Cod liver oil (CLO) is a nutritional supplement valued for its Omega-3 fatty acids (EPA and DHA), along with naturally occurring Vitamin A and Vitamin D. The oil is extracted from the livers of codfish, primarily the Atlantic cod. While the source fish may contain environmental contaminants like mercury, modern refining techniques ensure that high-quality CLO products are virtually free of this heavy metal. The risk of mercury contamination in commercially available CLO is negligible due to rigorous processing standards.
Why Mercury is a Concern in Fish Oil
Mercury becomes a concern in marine environments because of bioaccumulation, where organisms absorb substances faster than they can excrete them. Microorganisms convert inorganic mercury into methylmercury, an organic form that easily binds to proteins in fish tissue. The concentration of this neurotoxin then increases as it moves up the food chain, a phenomenon known as biomagnification.
Larger, longer-lived predatory fish, such as sharks and swordfish, accumulate the highest levels of methylmercury. Cod are lower on the marine food chain and have shorter lifespans compared to these larger predators, meaning they naturally contain lower mercury levels. However, all wild-caught fish absorb some mercury, which necessitates the purification of their oils to ensure consumer safety.
Manufacturing Processes That Remove Mercury
Mercury is removed from cod liver oil through sophisticated industrial refining processes. The primary technique used by manufacturers of high-quality fish oil is molecular distillation, sometimes referred to as short-path distillation. This method utilizes a high vacuum environment, which lowers the boiling points of the oil’s components, allowing separation to occur at relatively low temperatures.
Methylmercury is not fat-soluble and has a significantly different molecular weight and boiling point than the beneficial Omega-3 fatty acids. Under the vacuum and controlled heat, the oil molecules are vaporized and condensed, while heavy metals like mercury are left behind in the residue. This process strips contaminants and other undesirable compounds, such as saturated fats and polychlorinated biphenyls (PCBs), from the oil. Other refining steps, including alkali refining and bleaching, also contribute to removing trace metals and impurities.
Regulatory Standards and Third-Party Verification
Consumers can confirm the purity of cod liver oil by looking for evidence of testing against strict industry standards. While regulatory limits exist for whole fish, the fish oil supplement industry often adheres to voluntary, stringent guidelines. The Global Organization for EPA and DHA Omega-3s (GOED) sets a limit for total mercury in fish oil that is far below established governmental safety thresholds.
Reputable manufacturers submit their products for independent testing by external organizations, such as the U.S. Pharmacopeia (USP) or the International Fish Oil Standards (IFOS) program. These verification programs analyze each batch for contaminants, including mercury, and publish a Certificate of Analysis (COA) to confirm purity. The results typically show mercury levels are “below detectable limits,” meaning the amount present is too low to be measured by standard laboratory equipment. A 2023 study found that the average mercury concentration in tested cod liver oils was extremely low, at 0.106 micrograms per kilogram, which did not exceed the acceptable standard for dietary supplements.