Background and methodology:
This study identified all peer-reviewed scientific studies published since 1980 that compared the nutrient levels of organic and conventional foods. The authors assessed how these studies defined and selected organic and conventional crops for nutrient-level comparisons. From 97 published studies, researchers identified 236 scientifically valid “matched pairs” of measurements that included an organic and a conventional sample of a given food. The first screening took into account factors such as the experimental design of each study, the need for the same cultivars to be planted in both the organic and conventional fields and the degree of differences in soil types and topography. For each crop addressed in a given study, authors determined whether the study was “high quality,” “acceptable” or “invalid” based on explicit inclusion and exclusion criteria and a rating system. As a result of these screens and selection criteria, 11 nutrients in organic and conventional foods were compared: four measures of antioxidants, three precursors of key vitamins (vitamins A, C, and E), two minerals (potassium and phosphorous), nitrates (higher levels are a nutritional disadvantage) and total protein..
Findings:
Among 236 valid matched pairs across the 11 nutrients, the organic foods were nutritionally superior in 61 percent of the matched pairs, while the conventional foods were more nutrient dense in only 37 percent of the cases. Matched pairs involving comparisons of potassium, phosphorous and total protein levels accounted for over three-quarters of the cases in which the conventional samples were nutritionally superior. The organic samples contained higher concentrations of the very-important polyphenols and antioxidants in about three-quarters of the matched pairs representing those phytonutrients. Across all 236 matched pairs and 11 nutrients, the nutritional premium of the organic food averaged 25 percent. The differences documented in this study are sufficiently consistent and sizable to justify a new answer to the original proposition regarding nutrient–level assessment of plant–based organic foods: yes, organic plant-based foods are, on average, more nutritious than their conventional counterparts.
Reference:
Benbrook, C., Zhao, X., Yanez, J., Davies, N., & Andrews, P. (2008). New evidence confirms the nutritional superiority of plant-based organic foods. The Organic Center publication. Available on-line at: http://www.organiccenter.org/science.nutri.php?action=view&report_id=126