There are plenty of good reasons to buy organic produce, but nutrition may not be one of them, suggests a new study.
The Danish study, published in the Journal of Agricultural and Food Chemistry, found no difference in antioxidant levels between organically and conventionally grown onions, carrots, and potatoes.
The researchers only looked at three types of vegetables and just a few select nutrients. But the experiment was part of a bigger project that was so systematic and rigorously controlled that the findings likely apply to other crops, too, says lead author Pia Knuthsen, a senior research scientist at the Danish National Food Institute's department of food chemistry, based at the University of Copenhagen.
"Giving preference to organic products because they contain more bioactive components is doubtful and not supported by scientific evidence," says Knuthsen. "Still, there are many good reasons for the consumer to select organic food products, including absence of pesticide residues in foods, animal welfare, and environmental protections."
Healthy plant hypothesis
Controversy over whether organic produce is more nutritious than conventional varieties has been around for at least a decade, says Jeffrey Blumberg, director of the Antioxidants Research Laboratory at Tufts University in Boston.
Some studies have suggested that organically grown plants contain higher levels of healthful antioxidants, possibly because some antioxidants act as natural fungicides, pesticides and insecticides," he says.
"The idea is that, without help from industrial chemicals, plants on organic farms need to work harder to protect themselves. They end up becoming more nutritious as a result.
But plenty of other studies have shown no such relationship.
"It's a nice hypothesis," says Blumberg. As for evidence, he says, "I can summarise the state of the science very well by saying it's very mixed."
One of the biggest challenges to doing these kinds of studies, he says, is that a huge number of confounding factors can affect the results, including the age of a plant, how long it is allowed to grow, and variable weather conditions from year to year.
There is also no clear definition of what "organic" and "conventional" actually mean. Farming practices range from industrial and chemically intensive to wild and overgrown.
Knuthsen and colleagues tried to set up as controlled of an experiment as possible. Tapping into a large project called OrgTrace, which is investigating how growing methods contribute to the content of minerals and other compounds in plants. The researchers analysed potatoes, carrots and onions that were cultivated in side-by-side fields over two growing seasons.
Temperature, moisture levels, soil type, and other environmental conditions were kept the same between fields. But some of the vegetables were grown with chemical pesticides and organic fertilisers. Others complied with Danish guidelines for organic farming.
At the end of the growing season, the researchers measured two types of antioxidants: flavonoids and phenolic acids. Both types have been shown to benefit human health. And both are abundant in onions, carrots, and potatoes.
The results showed no significant differences in the levels of either type of antioxidant between the organically and conventionally grown vegetables.
Whether the findings will apply to other kinds of produce, other types of nutrients, or to produce grown in real-world conditions is still unknown, says Blumberg, and will probably remain that way indefinitely.
Perhaps, he says, that's for the best. With so many Americans falling short on fruit and vegetable intake, he would hate to see people trying to skimp on a serving of broccoli because they figured they were making up for the nutrient deficit by eating organic.
"I understand that organic farmers would love to be able to say that theirs are super-fruits and super-vegetables because there are higher amounts of these compounds in them," says Blumberg.
"If you want to buy them because you think they're safer for the planet to grow things that way, or because you think pesticides are bad for you or your family, or you like that they come from a shorter distance away, that's great."
"But if you want to buy them because they're more nutritious, you're confused, and we don't know if that's true or not."