Published on 03/02/2018

Storing Omega-3 Liquids and Pills or Capsules

Fish oil supplements are available in two main forms: liquids or softgels (liquid gels). It can be difficult to tell by looking at the labels, but it is important to know that not all fish oil is created equally when it comes to quality.

Regardless of what format the fish oil is in, the high levels of the omega-3 fatty acids EPA and DHA—which are responsible for the health benefits of fish oil—are susceptible to oxidation.
What is oxidation?
The best way to explain oxidation is to give you the example of rust. Rust is just a generic term applied to the reddish iron oxide formed when iron reacts with oxygen in the environment. Similarly, oil reacts with oxygen and initiates an undesirable series of chemical reactions that eventually produces rancidity unpleasant taste/smell in oil.
All oils are susceptible to oxidation, but fish oil is highly susceptible. EPA and DHA are polyunsaturated fatty acids, which means they have multiple double bonds between their carbon atoms. This makes them very unstable, as the double bonds can readily react with oxygen to create unpleasant fishy flavour and odours.
What does oxidation do to fish oil?
The process of oxidation leads to the accompanying “off” flavours and smells that people often associate with fish oil. Because of this, you can often use taste and smell as indicators of quality. Interestingly, if you’ve ever gone fishing you may recall that when a fresh fish is caught it doesn’t have a fish smell or taste. These sensory properties are developed because of oxidation during the processing and packaging of the fish oil.
Ideally, products with the least amount of fishy taste and smell have typically undergone the least amount of oxidation, and most closely resemble fish oil in its most pure and fresh state. However, some fish oil products can have a lot of flavors to mask the taste and smell.
Producing a fish oil product is not necessarily a tremendously difficult task. This is evident when you walk into a drug, grocery, or health food store and see a vast array of different fish oil products. However, producing a high quality, great-tasting fish oil is an extremely difficult task because of the oxidation issue.
How quickly can oxidation happen?
Oxidation occurs as soon as the oil encounters oxygen from the environment. Therefore, as soon as fish oil supplements are opened, oxidation can take place. There are ways to reduce oxidation and attempts should be made to reduce oxidation at each stage of oil manufacture and storage.
Here are some things to look for when purchasing fish oil supplements:
  • Temperature is a factor that can increase oxidation. Always store your NutraSea liquid fish oils in the fridge after opening as it slows down oxidation.
  • Exposure to light can also cause oxidation, so fish oil that is packaged in dark brown glass bottles may help to address this.
  • Make sure the manufacturer has kept the fish oil covered with an inert gas such as nitrogen throughout production. Oxygen exposure is a catalyst for oxidation.
  • Look for fish oil that has antioxidants added to it.
  • Look on the label for the product’s Natural Health Product Number (NPN) issued by Health Canada.
Most fish oil products state a shelf life of 24 months from the date they are made, if they are not opened or exposed to bright light or high heat.
How is fish oil oxidation measured?
Fish oil oxidation is usually assessed by measuring levels of primary and secondary oxidation products. Primary oxidation products, as the name implies, are the first to form, and consist of compounds called hydroperoxides. These oxidation products can be measured using Peroxide Value (PV)1.
The Global Organization for EPA and DHA Omega-3 (GOED) Voluntary Monograph states that the PV of fish oils should be no higher than 5 milliequivalent/kg (mequiv/kg). Health Canada has the same specification on their fish oil monograph1,2. However, PV levels change as oxidation progresses—fresh fish oil starts with a low PV which, as oxidation progresses, increases, plateaus, and then decreases. This can lead to an oxidized fish oil product having a low PV, but very rancid taste.
To detect secondary oxidation products in cases like the above, p-anisidine (AV) testing is used. It measures secondary oxidation products called aldehydes. These oxidation products increase as the primary oxidation products are broken down. The GOED Voluntary Monograph and Health Canada fish oil monograph both state that the AV of fish oil should be no greater than 201,2.

What about fish oil capsules?
NutraSea fish oil capsules (liquid gels) can be stored at room temperature once the product is opened, if they are kept out of direct sunlight and are not exposed to high heat. The material used to encapsulate the fish oil acts as a barrier to oxygen and this barrier helps to prevent oxidation. If stored properly, fish oil capsules will maintain their quality for the duration of the shelf life stated on the product label.

How should I store and use my omega-3 fish oil products?
Liquid fish oil can oxidize  much faster than fish oil capsules after opening, because they are lacking the barrier that the capsule provides. This means that the liquid is exposed directly to air. While fish oil capsules can be stored at room temperature, liquid fish oils should always be stored in the refrigerator once opened because the cool temperature slows down oxidation.
It’s also important to remember that you should never cook with liquid fish oil.

How will I know if my fish oil is of the highest quality?
If you want to take it a step further, you should investigate whether your fish oil has been third party tested. This means that an independent testing organization tests the fish oil for quality according to the industry’s most stringent guidelines for quality. This is extremely important if you want to guarantee the quality of your oil.
The Pure Check program is a third-party testing program that is easy to use to retrieve the third-party test results simply by typing the lot # on a NutraSea product into a box at Instead of just testing one batch of fish oil at an independent laboratory, Pure Check makes sure that every batch of product available on the market is tested for quality and potency and the results are easily accessible online in a nice, easy-to-understand format.

What else should I know about my fish oil?
Aside from oxidation, a key point to consider with fish oils is potency. In other words, how much EPA and DHA are provided? This is important since they are the fatty acids responsible for the health benefits of fish oil. You want to make sure that you are getting what you are paying for.

1. GOED, GOED Voluntary Monograph - Version 5, 2015: p. 3.

2. HealthCanada. Fish Oil 2017 July 18 2017 [cited 20201 Sept 28th ]; Monograph. Available from: