Why does cod liver oil become rancid so easily?

In the presence of oxygen, light and heat, the oil will become rancid (oxidize), developing undesirable flavors and odors. Cod liver oil is particularly rich in healthy, polyunsaturated omega-3 fatty acids, but this also makes it particularly prone to becoming rancid as a result of oxidative damage. When the polyunsaturated fatty acids become rancid, they create a complex mixture of small molecules that have strong flavors and odors. This is what gives cod liver oil its undesirable taste and smell (other contributors may include the presence of certain amines). If cod liver oil is rancid, with an undesirable taste and smell, you may be less likely to consume it as a result of your natural instincts (unless the oils have added flavorings to mask their strong taste and smell).

Many local fishermen from northern Norway recall this type of rancid cod liver oil. At school the bottle of cod liver oil was placed on the window sill and left in the light and heat. Many times, the top of the bottle was not even secured properly. It would turn brown and rancid with an unpleasant odor and taste and typically left a “scratchy sensation” in the back of the throat.