A shopper surveys the produce at Pacifica Farmers Market in Pacifica, Calif., in 2011. AP hide caption

A shopper surveys the produce at Pacifica Farmers Market in Pacifica, Calif., in 2011.

Yes, organics is a $29 billion industry and still growing. Something is pulling us toward those organic veggies that are grown without synthetic pesticides or fertilizers.

But if you're thinking that organic produce will help you stay healthier, a new finding may come as a surprise. A new study published in the Annals of Internal Medicine finds scant evidence of health benefits from organic foods.

"There's a definite lack of evidence," says researcher Crystal Smith-Spangler at Stanford University School of Medicine, especially when it comes to studies of people.

She and her colleagues collected 200 peer-reviewed studies that examined differences between organic and conventional food, or the people who eat it.

A few of these studies followed people who were eating either organic or conventional food and looked for evidence that the choice made a difference in their health.

One study, for instance, looked at whether eating organic food while pregnant would influence the likelihood of eczema and other allergic conditions among children, and another looked at whether eating organic meat would influence the risk of a Campylobacter infection, a bacterial food-borne illness. When the researchers looked at the body of evidence, they found no clear benefits. But they say more research is needed.

It's important to note, though, that such studies have a really hard time uncovering subtle effects of our environment, or what we eat, on our health. Too many other powerful influences get in the way. Also, these studies only followed people for a very short time about two years or less. That's hardly enough time to document any particular health benefit.

Most of the studies included in this collection looked at the food itself the nutrients that it contained as well as levels of pesticide residues or harmful bacteria.

As you might expect, there was less pesticide contamination on organic produce. But does that matter? The authors of the new study say probably not. They found that the vast majority of conventionally grown food did not exceed allowable limits of pesticide residue set by federal regulations.

Some previous studies have looked at specific organic foods and found that they contain higher levels of important nutrients, such as vitamins and minerals. We've reported on one particularly ambitious experiment, which is supposed to go on for a hundred years, comparing plots of organic and conventional tomatoes. After 10 years, the researchers found that tomatoes raised in the organic plots contained significantly higher levels of certain antioxidant compounds.

But this is one study of one vegetable in one field. And when the Stanford researchers looked at their broad array of studies, which included lots of different crops in different situations, they found no such broad pattern.

Here's the basic reason: When it comes to their nutritional quality, vegetables vary enormously, and that's true whether they are organic or conventional. One carrot in the grocery store, for instance, may have two or three times more beta carotene (which gives us vitamin A) than its neighbor. That's due to all kinds of things: differences in the genetic makeup of different varieties, the ripeness of the produce when it was picked, even the weather.

So there really are vegetables that are more nutritious than others, but the dividing line between them isn't whether or not they are organic. "You can't use organic as your sole criteria for judging nutritional quality," says Smith-Spangler.

Of course, people may have other reasons for buying organic food. It's a different style of agriculture. Organic farmers often control pests by growing a greater variety of crops. They increase the fertility of their fields through nitrogen-fixing plants, or by adding compost instead of applying synthetic fertilizer.

That can bring environmental benefits, such as more diverse insect life in the field or less fertilizer runoff into neighboring streams. But such methods also cost money. That's part of what you are buying when you buy organic.

So if you really want to find the most nutritious vegetables, and the organic label won't take you there, what will?

At the moment, unfortunately, there isn't a good guide. But a lot of scientists are working on it.

They're measuring nutrient levels in all kinds of crops, and discovering some surprising things, as The Salt reported last week such as supernutritious microgreens. They're trying to breed new varieties of crops that yield not a bigger harvest but a more nutrient-rich harvest.

The problem is, farmers still get paid by the pound, not by the vitamin. And consumers buy their food the same way. What this really requires is a whole new food system that can track those extra-nutritious crops from farmer's field to consumer's shopping basket.

Maybe, down the road, you will actually see signs in the supermarket that advertise, for instance, iron-rich beans. Maybe they'd be organic, or maybe not.

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Why Organic Food May Not Be Healthier For You - npr.org