Trace element concentration in organic and conventional milk – what are the nutritional implications of the recently-reported differences?

We have been asked to comment on differences in trace element concentrations between organic and conventional milk found in the recent meta-analysis by Średnicka-Tober and colleagues(1). Such a comment is important because in fact the most significant difference revealed between organic and conventional milk, in terms of contribution to nutrient requirements, is that of iodine. In many countries, and particularly in the UK where iodised salt is rarely used(2), milk is the single biggest contributor to iodine intake(3). By contrast, milk is a relatively inconsequential source of fatty acids, particularly of those desirable long-chain n-3 PUFAs. This calls into question the emphasis placed on the n-3 PUFAs both in the paper and the press release. We will concentrate our comment on the difference in iodine, selenium and iron concentration. We will use the standard meta-analysis data presented by the authors as these are weighted according to the size of the studies (unweighted meta-analyses are generally not considered appropriate) and were the only analyses to find significant differences in mineral concentrations between organic and conventional milk samples. For the same reason, we will use the weighted mean percentage differences derived from the standard meta-analyses