The Food Standards Agency has published a systematic review which it commissioned which sought to determine the size and relevance to health of any differences in content of nutrients and other substances in organically and conventionally produced crops and livestock products. The focus of the review was the nutritional content of foodstuffs.
It did not address contaminant content (such as herbicide, pesticide and fungicide residues) of organically and conventionally produced foodstuffs or the environmental impacts of organic and conventional agricultural practices.
The systematic review search process identified 162 relevant articles published, with an English abstract, in peer-reviewed journals since 1 January 1958 until 29 February 2008. A total of 3,558 comparisons of content of nutrients and other substances in organically and conventionally produced foodstuffs were extracted for analysis. Articles included in the review were assessed for study quality (satisfactory quality studies provided clear statements on material and nutrients analysed, laboratory and statistical methods and a clear definition of organic agricultural practices), and one third of all studies (55) met the pre-defined satisfactory quality criteria. Analysis was conducted on nutrients or nutrient groups for which numeric data were provided in at least 10 of the 137 crop studies identified by the review.
In analysis including all studies (independent of quality), no evidence of a difference in content was detected between organically and conventionally produced crops for the following nutrients and other substances: vitamin C, calcium, phosphorus, potassium, total soluble solids, titratable acidity, copper, iron, nitrates, manganese, ash, specific proteins, sodium, plant non-digestible carbohydrates, β-carotene and sulphur.
Significant differences in content between organically and conventionally produced crops were found in some minerals (nitrogen higher in conventional crops; magnesium and zinc higher in organic crops), phytochemicals (phenolic compounds and flavonoids higher in organic crops) and sugars (higher in organic crops). In analysis restricted to satisfactory quality studies, significant differences in content between organically and conventionally produced crops were found only in nitrogen content (higher in conventional crops), phosphorus (higher in organic crops) and titratable acidity (higher in organic crops).
The study was undertaken by the London School of Hygiene and Tropical Medicine. You can download the study here. The Chief Executive of the Food Standards Agency has also written an open letter to interested parties following the furore surrounding the report's publication. You can read it here.