Organic foods are no more nutritious than conventionally grown foods, and no less likely to be contaminated with certain bacteria, according to a new review of studies.
However, organics were less likely to contain pesticide residues, or harbor bacteria that were resistant to antibiotics, compared with conventional alternatives, the study found.
Though farming practices vary, organic plants are generally grown without the use of pesticides or industrial fertilizers, and organically raised animals are not routinely treated with antibiotics or growth hormones. Organic foods typically contain no genetically modified organisms.
Consumers purchase organic foods for a number of reasons, including perceptions that organic foods may be safer or more nutritious than conventionally grown foods. However, the health benefits of organic foods remain unclear.
"Our aim was to understand the evidence about differences in nutrient and contaminant levels between organic and conventional foods," said study researcher Dr. Crystal Smith-Spangler, an internist at the Stanford School of Medicine.
Though prices vary, consumers may pay up to twice as much for organic as conventional foods.
What the researchers found
Smith-Spangler and her colleagues analyzed data from more than 200 studies comparing nutrient and contaminant levels in organic and conventional foods, including fruits, vegetables, grains, meats, poultry, milk and eggs.
They found no significant differences between organic and conventional products, in terms of their vitamin content.
"Despite the widespread perception that organically produced foods are more nutritious than conventional alternatives, we did not find robust evidence to support this perception," the researchers wrote.
Organic and conventional foods were about equally likely to be contaminated with disease-causing bacteria, such as E. coli and Salmonella. About 7 percent of organic produce, and 6 percent of conventional produce was contaminated with E. coli. For chicken, 35 percent of organic, and 34 percent of conventional samples were contaminated with Salmonella.
But when the researchers looked at pesticide contamination and antibiotic resistance, conventional and organic foods differed.
The researchers found pesticide residue on 7 percent of the organic produce samples, but 38 percent of conventional produce samples. In all, organic produce had a 30 percent lower risk of containing pesticides than conventional produce.
The researchers also found that conventional chicken and pork were 33 percent more likely than organic products to harbor bacteria that were resistant to three or more antibiotics.
"The data on pesticides and antibiotic-resistant bacteria is very compelling, and in favor of organic foods," said Charles Benbrook, chief scientist at the Organic Center, a Boulder, Colo. organization that promotes the benefits of organic food and farming.
However, the researchers said the difference between organic and conventional produce were only slight, in terms of how likely it was that the pesticide levels on the food reached the maximum acceptable limits. The risk of either type of produce exceeding regulatory limits may be small, Smith-Spangler said.
What the findings mean
Experts have debated the routine use of antibiotics in animal farming. The extent to which antibiotic use in livestock contributes to antibiotic-resistant infections in people remains unclear. Overuse of antibiotics in human medicine is likely the major cause of antibiotic-resistant infections in humans, according to the study authors.
"It is impossible to say from this study whether one method of farming is better than the other, though we are not seeing the negatives associated with organics that we are with some of the conventional products," said Gene Lester, a plant physiologist for the Agricultural Research Service of the United States Department of Agriculture in Maryland.
While the findings are interesting, he cautioned, they are far from definitive. Variation within organic farming practices, and differences in the way previous studies reported their findings make it difficult to draw conclusions, Lester said.
"We found very few studies that compared the health of human populations consuming largely organic versus conventional diets, so it is difficult to interpret the clinical significance of the findings," Smith-Spangler said.
Future studies should investigate whether the decreased risk of exposure to pesticide residues in organic foods leads to real health improvements, particularly for pregnant women and children, Smith-Spangler said.
The review is published today (Sept. 3) in the Annals of Internal Medicine.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.