Fish oil supplements may be a good back-up plan if you’re not getting the recommended two servings of fish each week, but do they contain the same toxic mercury we’re told to avoid in certain large-fish species?
This question has been addressed in a couple of studies, both of which concluded that fish oil supplements contain less mercury than most fish.
For example, biochemists from the Czech Republic analyzed 19 different samples of fish oil supplements, and found mercury levels to be insignificant. Levels of methylmercury, the chemical formed when mercury is processed by microorganisms in lakes and streams, were negligible; in fact, they were below the detection limit. Their results were published in 2009 in Neuroendocrinology Letters.
Similarly, a 2003 review of five popular brands of fish oil supplements—including Natrol, CVS and Kirkland—found negligible amounts of mercury. In three of the samples (Nordic Ultimate, Sundown and Kirkland brands) no mercury was detected at all. The researchers, from Harvard Medical School and Massachusetts General Hospital, concluded that either mercury in the fish oil was removed during the manufacturing process, or that it was extracted from species containing little of the poisonous metal. They suggest that fish oil supplements may offer a safe, relatively mercury-free source of healthy omega-3 fatty acids—especially for pregnant or nursing women, as well as for children, for whom mercury exposure is particularly dangerous.
Foran SE, Flood JG, Lewandrowski KB. "Measurement of mercury levels in concentrated over-the-counter fish oil preparations: is fish oil healthier than fish?" Arch Pathol Lab Med. 2003 Dec;127(12):1603-5.
Smutna M, Kruzikova K, Marsalek P, Kopriva V, Svobodova Z. Fish Oil and Cod Liver Oil as Safe and Healthy Food Supplements. Neuro Endocrinol Lett. 2009;30 Suppl 1:156-62.