Jul 18 2014

New Organic Farming Meta-analysis – What Does it Really Show?

The Guardian’s headline reads: Clear differences between organic and non-organic food, study finds. While this article was better than most in including some caveats, it was clearly favorable to the conclusions in the study, and failed, in my opinion, to properly put the new study into an informative context.

How does this new study add to the literature looking at the safety and health effect of organic produce vs conventional produce?

First, the study is a meta-analysis of 343 prior studies looking at nutrient content, pesticide, and heavy metal contamination of produce. It is not a collection of any new data. A meta-analysis is very tricky to conduct well – it does not improve the quality of the data going into the analysis, only the statistical power. Further it introduces another layer of potential bias (another researcher degree of freedom) in which studies are chosen for the analysis.

This study used very open criteria, and therefore included more lower-quality studies (likely to be false positive or show the bias of the researchers) than other meta-analyses.

Whenever I am trying to quickly grasp the bottom line of any scientific question, I look for a consensus among several independent systematic reviews. If multiple reviewers are looking at the same body of research and coming to the same conclusion, that conclusion is likely reliable.

In this case, there are three other recent large systematic reviews and meta-analyses of the same research on the nutritional content and safety of organic vs conventional produce. The other three studies all came to the opposite conclusion as the current study. A 2009 review by Dangour et. al. concluded:

“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.”

A 2010 review also by Dangour found:

“From a systematic review of the currently available published literature, evidence is lacking for nutrition-related health effects that result from the consumption of organically produced foodstuffs.”

 A 2012 review by Smith-Spangler et. al. found:

“After analyzing the data, the researchers found little significant difference in health benefits between organic and conventional foods. No consistent differences were seen in the vitamin content of organic products, and only one nutrient — phosphorus — was significantly higher in organic versus conventionally grown produce (and the researchers note that because few people have phosphorous deficiency, this has little clinical significance).”

Older reviews by other researchers came to the same negative conclusions. This latest review, therefore, is the outlier. While I don’t think that possible conflicts of interest are definitive in analyzing research, it is informative, especially when there are disagreements. The negative reviews of the data were independently funded. This latest study was partially funded by the Sheepdrove Trust, which funds research supportive of organic farming. If, for example, the only study out of several that was favorable to a particular drug was funded by the drug manufacturer, that would be significant.

It is likely that the inclusion of weaker studies biased the results of this latest analysis in favor of a false positive. The authors claim that they did an analysis and removing the weaker studies did not have a significant effect on the results, but I find this unlikely as removing the weaker studies would essentially make the current analysis more similar to the other analyses, which found no significant differences.

The reporting about this study, based on how it was presented, is also misleading in other respects. They claim that anti-oxidants have proven health benefits, but this is misleading. The effect of antioxidants on health is more complex, and I don’t think it’s fair to conclude that higher antioxidant levels are clearly beneficial.

The study also found that organic produce has lower levels of protein, fiber, and nitrates, but these findings were strangely missing from the abstract’s conclusions and from uncritical reporting about the study.

Regarding pesticides, there is agreement that the data shows higher residue levels of those pesticides tested in conventional produce. However, these levels are well below safety limits, so this likely has no health effect. Organic proponents argue that the cumulative effect is what matters, even if individual levels are safe, but this is pure speculation.

Further – these results are rigged and misleading. The studies only test for conventional pesticides, so of course there are more of these on conventional produce. They don’t test for organic pesticides, those used in organic farming. There is absolutely no reason to conclude that organic pesticides are safer than synthetic pesticides, their safety is assumed by organic proponents because they are “natural,” a clear example of the naturalistic fallacy.

The cadmium issue is similar in that, while levels were higher in conventional produce, the levels are still well below safety limits. Again organic proponents argue that the levels accumulate, but they have no evidence for this.


Three recent systematic reviews of hundreds of studies concluded that there is no significant difference in nutrient content between organic and conventional produce. Now this is one meta-analysis that concludes that there are higher levels of anti-oxidants (and lower levels of protein and fiber). The simplest explanation for the difference is that the independent studies were of better quality by being more exclusive of lower quality studies.

The pesticide issue is fearmongering, in my opinion, as the levels in conventional produce are well below safety limits. The analysis also ignores organic pesticides, without any justification, in my opinion.

Even if you believe that there are differences in organic vs conventional produce, it is not clear that any particular method of organic farming is the cause. There are many differences in practice, and combining them together philosophically into “organic” farming is misleading and counterproductive. Further, it is possible that the overall smaller size of organic produce simply concentrates some nutrients, but this does not mean the consumer is getting overall more nutrients.

I also agree with Richard Mithen, leader of the food and health program at the Institute of Food Research, who is quoted as saying:

“The additional cost of organic vegetables to the consumer and the likely reduced consumption would easily offset any marginal increase in nutritional properties, even if they did occur, which I doubt,” Mithen said. “To improve public health we need to encourage people to eat more fruit and vegetables, regardless of how they are produced.”

The higher cost of organic produce, due to the lower productivity or organic farming and the premium created by the marketing hype of organic food, could potentially lead to overall reduced consumption of fruits and vegetable, and therefore, ironically, be a net negative to nutrition.

Just eat plenty of fruits and vegetables, and if you’re worried about the tiny residue of pesticides, wash them before eating (whether organic or conventional).

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