A large review found very few studies that systematically compared the health outcomes of eating organics or conventional foods.
In the largest review yet of studies that compare conventional to organic foods, researchers found no major evidence that one type is healthier than the other in measurements like nutrient content, allergic response or infection rates.
Some research did show differences in pesticide levels of conventional foods and of children that ate them, but even conventional produce came in way below allowable safe limits.
Perhaps more revealing, the review found very few studies that systematically examined the health outcomes of eating one kind of diet compared to the other.
Out or more than 200 studies that were ultimately included in the new analysis, many were small or funded by interest groups. Each was designed in a different way and focused on different kinds of measurements or health outcomes, which were complicated by differing definitions of "organic" or incongruous agricultural practices.
And none of the studies lasted longer than two years, making it hard to draw definitive conclusions. The shortest lasted just two days.
"There are many reasons why people might choose an organic apple compared to a conventional one, like environmental concerns or animal welfare or they like buying from their local farmers," she added. "To the extent that you're doing so for health-related effects, I would say there probably is not a ton of evidence. For myself, I don't go out of my way to buy organic food anymore."
In her medical practice, Bravata repeatedly fields questions from patients about what kinds of foods they should eat, including whether they should prioritize organics. At a loss to offer evidence-based answers, she and colleagues embarked on a major effort to figure out what science had to offer.
They cast a wide net. Beginning with about 5,900 studies, the team reviewed the full text of 460 papers that seemed to fit their criteria of focusing on health benefits or risks and clearly comparing the two categories of foods or health measures of the people that ate them. Of those, 237 made the final cut, based on solid study design.
The vast majority of studies that made it into the analysis looked at properties of food itself. And overall, the researchers report today in the journal Annals of Internal Medicine, results showed few differences in the content of vitamins, minerals, antioxidants, proteins or other nutrient measures.
The two groups did show some differences, though.
Organic produce contained more phosphorous than conventional versions, for example, though Brevata pointed out that phosphorous deficiency only occurs in people who are near starvation.
A few very small studies also suggested that organics contained higher levels of a type of antioxidant called phenols. And organic milk and chicken contained more omega-3 fatty acids than conventional varieties did, but Brevata added a caveat there as well: Omega 3s varied equally as much with brand and harvesting season.
Pesticide levels varied between the two groups, with conventional foods carrying a 30 percent higher chemical load than organics. Despite what sounds like a large difference, Brevata said, both groups were well below what are currently considered safe limits.
None of the studies showed that any particular kinds of produce contained more pesticides than others. And none looked systematically at the effects of washing or peeling.
Only 17 studies compared groups of people eating different diets, and most showed no difference on measures like sperm motility, levels of fatty acids in breast milk or antioxidant levels in blood.
Two studies looked at children and both showed higher levels of pesticides in kids who ate a greater proportion of conventional foods. But again, measurements in both groups of kids were well below concerning levels. And some studies showed that people were exposed to more pesticides from household use than from food.
Some research showed that conventional chicken and pork contained 33 percent more antibiotic-resistant bacteria than organic alternatives. It's hard to translate those numbers into health outcomes, Brevata said, because cooking well kills even drug-resistant microbes.
Together, the results are too inconclusive and disparate to draw any major conclusions, said Betsy Wattenberg, a toxicologist at the University of Minnesota, Twin Cities.
In order to really know anything about food-related risks that people tend to care most about, such as cancer or reproductive and developmental health issues, we would need carefully controlled studies that last for years or even decades.
Those kinds of studies don't exist, and they are likely impossible to do.
"It is really difficult to draw a conclusion from the paper except that there is a lack of good comprehensive studies," Wattenberg said. "I wouldn't say the conclusion is there's no difference (between organics and conventional foods). It's that we just don't have the evidence to say there's no difference, which is different from saying there's no difference."