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However, most authorities recommend an upper limit of 2 portions per week for women who are lactating or who are of child-bearing potential. Women with no plans for having children and men are given an upper limit of 4 portions a week. These guidelines are a balance between the beneficial effects if eating oily fish and the theoretical risk of accumulating toxins such as dioxins, polychlorinated biphenyls and mercury.
Note that Sardines are well down the food chain, and therefore do not accumulate Mercury in the way or to the extent that Tuna do, also that a proportion of the oil is pressed out of tinned Tuna, nice though that is, and therefore the effectiveness of it is diminished. (The oil goes to animal feed, and presumably a proportion continues up our food chain.)
A hypothesis about our evolution, popularised by Elaine Morgan, has our ancestors living along the strand, thus having no selective disadvantage to being unable to synthesize as much of these specific fatty acids as we ideally need.