Animal production, problems with organic

Regarding the compositional differences shown for animal products, whether they are important for health depends, as for the plants, on the overall composition of the diet of the people who eat them. The vitamin E content in milk is far too small for relevant differences to affect health (Nielsen ef al. 2004), and too little is known about the dose-response relations of the impact of conjugated linoleic acids on health. However, the increased vitamin E level may still be important to prevent oxidation of the fat, a problem that can be exacerbated by increased levels of polyunsaturated fatty acids (Dhiman ef al. 1999, Nielsen ef al. 2004). However, while oxidised milk is clearly not good for health, its rancid taste and smell allows detection and rejection before consumption and thus prevents harm to health, similar to plant toxicants. Also in line with the plants, while the use of roughage is clearly more extensive in organic farming, some conventional farmers use almost identical feed compositions and are therefore likely to produce the same quality of products in this respect. 

In contrast, fish and fish products can be highly burdened with mercury, especially methylmercury. The highest concentrations are found both in marine and freshwater fish at the highest trophic levels of the aquatic food chain, caused by bioaccumulation [4-6] (see Sec. 2.1). The mean mercury concentration in the edible parts of these problem fishes is approximately 1 mg/kg, but peak values even of 10 mg/kg have been reported [16,17]. Shellfish also accumulate methylmercury to a high extent. For instance, the weekly consumption of 200 g of fish containing only 0.5 mg mercury/kg results in the intake of 100 p,g mercury per week [2]. This amounts to one-half of the PTWI (provisional tolerable weekly intake), as recommended by the World Health Organization (WHO) [4,18]. The feeding of animals like chickens with fish meal may raise the mercury burden of terrestrial animal foodstuffs too. 

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