There have been four progressively rigorous meta-analyses published since 2009 focusing on differences in the nutritional quality and safety of organic versus conventional food. The latest comes out July 15, 2014 in the British Journal of Nutrition (BJN). I was the sole American scientist on the mostly European research team that produced the BJN paper:
Higher antioxidant and lower cadmium concentrations and lower incidence of pesticide residues in organically grown crops: a systematic literature review and meta-analyses. Baranski, M., D. Srednicka-Tober, N. Volakakis, C. Seal, R. Sanderson, G. B. Stewart, C. Benbrook, B. Biavati, E. Markellou, C. Giotis, J. Gromadzka-Ostrowska, E. Rembiałkowska, K. Skwarło-Son, R. Tahvonen, D. Janovska, U. Niggli, P. Nicot and C. Leifert.
I have been asked by the team to help disseminate our study’s results in the U.S. Toward this end, we have posted on the M2M website an extensive set of resources on the study including:
- The abstract and links to the full, open-access paper;Press releases from WSU and Newcastle University in the U.K., home to several of the co-authors and study leader Carlo Leifert;
- Extensive FAQs covering major findings, methodological details, and why this study’s findings are more robust than earlier ones;
- Discussion of the findings, limits, and flaws in earlier reviews;
- A set of graphics and tables summarizing methods, data sources, and major findings (available as a downloadable Powerpoint), and
- Links to other resources on the study.
The first of the four meta-analyses was published in 2009 by a team led by Alan Dangour at the London School of Hygiene and Tropical Medicine. The Dangour et al. review was funded by the U.K. Food Standard Agency, and based its unweighted meta-analysis on 137 studies focused on plant-based foods. The second came out in 2011 and was carried out by a team led by Kirsten Brandt at Newcastle University. The Brandt et al. review drew upon the results of 102 studies published through October 2009. A team based at the Stanford Medical School conducted a third systematic review published in 2012 in the Annals of Internal Medicine. The Smith-Spangler et al. review covered about 210 studies on plant-based foods published through May 2011.
Our 2014 review was funded primarily by the European Commission’s science and technology program and is based on 343 published studies on plant-based foods – over 200 more than the Dangour et al. review, 241 more than Brandt et al., and about 130 more than more than Smith-Spangler et al.
Each of the four analyses make progress in overcoming limits and methodological problems identified in the earlier studies, and in all likelihood, the next study will improve upon ours. In this way science marches on. The sensitivity of the methods used has steadily improved, as has the sophistication of the inclusion criteria applied in deciding which studies are of sufficient quality to include in the meta-analyses. More refined decision rules have also been developed and applied in extracting data from papers and incorporating more factors in meta-analyses (e.g., sample size and standard deviations around means).
Carlo Leifert, a Newcastle University professor and the project leader, explained in the WSU press release that “We benefited from a much larger and higher quality set of studies than our colleagues who carried out earlier reviews.”
Our study identified three significant benefits of organic plant-based foods – far fewer pesticide residues, about 50% less cadmium (a toxic heavy metal), and 20% to 40% higher levels of antioxidant polyphenols.
Only two of the four studies addressed pesticide residues and cadmium (Baranski et al. and Smith-Spangler et al.). In terms of the frequency of pesticide residues in organic food versus conventional food, both meta-analyses show that residues are about 4- to 5-fold more common in conventional food.
In the case of cadmium, a toxic heavy metal, Smith-Spangler et al. found 15 studies reporting 77 comparisons of cadmium levels, of which 21 favored organic (i.e., lower levels) and one favored the conventionally grown food. Based on their unweighted meta-analysis though, Smith-Spangler reported no significant difference in average cadmium levels. Unfortunately, some of these 15 studies in cadmium assessment by Smith-Spangler et al. involve animal products, making the direct comparison of results to our newly published Baranski et al. paper impossible.
Our study found 25 direct comparisons of cadmium in organic and conventional plant-based foods that included mean levels, sample sizes, and standard deviations/standard errors – all essential data to conduct a weighted (recommended) meta-analysis. Baranski et al., in contrast to Smith-Spangler et al., reports a statistically significant, 49% difference (lower) level in organic foods.
In is likely the most intense debate in the wake of Baranski et al. will focus on our third key finding involving higher levels of antioxidants and improved nutritional quality in organic plant-based food.
Two of the four published studies – Baranski et al. and the Brandt et al. review – conclude that there are significant differences in the nutritional quality of organic versus conventional plant-based foods, largely because of enhanced polyphenol levels. Dangour et al. report higher average levels of some polyphenols in organic food, but dismiss the differences as too small to matter in terms of human health outcomes.
Smith-Spangler et al. focused their review on evidence of significant improvements in clinical outcomes among people consuming organic food. This is a high hurdle for any study, or set of studies, focused on the nutrient content of individual foods, because some many factors drive diet-health outcomes and nutrient levels in any one food rarely makes a significant difference. Smith-Spangler and co-authors also note, as we do, that there have been no large-scale, properly designed human-intervention studies comparing health outcomes for people consuming mostly organic versus mostly conventional diets. There have been, fortunately, some well-designed intervention studies in experimental animals that have produced promising findings linked most directly to increased antioxidant intakes (for more details, see the “Discussion” section in Baranski et al.).
There is, actually, considerable agreement across the four studies on the presence, and even magnitude in most cases, of differences in nutrient levels in organic and conventional foods. The focus of debate has been and will likely remain the key question – Would increasing antioxidant intakes 20% to 40% in plant-based foods improve public health outcomes?
Our team, and indeed all four reviews, acknowledges that many questions remain about the bioavailability of plant-based antioxidants, how necessary they are at different life stages, and how inadequate intakes shift the burden of disease. But our view is that the weight of evidence supports linkages between higher antioxidant intakes and improved health outcomes, despite inability to quantity such linkages or predict fully which factors drive them.
And so, we conclude that every effort should be made now to increase fruit and vegetable intakes, as well as the concentrations of antioxidants in fruits and vegetables. We also hope scientists will be given the resources needed to establish more precisely how, under what circumstances, and to what extent antioxidants in plant-based foods enhance positive health outcomes, for example, through organic farming and/or switching servings of fruits and vegetables to those that are more nutrient dense and deeply colored.
Why Are There Generally Higher Nutrient Levels in Organic Food?
Baranski et al. explains that the level of nitrogen available to plants, and the form in which nitrogen is supplied, plays a major role in driving antioxidant and other nutrient levels up or down. In general, the higher the nitrogen level, and the greater the percentage of nitrogen applied in a readily available form, the greater the risk of diluting, or lowering, the concentrations of health-promoting plant phytochemicals in plant-based foods.
This conclusion leads to a vital insight — how farmers feed their plants helps determine the nutritional profile of the food harvested from them.
Our December 9, 2013 paper in PLOS ONE on the more nutritionally desirable profile of omega fatty acids in organic versus conventional milk also can be largely explained by what dairy cows are fed – the higher the percentage of grass and legume pasture and forage-based feeds in the cow’s diet, the higher the level of health-promoting omega-3 fatty acids in her milk.
Despite many gaps in knowledge on how diets impact health, we share the view of most governments around the world that human health trajectories will be improved if people consume more servings of nutrient-rich fruits and vegetables and whole grains, while avoiding excessive amounts of sugar, omega-6 heavy oils, and carbohydrates.
Indeed, it seems that several fields of science are pointing towards a common theme applicable to plants, people, and cows – what and how we are fed helps determine how, and how well, we live.