“Like any oil (including the olive oil you cook with), rancidity can be a major issue if quality-control parameters are not top-notch,” mbg’s vice president of scientific affairs Ashley Jordan Ferira, Ph.D., RDN previously shared. And similar to cooking oils, heat, light, and oxygen (aka air exposure) are the culprits of fish oil oxidation.   

Many fish oils mix anchovy, sardines, mackerel, etc. from all over1— like Chile, Peru, Turkey, Morocco, and even Norway. The act of processing these oils in addition to shipping them sometimes thousands of miles for further manufacturing (all of this occurring before the supplements even make it to your home) naturally creates space for oxidation processes to occur—especially in hot temperatures.

Sometimes manufacturers cut corners and “spike their products with cheaper, lower quality, dare I say, oxidized, types of fish oils with higher contaminant levels.” Ferira said on the mindbodygreen podcast. “Oxidation of fish oil is the crux of unpleasant fish burp.”