Rancid Oil – What You Don’t Know About Omega-3 May Be Harming Your Dog

Omega-3 is essential for dog health but quality and storage matters. Here's how to detect and avoid rancid oil.

Omega 3 gel capsules.


As a dog owner, you’re probably aware that omega-3 is essential for your dog’s health. We’re told by pet food manufacturers and supplement companies that we must include omega-3 (usually in the form of fish oil) for optimal health.

And I agree…

Omega-3 is absolutely essential for the health and well being of your dog.

Much like humans, dogs must get omega-3 from food. Their bodies cannot make it without eating the right foods rich in omega-3.

But be careful!

What you may not realize is that most animal or plant-based omega-3 supplements can do more harm than good.

In fact, you are better off NOT feeding your dog omega-3 oil over feeding him rancid oil!

Rancid oil is something I have a lot of experience with and I want you to learn from my mistakes.

Causes of Rancid Oil – Why Does Omega-3 Oil Go Rancid?

Rancid oil definition: Oil is considered rancid when there is an unpleasant, stale smell or taste usually from the chemical change of the fat and or decomposition. Oils extracted from fats are most susceptible to light, heat, and oxygen.

So what causes oil to become rancid?

Three main reasons oil will go rancid:

  1. Oxygen – Oil exposed to the air is the most common way oil becomes rancid. It can happen quickly. So make sure that your oil is capped tight and minimize the exposure to oxygen. If you have supplements, this should not be an issue as the gel caps will prevent oxygen from entering the oil. This applies to bottle containers and any whole foods like sardines or raw fish.
  2. Light – When oil is exposed to ultra violet light, it can become rancid. Livescience explains the chemistry behind light and rancid oil. If your omega-3 fish oil for your dog is sold in a bottle, make sure it is a dark amber color glass bottle (preferred over plastic) or a plastic BPA-free bottle void from any light.
  3. Heat – Cooking oil or exposing it to any amount of heat, even out in the sun, is a big no no! Heat will cause the oil to go rancid quickly. So make sure you store your oil in a cool (I use the refrigerator) dry place.

Prevent Rancid Oil

How To Store Omega-3 Supplements For Your Dog

Whether you are feeding canned sardines or an omega-3 supplement for dogs, it’s very important that you store it properly to avoid rancidity.

I like to store oil in a cool dry place away from light, often in the refrigerator.

A good manufacturer will place the oil in a bottle that is either dark (dark amber bottles) or in a bottle that is completely secluded from light. I prefer glass bottles over plastics but most come in the form of plastic bottles.

When you are buying omega-3 for your dog its very important that you know how to spot and identify rancid oil and that you avoid manufacturers that don’t have protocols in place for testing for rancidity and impurities.

Tips to avoid rancid oil for your dog:

  1. Make sure the brand of omega-3 fish oil you feed your dog has good manufacturing protocols to avoid impurities in the oil.
  2. Ensure the packaging excludes light.
  3. The source should be from a fish like sardines or herring.
  4. Before purchasing make sure the oil was stored in a cool and dry place.
  5. Never leave your oil near heat like a stove or out in the sun when receiving a package in the mail.
  6. Avoid keeping your oil open to the air (oxygen exposure).
  7. Use the smell test, if it smells “off” or has a fermented smell to it, that’s your first clue it is rancid.
  8. Take a taste test. If it tastes burnt or “bitter-sweet” and if you get a headache after taking a teaspoon, the oil is most likely rancid! Taste and smell before giving to your dog (yes humans need omega-3 too!).

Some Oils Become Rancid Quicker Than Others

The chemical composition and/or nutritional profile of the oil plays a critical role in determining if your oil is going to go rancid or not.

The more saturated the fat in the oil, the less susceptible it is to rancidity. Also, nutrients in the originating source of the oil play a role.

Since most omega 3 supplements sold for dogs contain just the oil (pressed out and purified) you don’t have any antioxidants or vitamins there to protect from rancidity. Some companies will add vitamin E for example to help stabilize the oil.

There are a few exceptions to this, mainly just one: that is a plant-based omega-3 oil like chia oil. But that is for another discussion all-together.

Why I Prefer Canned Sardines

I love sardines as a source of omega-3 for my dogs.

Because I’m feeding the whole fish and not the extracted oil (which is more vulnerable to rancidity), there’s less of a chance of decomposition.

Whole-Fish vs Extracted Oil From Fish

When a manufacturer presses oil, it is extracting the oil from the fish and isolating it. Thus making the precious fatty oil even more susceptible to becoming rancid.

Important: When the oil is kept with the whole fish, other nutrients like Vitamin E and antioxidants will protect the precious oils in the fish from going rancid quickly!

I find that sardines are one of the best sources for omega 3. They are a small fish, high up on the food chain which means they probably won’t have high levels of mercury either. And most companies now are sustainably fishing the population of sardines.

But yes even canned sardines can become rancid!

Here’s my story about receiving a box of rancid sardines (probably my fault)…

One day I ordered a box of sardines online and the mailman left the box outside in the heat. It was over 90 degrees outside and I didn’t know the package had arrived until about 3 hours later!

The difference in color and appearance of these sardines after being exposed to the heat was clearly noticeable. They had a dark brown color with weird spots oozing from the fish. One quick smell and taste I could tell the fish were becoming rancid fast. But not wanting to waste the fish, I fed them to my dogs anyway.

I didn’t notice anything right away.

That’s the problem, you won’t be able to tell if your dog has a bad reaction to the rancid oil because it damages your dog’s cells and DNA over time.

But I did notice an inflammation response.

I feed omega 3 oil for anti-inflammatory purposes and hip and joint support. I noticed the dogs skin and coat became itchy and dry as opposed to oily and soft after feeding this batch. My dogs also didn’t like the fish immediately. Normally they can’t turn sardines away and will eat them with a ferocious appetite.

But even my non-picky eater spit it out before finally eating the rancid fish.

I am not trying to scare you here.

Like I said, I paid a lot of money for these sardines and was not about to let them go to waste. But at the same time, I realized I was doing more harm than good by feeding them these rancid sardines.

But now I know not to leave sardines out in the hot sun!

Bottom line

  • Don’t feed your dog rancid oil if you think it is rancid.
  • Don’t buy omega-3 dog supplements from manufacturers who don’t test the purity of the product.
  • Avoid omega-3 oils from large fish like cod, mackerel, salmon.
  • Don’t store your oil near heat and avoid light exposure and oxygen exposure when possible.
  • Use a taste and smell test to make sure what you are giving your dog is helping and not harming their precious DNA and cells.

I’d also like to point out something (I plan to write more on this later). Vitamin E plays an important role in your dog’s ability to digest and process fat, especially omega-3.

Too much fish oil can lead to a depletion of Vitamin E. But too much vitamin E isn’t a good thing either. The key is a balance.

Tips to keep your dog’s diet balanced:

  1. Avoid grains. Feed your dog a meat-based diet preferably with a balanced nutritional profile from a reputable dog food brand. I have fed both kibble and raw over the years and currently feed primal.
  2. Avoid heavy omega-6 oils like found in most vegetable oils. These oils become rancid quickly and can also cause inflammation.
  3. Always feed grass-fed, antibiotic free meats whenever possible. They have a better balance of PUFAS.
  4. Consider adding other phytonutrients and supplement with algae like spirulina, phytoplankton, and herbs like milk thistle and dandelion for detoxing the liver.
  5. Also, look for a high-quality digestive enzyme formula which helps digest fats and other food.

Have you experienced rancid oil before? Let me know your thoughts below!

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