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Do prepackaged salad greens lose their nutrients?

“Anyone who’s eating greens instead of potato chips is already ahead of the curve in terms of their health,” said Mary Ann Lila, a food scientist at North Carolina State University. But some vitamins in salad greens, especially vitamin C and B vitamins, are water soluble and can diminish somewhat through the washing and chopping of commercial processing and from sitting in the fridge for too long, she said.

One study found that packaged spinach lost nearly half its folate, one of the B vitamins, after eight days of refrigeration, though another study found that lettuce maintained most of its folate content with storage.

But most other nutrients, including fat-soluble vitamins A, E and K and minerals such as iron and calcium, aren’t lost in water and are retained during storage, experts say.

As for vitamin C, a recent study in Food Chemistry found that spinach lost relatively more than other greens. Mature spinach lost about 80 percent of its vitamin C after three days of storage, while baby spinach lost only 25-45 percent. By contrast, watercress maintained nearly 60 percent of its vitamin C content after 10 days of storage, and arugula lost a statistically insignificant amount.

But the loss of vitamin C “is not a serious health hazard,” said Stephen Fry, the study’s senior author and a scientist at the University of Edinburgh, because most people get more than adequate amounts of it in their diet.

Christine Bruhn, director emerita of the Center for Consumer Research at the University of California, Davis, said people should “focus on the big picture of eating lots of fruits and vegetables.”

“Spinach is more than just vitamin C,” she said. “It’s a rich source of vitamin A and fiber and a little bit of iron. It’s the total package that’s important.”

If you’re concerned about the nutrients in your salad greens, keep them refrigerated and eat them while they’re still fresh and the leaves have not wilted, Lila said. “Purchase greens when you plan to eat them,” she said, “and eat them as soon as possible.”