Fish oil is primarily consumed for its high concentration of Omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). While fish is a known source of iodine, the oil is derived from the tissues of oily fish. However, the manufacturing process used to create the supplement changes the composition significantly from the whole-food source. This explains why virtually all purified fish oil contains little to no iodine.
The Distinction Between Fish Oil and Whole Fish
Whole fish and other seafood are naturally rich in a variety of minerals, including iodine, but the amount depends heavily on the species. Leaner white fish like cod often contain higher concentrations of iodine than fattier fish like mackerel or tuna. The key difference lies in where the iodine is stored versus what part is used for the oil supplement. Iodine is a water-soluble mineral, meaning it resides mainly in the non-fatty tissues, organs, and bones of the fish. Fish oil is extracted almost exclusively from the fatty tissues containing the beneficial Omega-3s, meaning the raw, unrefined oil contains only trace amounts.
Iodine Levels in Purified Fish Oil Supplements
The process of turning crude fish oil into a purified supplement is designed to remove everything but the beneficial fats. This multi-step manufacturing process, which often involves molecular distillation or other extensive refining techniques, is primarily used to strip away environmental contaminants. These contaminants include heavy metals like mercury, persistent organic pollutants, and other undesirable compounds.
Since iodine is an ionic mineral and is not soluble in oil, it is effectively separated and removed during the purification and filtration stages. The result is that high-quality, purified fish oil supplements contain negligible or undetectable amounts of iodine. Some analyses of commercial fish oil have found iodine content as low as 0.5 to 1.3 micrograms per gram of oil, which is an infinitesimal amount compared to most whole-food sources.
Consumers who are concerned about iodine intake should look for supplements that have been tested and certified for purity. Third-party testing programs, such as the International Fish Oil Standards (IFOS) program, verify the absence of harmful contaminants and ensure the oil is ultra-refined. While extremely rare, a slight trace of iodine may sometimes be present if the supplement uses a gelatin capsule derived from marine sources, but this contribution is generally considered insignificant.
Primary Dietary Sources of Iodine
Individuals seeking a reliable source of iodine should focus on whole foods where the mineral naturally resides. The most concentrated source of iodine is seaweed, such as kelp, nori, and wakame, which absorbs high amounts of the mineral from seawater. Beyond seafood, a primary source of iodine for many people is iodized table salt, which is fortified to help prevent deficiency. Dairy products, including milk and yogurt, and eggs also provide iodine.