You know what we mean by rancid. When food has gone bad. And the same goes for some supplements—like fish oil. With apologies to Dr. Seuss, if the smell isn’t swell, well, that’s a tell, that the fish oil’s not necessarily well.

All kidding aside, fish oil rancidity is no joking matter. A mild fishy smell for fish oils (and other marine-based oils like krill and algae) is not unusual, although some companies spend more (and charge more) to make sure their fish oil supplements have no fishy smell. Others flavor their fish oil supplements to make them more palatable to the nose, which in turn makes them more palatable to swallow.

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It’s when your fish oil stinks like fish gone bad that it might be rancid.

It’s not just the smell. Rancid fish oil can also negatively impact the taste and texture. (You try rhyming with texture.)


What Causes Fish Oil to Become Rancid?

The story of rancid fish oil supplements starts with oxidation.

Oxidation is a fact of life for unsaturated fatty acids, for all lipids, in fact. Here’s how it happens: when the unsaturated fatty acids in fats and oils are exposed to the air’s oxygen, or light or heat, the chemical bonds in those fatty acid molecules break down, forming other oxidative products like fatty acid peroxides, alcohols and aldehydes.

The higher the amount of the unsaturated lipids, the more likely that the fish oil will go rancid over time, especially if certain precautions aren’t taken by manufacturers and by those purchasing/consuming the products. And while it is the highly unsaturated fats found in omega-3s EPA and DHA that provide so many of the benefits of fish oil supplements, it’s those same highly unsaturated fats that can lead to rancidity.

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No (legitimate) company starts out thinking “let’s make some rancid fish oil capsules.” But let’s face it: some companies take more care than others to make sure that their customers are buying high quality fish oil supplements.


Factors That Can Affect the Freshness of Fish Oil Products

Fortunately, fish oil supplement manufacturers have developed effective strategies to help combat oxidation in the production process. Some of these steps include limiting exposure to ambient air during production, refining oils in a vacuum, blanketing storage containers with inert gases like nitrogen that displace oxygen and using antioxidants to slow the rate of oxidation.

There’s still more that responsible companies are doing to help ensure the highest standards for omega-3 EPA and DHA. First, a little history to bring you up to date.

Back in 2002, the Council for Responsible Nutrition (CRN), a leading dietary supplement industry trade association, worked with Omega-3 companies to develop a Voluntary Monograph for Omega-3s EPA and DHA as a way to help ensure safe, high quality products for consumers worldwide. The monograph particularly focused on developing standards for oxidative quality, environmental contaminants and the measurement of EPA and DHA, incorporating the strictest quality and purity standards from regulations worldwide.

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The companies who worked to produce the monograph later formed GOED, a not-for-profit trade group, which today represents more than 170 companies from fisheries and crude oil suppliers to refiners, concentrators and brands exclusively dedicated to increasing omega-3 EPA and DHA consumption with quality products that consumers can trust.

It made sense for GOED to rebrand the monograph under its banner, as the GOED Voluntary Monograph, and to review and update it as needed to help ensure that omega-3 oils used by consumers are both safe and of high quality. The monograph was most recently updated in January 2022 and today still serves as the benchmark for quality in the omega-3 industry worldwide.

The GOED monograph identified three primary analytical measures for manufacturers to determine oxidation levels in omega-3 fish oils: the Peroxide Value (PV), the para-Anisidine Value (pAV) and TOTOX.

A companion piece, known as the GOED Guidance Documents, serves as a technical blueprint for manufacturers to comply with the GOED Monograph, including oxidation recommendations.


Why is Freshness So Important with Fish Oil?

Fish oil supplements are among the most researched and their potential health benefits are well-established in the areas of heart health and reducing inflammation. Further, research has also shown benefit in the areas of eye health, brain health, and for pregnant women and their babies-to-be. There’s also emerging research in several other health areas, including sports performance, mood support, cognition, skin conditions like eczema, and more. Fish oil supplements are available in soft gels, capsules, gummies and liquids and are among the most popular supplements on the market today.

But sometimes some of the food or supplements that can be good for you, can also go bad for you. Have you ever drunk rancid milk or tasted fish that’s gone off? When something is rancid, that unpleasant smell, taste, and the memory of all of it is something your nose, mouth and possibly stomach won’t soon let you forget. And that’s not the only problem. Some experts say that rancid fish oil can be harmful to your health.

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That might be true, but not necessarily in the way you might be thinking.

While rancid fish oil supplements may not be good for you—in part because that kind of bad experience might deter you from trying fish oil again, thereby depriving you of all the potential benefits—GOED says the science does not support the claims that suggest rancid fish oil is toxic.

Yes, there are some animal studies that have associated consuming rancid oil with organ damage and atherosclerosis, and other researchers have found that oxidized fatty acids have a pro-inflammatory and mutagenic effect. And we don’t mean to downplay those studies.

At the same time, we don’t expect them to be overplayed, as some in the mainstream press have done.

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And as for concerns by some that fish oil rancidity is widespread, GOED disagrees. A 2015 study that found 92% of products in New Zealand exceeded one of the recommended oxidation limits. In 2017, GOED replicated that study and found less than 20% of companies exceed the limit. Gerard Bannenberg, GOED’s Director of Technical Compliance at GOED, was more recently quoted in The Guardian, a British newspaper, as saying “I don’t think people need to be concerned about oxidation. The level we’re talking about is very low. Even if the product is slightly oxidized, it’s very unlikely that it will harm our health.”

Even 20% is too high, but it’s certainly a far cry from 92%.

GOED also says that “it’s important to note that even a very fishy-smelling product is not unsafe, just unpleasant. If the product smells bad and is beyond its expiration date, then it should be thrown away.”

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8 Tips for Fish Oil Freshness

Let’s be clear: no one should be taking rancid fish oil.

We’ve already talked about what companies are doing. Here are some things you can do to avoid rancid fish oil supplements:


  1. Expiration date. Here in the U.S., dietary supplements are regulated by the Food & Drug Administration (FDA), among other agencies. FDA does not mandate that companies list expiration dates on dietary supplements. However, many companies choose to do so in an effort to be transparent. In the case of fish oil supplements, an expiration date is one way to protect you from consuming fish oil that could be rancid. Other companies may stamp the packaging with “best by” date or the manufacture date (call to find out what that means in terms of end use date). Fish oil will go rancid—it’s just a matter of time. Throw out the product when it reaches the expiration date.


  1. Don’t stockpile. It’s hard to resist a sale; however, with fish oil products, you don’t want them hanging around for too long. And if you buy more than one large bottle at a time, certainly don’t open them both. Stocking up on a fish oil sale may mean you’ll end up with expired (or possibly rancid) product that you’ll need to throw away. So much for the sale. Do use your fish oil (per label instructions) consistently and on a daily basis. That’s another way to ensure you won’t be left with expired product.


  1. Sniff away. It’s okay to break open a fish oil soft gel or capsule to do a sniff test, but then you don’t want to swallow or use the oil. Throw that capsule away. And if it smells super fishy or you recoil from the smell, throw the whole bottle away. (If it’s still within expiration date, call the company to see if they’ll offer you a refund, or at the very least, an explanation.) And by all means, if you’re using fish oil supplements in the liquid form (generally cod oil), it’s fair game to also take a whiff.


  1. Rely on your other senses. If your fish oil supplement tastes bad, makes you gag when you swallow or gives you the burps, don’t use it. Just as there are plenty of fish in the sea, there are plenty of companies selling fish oil supplements. Excessive burping or bad taste may be a sign of rancidity or just a hint that this is not the brand for you. Try another brand. There are flavored fish oils that have been developed to cover unpleasant sensory reactions, but some say that these added flavors may also unwittingly mask the smell or taste of a rancid product.


  1. Proper storage You want to keep your fish oil supplements away from heat, light, and air (for the latter, make sure your bottle is tightly closed). Store your package in a cool, dry place—not the bathroom cabinet and not next to the stove. Some people choose to keep their fish oil supplements in the refrigerator, especially if you’re consuming fish oil liquid. You know how your olive oil or oil-based salad dressing can get cloudy when you take it out from the refrigerator? The same may be true with your fish oil. Put it out on the countertop for about 5 or 10 minutes and let it return to its normal state. But if it doesn’t, call the company’s customer service and ask about it.


  1. Packaging considerations. Look for products sold in bottles with opaque plastic or dark or UV-blocking glass, or in blister packs. These options help to limit the amount of light reaching the fish oil capsules or fish oil.


  1. Company counts. Do your research because who you buy from matters. Check for company reputation and test out the customer service before you buy that brand by asking some questions of the company or of the retailer if you purchase from a trusted retailer. What do they have to say about how the product should smell? How will you know if the product is rancid? How should their product be stored? Do they comply with the GOED Voluntary Monograph? What about their test results for PV and pAV? Do they use an independent testing company or a third-party certifier?


  1. Keep doc in the loop. Be sure your doctor is looped in your decision to use (or not to use) fish oil supplements in case you’re contraindicated (e.g., are you allergic to fish) or are taking medications that could interact with fish oil supplements. Your pharmacist can also advise about those potential interactions.


Keep your doctor in the loop about all of your supplement use. And, remember, when it comes to concerns about fish oil supplements and rancidity, common sense and smart choices go a long way.

VIDEO: 3 Tips on How to Pick an Omega-3 Supplement

These statements have not been evaluated by the Food and Drug Administration. This test is not intended to diagnose, treat, cure, prevent or mitigate any disease. This site does not offer medical advice, and nothing contained herein is intended to establish a doctor/patient relationship. OmegaQuant, LLC is regulated under the Clinical Laboratory improvement Amendments of 1988 (CLIA) and is qualified to perform high complexity clinical testing. The performance characteristics of this test were determined by OmegaQuant, LLC. It has not been cleared or approved by the U.S. Food and Drug Administration.

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