According to an exposé by CBC’s Marketplace and The Fifth Estate, some fish oil supplements may be rancid. In one brand of fish oil supplements that they tested it showed twice the Health Canada allowable level of oxidation. This was well before the expiry date on the bottle.
Unfortunately, when fish oil capsules, or even many liquid supplements, go rancid it’s not so easy to tell without access to a lab. This is because many manufacturers add flavours to mask the fishy taste. Your fish oil might even be rancid before you get it home so your best efforts to store it in a cool dark place may be for naught.
The harm caused by consuming rancid fish oil supplements likely exceeds any potential benefits you might obtain from them. While there is limited research in humans, oxidized fish oils (or any oxidized oils for that matter) contain free radicals which could potentially cause damage to your cells and contribute to the development and progression of some chronic diseases.
As a dietitian I’m generally always preaching that we should aim to obtain the majority of our nutrients from whole foods, as opposed to supplements. It’s beginning to look like fish is another case where this may be true. Until more conclusive research is conducted I’ll be hedging my bets, clearing the fish oil capsules from the fridge, and aiming to eat fish more regularly.