peppers to your salad.