The study was the work of researchers from the London School of Hygiene and Tropical Medicine and appears in the 29 July issue of the American Journal of Clinical Nutrition.
Based on figures from 2007, the organic food industry is estimated to be worth 29 billion pounds (about 48 billion US dollars) worldwide and continues to grow while consumers appear willing to pay premium prices for food they believe to be superior in health and nutritional benefits.
Although some previous reviews have concluded that organic food is superior in nutritional content compared to conventionally produced food, nobody has yet done a systematic review of the literature, said a press statement from the London School of Hygiene and Tropical Medicine.
For this study, which was funded and commissioned by the UK Food Standards Agency (FSA), the authors sought to:
"Quantitatively assess the differences in reported nutrient content between organically and conventionally produced foodstuffs."
They systematically searched abstracts databases for details of papers dating from the beginning of 1958 to the end of February 2008, contacted experts on the subject, and also manually searched bibliographies.
They included reports of research where the abstracts were written in English and the researchers had measured and compared the nutrient content of organic and conventional food.
To be of satisfactory quality and be included in the analysis, a study had to show it was designed with rigour. For instance, it had to include evidence of the organic certification scheme from which the studied foodstuffs were derived, the breed of livestock, the crop cultivar, plus details of lab methods and statistical tools used in the analysis.
The reviewers then analysed data on 13 nutrient categories.
They did not examine the content of contaminants or chemical residues.
The results showed that:
- From a total of over 52,000 articles, there were 162 (137 on crops and 25 on livestock products) that met the researchers' first level of inclusion criteria but only 55 of these were of satisfactory quality and went into the analysis.
- Conventionally produced crops had a significantly higher content of nitrogen.
- Organically produced crops had a significantly higher content of phosphorus and higher titratable acidity.
- There was no evidence of a difference among the remaining 8 crop nutritient categories.
- Analysis of the few quality studies on livestock products showed no evidence of differences in nutritient content between those that were organically and those that were conventionally produced.
"On the basis of a systematic review of studies of satisfactory quality, there is no evidence of a difference in nutrient quality between organically and conventionally produced foodstuffs."
"The small differences in nutrient content detected are biologically plausible and mostly relate to differences in production methods," they added.
In the press statement they said that the differences detected were most likely due to differences in fertilizer use (nitrogen and phosphorous) and ripeness at harvest (acidity).
Co-author Dr Alan Dangour, of the London School of Hygiene and Tropical Medicine's Nutrition and Public Health Intervention Research Unit told the media that:
"A small number of differences in nutrient content were found to exist between organically and conventionally produced foodstuffs, but these are unlikely to be of any public health relevance."
"Our review indicates that there is currently no evidence to support the selection of organically over conventionally produced foods on the basis of nutritional superiority," he added, but also said that:
"Research in this area would benefit from greater scientific rigour and a better understanding of the various factors that determine the nutrient content of foodstuffs."
The researchers' work was externally reviewed by a panel of experts that included Dr Julie Lovegrove of the University of Reading in the UK, and who is an expert in public health nutrition with systematic review experience, and Professor Martin Wiseman of the University of Southampton in the UK and who also works with the World Cancer Research Fund International in the UK.
A spokesman for a leading organic organization said he was disappointed with the study conclusions.
Peter Melchett, policy director for the Soil Association told the BBC that:
"The review rejected almost all of the existing studies of comparisons between organic and non-organic nutritional differences."
However, he also added that it is difficult to arrive at firm conclusions without good quality, large scale longitudinal research, something which the study authors also said. And he added that we don't have enough good research on the long term effects of pesticides on human health.
Gill Fine, director of consumer choice and dietary health at the FSA, the body that sponsored the research, said that the FSA was neither for nor against organic food, but ensuring people have:
"Accurate information is absolutely essential in allowing us all to make informed choices about the food we eat."
She told the BBC that the study did not say people should not eat organic food, but it did show that there was no evidence of a nutritional difference between organic and conventionally produced food.
Fine also said that there were many reasons why people might choose to eat organic, such as being concerned about animal welfare and the environment.
"Nutritional quality of organic foods: a systematic review."
Alan D Dangour, Sakhi K Dodhia, Arabella Hayter, Elizabeth Allen, Karen Lock and Ricardo Uauy.
Am J Clin Nutr, July 29, 2009.
Additional sources: London School of Health and Tropical Medicine, BBC.