I got an article in my inbox today about the risk of oxidized fats in fish oil capsules, even those that are well within the expiration date on the bottle.

We wrote about this very thing in the Protein Power LifePlan back in 2000.

Fish oil is primarily EPA and DHA, both of which are extremely unsaturated. And, as we all know, the greater the degree of unsaturation, the greater the propensity to go rancid. When these oils go rancid (or “go off” as the Brits put it) they don’t go from being healthful to simply becoming neutral, they actually convert to harmful oxidized fats called lipid peroxides.

Lipid peroxides can (and do) start free radical cascades that can damage fatty cellular membranes. At the very least lipid peroxides consume the body’s stores of vitamin E and other antioxidants to neutralize them, leaving the body short of many of its natural defenses.

Another important point that the scientist in this article makes is that often rancid fish oil is used in scientific studies. You will often see in the “Methods” section of a scientific paper that an off-the-shelf brand of fish oil was used. Since many of these capsules will be rancid, how can the study have any validity? I’m convinced that a number of studies showing little or no beneficial effect from fish oil consumption are generated because the fish oil used is rancid.

How do you insure that the fish oil you get is not rancid? It’s actually pretty easy. First, purchase the freshest fish oil capsules you can find. Take them home, and if they are in plastic bottles, put them in glass bottles. Plastic bottles, surprisingly enough, are not totally impervious to air. Glass bottles are impervious to air. Most fish oil is encapsulated using gel caps, which are also not impervious to air. You can’t really re-encapsulate the fish oil, so you’ve got to live with the gel caps, but putting them into a glass bottle keeps the air from getting to the gel caps in the first place. Second, put the glass bottle in the refrigerator. The cold will markedly slow down the oxidation process even if a little air gets in the bottle. Refrigerated fish last a lot longer than fish left out on the counter.

The final step you can take to insure freshness of your fish oil capsules is to bite into one and chew it. If it is rancid, you’ll know it. If it is, throw the whole batch out. If you perform the chew test every four or five days, you’ll always know you’re taking unrancid fish oil.

There are a couple of brands that I like that are packaged a little differently. One, OmegaBrite, packages its capsules in individual “blisters” that are filled with nitrogen to keep out the oxygen. I’ve performed the chew test many times on these capsules (which I still keep in the frig) without ever finding a bad one. The other brand, Nordic Naturals, has a single dose fish oil container that you twist the top off of and take straight. Since the unencapsulated fish oil goes directly into your mouth, it’s easy to tell whether it’s rancid or not. I’ve never found rancidity in the Nordic Naturals product.

Another way to get good doses of EPA and DHA is in krill oil. Krill oil comes from krill, which are tiny shrimplike creatures that make up the lion’s share of the oceanic biomass. Although a number of companies are bottling krill oil capsules, all get it from one source: Aker Biomarine, a Canadian company that supplies the brand called Neptune Krill Oil (NKO). Krill oil has a number of antioxidants within it that prevent its going rancid, but I still put it in glass containers and keep it in the refrigerator. And I do the chew test.

If and when we ever get out website up, we’re planning on making all of the above products available.


  1. I have just purchased some Neptune Krill Oil over the internet and am surprised to find that the capsules are a darkish red colour. Is this a sign of rancidity or is it because of the present of astaxanthin which I think is what gives prawns and shrimps and perhaps also krill a pink colouring?

  2. All the krill oil I’ve ever seen are a deep, darkish red color. The best way to check for rancidity is to bite into one of them. If it’s rancid, you’ll know it.

  3. I’ve been taking the omega 3 zone fish oil of dr barry sears from usa. could one test for oxidation byproducts of fish opil by a more scientific method ie such as mass chromatography ?

  4. I’m not sure what the process is that most accurately tests for oxidation of oils. I taste test them. If they’re mildly ‘fishy’ but still pleasant tasting, I figure they’re okay. If they taste strongly like rotten fish smell, I don’t take them.

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