It seems there is little evidence that organic foods are more nutritious than conventionally grown foods, according to the most comprehensive study to address the question to date. However, the findings by researchers at Stanford University, California, do suggest that eating organic foods can reduce the likelihood of consuming pesticides and antibiotic-resistant bacteria.

Lead author Crystal Smith-Spangler and a team of researchers looked at 240 published studies of the nutrient and contaminant levels in organic and conventionally grown foods, as well as studies of humans consuming the two types of food.

The researchers reviewed 17 studies (six of which were randomised clinical trials) of populations consuming organic and conventional diets, and 223 studies that compared either the nutrient levels or the bacterial, fungal or pesticide contamination of various products (fruits, vegetables, grains, meats, milk, poultry and eggs) grown organically and conventionally. The duration of the studies involving human subjects ranged from two days to two years.

Smith-Spangler writes in the Annals of Internal Medicine: "Despite the widespread misperception that organically produced foods are more nutritious than conventional alternatives, we did not find evidence to support this perception."

They found no consistent differences in the vitamin content between the two types of produce or that conventional foods pose any greater health risk than organic ones. They also found that the risk of bacteria Escherichia coli (E. coli) contamination was unrelated to farming method.

However, their analysis did find that conventional produce carries a 30% higher chance of pesticide contamination when compared with organic foods.

Commenting on these results, Smith-Spangler told me: "This study is not able to give consumers hard information about the impact of pesticide contamination on health because there are so few human studies available.

"Based on our result people who do not wish to have exposure to any pesticide residues may prefer to choose organic foods for that reason. However people who understand that there are regulations to ensure the safety of our food supply, those people may be less concerned because it is unusual to have pesticide residues exceeding our maximum allowed limits."

Interestingly, a 33% increase in the risk of consuming antibiotic-resistant bacteria in conventionally raised chicken or pork compared with their organic counterparts was also reported.

This research supports the findings of a 2009 review by the Food Standards Agency, which also concluded that there are no important differences in the nutrition content or additional health benefits of organic food when compared with conventionally produced food.

Dr Alan Dangour, a nutritionist at the London School of Hygiene and Tropical Medicine and lead author of the FSA study, told me: "Like our 2009 review, [this study] again demonstrates that there are no important differences in nutrient content between organic and conventionally produced foods. This latest review identifies that at present there are no convincing differences between organic and conventional foods in nutrient content or health-benefits. Hopefully this evidence will be useful to consumers."

When I emailed Emma Hockridge, head of policy at the Soil Association, she replied that organic consumers' concerns about chemicals were well-founded. "An FSA report earlier this year revealed the presence of carbendazim, a suspected hormone-disrupting chemical banned in the US, in fruit and vegetables even after washing and cooking. Under organic regulations, these chemicals are banned, making buying organic food a simple solution to avoiding chemicals which remains the top reason for consumers to do so."

That said, it should be noted that there are currently no long-term studies of the health outcomes for people consuming organic versus conventionally produced food. Given that people who regularly consume expensive organically grown products tend to be more affluent – a factor strongly associated with better health outcomes – this might be a tricky study to design.