Pre-packed salad leaves may not be as nutritious as they look.
Packaging may not preserve all of the goodness
Research suggests a processing technique used to prolong shelf-life may remove vital nutrients.
Tests showed salad bags did not boost vitamin C levels in the body like fresh lettuce.
Some nutritionists say this should not deter people from eating pre-packed salad leaves as they contain other goodness such as fibre.
According to the British Nutrition Foundation, there are still nutritional reasons for consuming them.
"It does contribute to your five a day portions of fruit and vegetables," said spokesperson Sarah Stanner.
She pointed out that storing fresh lettuce in the fridge for a week or so would lead to a decline in vitamin content.
The study relates to pre-packed bags of salad leaves, such as lettuce, rocket and more unusual varieties.
To keep the products fresh they undergo a process known as modified-atmosphere packaging.
According to scientists in Italy, this may remove antioxidant nutrients which protect against disease.
Blood samples taken from 11 volunteers fed pre-packaged lettuce showed no increase in vitamin C levels and certain other nutrients.
With fresh lettuce, however, vitamin C and other antioxidants rose significantly in the blood.
The team, from the human nutrition unit at the National Research Institute for Food in Rome, says further research is needed to develop techniques "able to preserve the bioactive molecule content of plant food."
The study is published in the British Journal of Nutrition.