Higher nutrient levels are a much-touted benefit of organic fruits and vegetables, but a recent study casts doubt on that idea. Researchers from the National Food Institute at the University of Copenhagen in Denmark put the antioxidant content of organic potatoes, onions, and carrots to the test and found them no better than their conventionally grown counterparts. The research was published in the Journal of Agriculture and Food Chemistry.
Explore this issueFebruary/March 2011
Advocates of organic farming have long held that organic produce contains higher levels of antioxidant compounds such as flavonoids and phenolic acids, because the plants must produce more bioactive compounds to protect themselves from pests and disease. Previous scientific studies have yielded conflicting results. In order to resolve those conflicts, Pia Knuthsen, PhD, a senior research scientist in the department of food chemistry at Denmark’s National Food Institute, set up an experiment designed to rigorously compare the content of flavonoids and phenolic acids in organically and conventionally grown onions, potatoes, and carrots.
The group led by Dr. Knuthsen controlled temperature, moisture levels, soil type, and various environmental conditions. The experiment was carried out over two growing seasons to control for the known effect of different growing seasons on the polyphenol content of foods.
Onions are an important source of dietary flavonoids, especially quercetin mono- and diglucosides. No differences were observed in the content of flavonoids between the conventionally grown onions and either of the groups of organic onions.
Other plant compounds, e.g., different vitamins, are metabolized in the plants in different ways, and all possess different chemical properties. Thus, different compounds should be studied in carefully designed experimental field trials before drawing such conclusions.
—Pia Knuthsen, PhD, Denmark’s National Food Institute
Carrots produce the polyphenol 5-O-caffeoylquinic acid (5-CQA). Again, no significant differences in 5-CQA content were found between organically and conventionally grown carrots. The carrots did not show significant year-to-year variation.
The only significant signal detected by the study was in potatoes, where one of the organic fields, with potatoes fertilized by a cover crop, produced higher levels of 5-CQA. The authors wrote, however, that “it cannot be concluded that organically grown onions, carrots, and potatoes generally have higher contents of health-promoting secondary metabolites in comparison with conventionally cultivated ones.”
It is not clear what implications, if any, the study has for other vegetables and other nutrients. “Other plant compounds, e.g., different vitamins, are metabolized in the plants in different ways, and all possess different chemical properties. Thus, different compounds should be studied in carefully designed experimental field trials before drawing such conclusions,” said Dr. Knuthsen.
The experiment was part of a larger project called OrgTrace, a field experiment run by the International Centre for Research in Organic Food Systems, based in Denmark. OrgTrace’s purpose is to study the effects of organic and conventional agricultural systems on the assimilation of minerals and the synthesis of bioactive compounds.
Other projects being carried out by OrgTrace include a study to identify bioactive selenium and sulphur metabolites in plants, including spring barley, winter wheat, white cabbage, carrots, onions, potatoes, and fava beans; a study to characterize the health status of rats fed on plant products from different locations and agricultural systems; and a human bioavailability study in male subjects consuming two different organic diets and one conventional diet.
Even in the absence of antioxidant benefits, there are still good reasons to choose organic vegetables, Dr. Knuthsen said. Those reasons might include a commitment to sustainable or local agriculture, a philosophical objection to conventional agriculture methods, or a desire to protect the environment.