• Hahnbee Choi

To Feed Or Not To Feed: Fish Oils

Updated: Oct 28, 2020

Fish oils are high in polyunsaturated fatty acids (PUFAs) also known as Omega-3s that have become popular worldwide.

This is because Omega-3s are beneficial for reducing inflammation in the body which decreases the chances of metabolic disease. Many studies have shown that incorporating omega-3s into the diet can reduce the risk of heart disease, prevent arthritis, aid in growth and development, and more! Which is why American easily spend over $1 billion dollars a year on over the counter fish oils.

But, fish oils are highly susceptible to oxidation due to their large number of double bonds and position in the fatty acid chain... which makes the fish oil market one of the most immutable markets in the world. unfortunately, like the rest of the pet world, fish oil companies can make it hard to specify what makes a "good fish oil" or even if they're good at all...

Before you read on I just want to mention that fish oil literature is very conflicting, especially on the topics that are so heavily researched on such as cardiovascular disease. So, keep an open mind and remember there's so much we still don't know!


Some terminology you'll need to know before going in is "oxidation" aka oxidative stress. The fatty acids in fish oils are very sensitive to oxidation. When they are exposed to oxygen they create free radicals. Free radicals are the cause of premature aging and many organ diseases.

This process is called oxidative stress. Free radicals take a healthy normal cell, attack it, and damage the cell with oxidative stress, therefore, killing the cell. Free radicals can also cause gene mutations, cancer, as well as inflammatory health problems.

When l fish oil oxidizes, it mutates into a new by-product called lipid peroxides which is a free-radical chain reaction that then reacts with un-oxidized PUFAs to form addition peroxides and also breaking down a range of secondary oxidation products such as ketones and alcohols. This is what gives the unpleasant smell and taste of fish.

The rate at which the fish oil oxidizes depends on multiple factors such as

But once the oil begins oxidizing there is no reversing the process. It is simply a matter of time before all the beneficial and medicinal properties of the omega-3s are oxidized away.

Sourcing & Processing

It is a very long and complex processing situation to get the fish oil in the bottle which is why it is not surprising that so many fish oils are severely oxidized before even getting to the consumer. A simplified version of how a typical standard quality fish oil company will source and the process is shown below...

A majority of standard fish oils are sourced from the coast of Peru or Chile and are composed of small pelagic fish. Then, each catch is moved to a vessel to shore where the fish are processed by fractionation into fish meal and crude fish oil.

Fractionation is a process of separation of mixtures of liquids, gases, or solids into their components.

The oil extracted is then stored in very large tanks to be shipped off for further processing which involves several more steps but most notable repetitively heating the oil at high temperatures. The last step is deodorization to remove aldehydes and ketones which all play a part in the rancid smell and taste of fish and fish oils.

Deodorization means to neutralize.

Did you know... less than 25% of the total crude fish oil supply actually goes into the human market?

The end result of the common bottles of fish oils sold for pet consumption is an oxidized supplement where there is often little information on the oil's sourcing, age, and level of refinement. The variety of studies on oxidation have shown that the general population is consuming (or feeding) oxidized oils that are "exceeding voluntary industry-standard levels".

Now that you know how fish oils are made... what are the effects of feeding oxidized fish oil?

Health Effects

Now that you know why and how quickly fish oil oxidizes and goes rancid, what are the health effects?

There has yet to be human studies to be done of the effects of rancid fish oils and it may or may not be due to the results in multiple animal studies...

More and more literature shows that oxidized marine oils can cause organ damage, severe inflammation, carcinogenesis, advanced atherosclerosis, and more. The excess inflammation in the body also activates certain pathways called the NF-κB pathway and increases the production of pro-inflammatory cytokines. Chronic low-grade inflammation is the root cause of almost all metabolic diseases such as diabetes, insulin resistance, and metabolic syndrome.

Animal studies have clearly shown rancid fish oils have harmful effects of feeding oxidized PUFAs. A study done by The American Journal of Clinical Nutrition discovered that the prolonged feeding of rancid fish oils to rats leads to growth retardation, intestinal irritation, liver and kidney enlargement, hemolytic anemia, decreased vitamin E, increased lipid peroxides, and inflammatory changes in the liver, cardiomyopathy, and potentially malignant colon cell proliferation.

A study done in rabbits showed the addition of fish oils led to rapid atherosclerosis which is the build-up of fats, cholesterol, and other substances in the artery wall (plaque) which can restrict blood flow. It is speculated that this may be a cause of atherosclerosis in humans but since there are minimal studies with fish oils and humans, further research must be done.

Another study that was done with mice and humans showed that consuming marine oils lead to increased plasma and urinary problems because of the absorption of oxidized oil.

It's hard to truly tell if one of the world's most popular supplements is safe to feed after oxidizes. What happens when the rancid oil enters the body is complex but from the little we do know, it can do a lot more harm than good.

With many suppliers realizing the concern of rancid oils, many companies will do what they can to stop the oil from oxidizing and one of the most popular ways is adding in artificial antioxidants which can add even more health effects.

★ To read more about artificial antioxidants click here

What about Cod liver oil or Krill oil?

Cod liver oil and krill oils tend to oxidize slower than the traditional fish oils, they still do oxidize and cause the same health effects as normal fish oil does under the right circumstances. And even when an antioxidant was added it was reported to "not show any protection against oxidation"

According to the Journal of Agricultural and Food Chemistry, Antarctic krill oil (AKO) highly susceptible to oxidation due to the high unsaturation degree of bioactive substances. Which basically means it oxidizes, becomes rancid, and has negative health effects on the body.

These oils are slightly higher quality than the traditional fish oil but still will oxidize in the end.


If fish oils aren't the best sources of omega-3s what are some alternatives?

I personally use whole fresh and/or canned fish (preferable whole & wild-caught) and phytoplankton!

Whole and canned fish provides a bioavailable and fresh source of omega-3s to your dog.

Fish is a staple in any diet whether its kibble, raw cooked! Plus, fish counts as whole prey since it has all the glands and organs included. As well as being filled with omega-3s, fish is also filled with vitamin D which can be a commonly deficient nutrient in raw diets.

BUT the sourcing and type of fish you feed are vital! Higher predator fish such as tuna and king mackerel will be higher in heavy metals, toxins, and radiation. You also don't want to source your fish out of the Pacific ocean since that's where most pollutants are found! Stick to lower level fish such as sardines, smelt, and salmon to avoid high levels of mercury & other toxins. And if you happen to catch any fish that are suited to feed, make sure to freeze the fish for at least 3 weeks to kill off any parasites that may be present.

The next item I love feeding is called phytoplankton which is what fish feed off that gives fish such high levels of omega 3s. The zooplankton will eat the phytoplankton then the fish eat the zooplankton then our dogs eat the fish. So when I feed phytoplankton I'm going straight to the source of omega 3s and cutting out all the middlemen. Phytoplankton is half of omega 3s by weight (that's twice as more than we get from fish) and has a very powerful antioxidant called superoxide dismutase (SOD) which protects cells from free radicals. Plus, phytoplankton can be sustainably grown in filtered water on farms so we don't have to worry about heavy metals or radiation.

I personally don't prefer one over the other and feed both together on a frequent basis. I like to keep phytoplankton on hand year-round, stock up on canned sardines in the water when they're on sale and buy fresh fish seasonally!

My favorite phytoplankton brands are Adored Beast or Mr. Ross and my favorite canned sardines are from Wild Planet.

What to look for in fish oil (if you have to buy it)

While I personally don't recommend feeding fish oils it's understandable that there are circumstances that you may have to continue feeding fish oil... so here are some tips and tricks to make a smart purchase.

Evaluate Your Current Product

Assess whether the fish oil your feeding right now is already rancid by the taste and smell test. Fresher fish oil should have no fishy taste or smell, just like fresh fish. If you have fish oil capsules just break them open to test the oil.

Make Smart Purchases

As mentioned above, fish oil goes rancid very quickly. Make sure to check expiration dates and buy smaller bottles more frequently to try and reduce oil rancidity.

Oxidation Values

Oxidation of an omega-3 product can often be determined by looking at the oxidation values also known as peroxide, p-aniside, and TOTOX levels.

The peroxide value indicated the number of lipid peroxides that have formed in the oil. P-aniside measures the levels of secondary oxidation by-products such as aldehydes. Lastly, the TOTOX value is calculated based on peroxide and aniside levels and shows a complete summary of the freshness of the oil. The lower all of these numbers are the fresher the oil is.

It's important to keep in mind that these measurements are not perfect...especially if there are flavoring and other additives. This test is also only a snapshot of the oil the moment it was tested. A fish oil's oxidation level can be significantly higher than shown on the report by the time it reaches the shelves. If a fish oil brand doesn't show their oxidation values, request them. And if they refuse to give them to you... run.


Low temperatures have been shown to slow down the rancidity bomb. So rather than leaving it out on the counter, stick it in the fridge.

All in all, there is so much more to learn and study on this topic and nutrition in general. The purpose of this blog is to give you the power to do what's best for you! Now that you have the facts and studies, it's up to you to choose!

As always, I hope you enjoyed and Always Keep Exploring!

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