Fish Oil Storage, Freshness and Rancidity

Key overview

  • Omega-3 oxidises readily. Heat, light and air are what degrade it.
  • A strong fishy smell or taste, or fishy burps, usually indicate oxidation rather than a normal property of the product.
  • Store in a cool, dark place. Refrigerate liquid oils once opened.
  • Buy quantities you will finish within a few months rather than the largest bottle available.

Freshness is the specification nobody checks and the one most likely to make an otherwise appropriate supplement useless. Omega-3 fatty acids are highly unsaturated, which is what makes them biologically useful and also what makes them chemically fragile.

What oxidation does

Exposure to oxygen, heat and light breaks down the long-chain fatty acids and produces the compounds responsible for the smell of old fish. Two consequences follow. The oil delivers less of what you bought it for, and the oxidation products themselves are pro-inflammatory — which is the opposite of the reason you are taking it.

Independent testing of retail fish oil has repeatedly found products exceeding recommended oxidation limits, sometimes well before their stated expiry. This is not a fringe concern.

How to tell

  • Smell. Fresh fish oil smells mildly of the sea or of very little. A strong, sharp fishy smell on opening a capsule is the clearest sign.
  • Taste. Bite a capsule if you are unsure. Rancid oil tastes acrid and unpleasant in a way fresh oil does not.
  • Fishy burps. Frequently blamed on fish oil generally, these are more often a sign of an oxidised product than an inevitable feature. A fresh, good-quality oil taken with food produces them far less.
  • Appearance. Darkening or cloudiness in a liquid oil is worth noting, though colour alone is not reliable.

Storing it

  • Cool and dark. A cupboard, not a windowsill and not next to the oven.
  • Refrigerate liquid oils after opening. Capsules are better protected but also keep better cold; the fridge is a reasonable default for both.
  • Keep the lid on. Air is the main variable once a bottle is open.
  • Do not decant into a pill organiser for the week. Capsules out of their bottle, at room temperature, in the light, degrade quickly.
  • Buy sensible quantities. A twelve-month supply on offer is a worse deal than it looks if the last four months of it are oxidised.

What to look for when buying

Some manufacturers publish peroxide value and anisidine value, or a TOTOX figure combining them. Lower is better, and a brand that publishes these at all is signalling something. Added antioxidants such as vitamin E are common and sensible. A stated manufacture date is more informative than an expiry date alone.

For the other specifications that determine whether a product is comparable to research — dose, ratio and chemical form — see how to choose a supplement and triglyceride versus ethyl ester.

If yours has gone off

Discard it. There is no salvaging oxidised oil, and continuing a twelve-week protocol on a rancid product means twelve weeks that tell you nothing. If a supplement has consistently tasted or smelled strongly fishy from the point of purchase, that is a supplier problem rather than a storage one.

Part of our guide to Omega-3 & Eye Nutrition.