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differences between organic and conventional milk and meat

Feeding into the debate and uncertainty over whether the use of organic production standards affects food quality, researchers have published two meta-analyses in the journal British Journal of Nutrition which report the differences in nutrient content of organic and conventional bovine milk and meat. Roundup comments accompanied this analysis.


Title, Date of Publication & Journal

Composition differences between organic and conventional meat: a systematic literature review and meta-analysis


Higher PUFA and n-3 PUFA, CLA, α-tocopherol and iron, but lower iodine and selenium concentrations in organic milk: a systematic literature review and meta- and redundancy analyses

by Srednicka-Tober et al.

British Journal of Nutrition, 16 February 2016


Study’s main claims – and are they supported by the data


This is a large meta-analysis which considers all available published evidence and appropriate statistical methods have been used to combine the results from the different studies (although see previous comments about the unweighted analyses).

The explicit focus on fatty acids and a limited set of parameters leads to less likelihood of obtaining “false-positives” than a similar meta-analysis by the same team on organic vegetables in 2014.


Publication bias:

The authors highlight the problem of publication bias but do not suitably discuss what impact this may have had on the results. Generally publication bias is likely to result in an overestimate of the effect size – or, in some cases, finding an effect that is not real.

Heterogeneity (different studies found different things):

There was a large amount of heterogeneity between the studies for most of the outcomes considered; in other words, differences between organic and conventional systems were inconsistent between studies. The authors discuss various factors which are likely to have contributed to this. This means the average difference between organic and conventional systems for these outcomes may be fairly meaningless in terms of modifying behaviour, since the actual true difference (as indicated in the paper) varies from country to country.


Before The Headlines is a service provided to the SMC by volunteer statisticians: members of the Royal Statistical Society (RSS), Statisticians in the Pharmaceutical Industry (PSI) and experienced statisticians in academia and research. A list of contributors, including affiliations, is available here.

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