Skip to main content

Natural Awakenings

Is Your Fish Oil Rancid?

Sep 28, 2021 03:36PM ● By Anne-Marie Chalmers, MD
Tipped over bottle of fish oil supplements spilling out


Think about fish oil. If your first reaction is “yuck,” you are in good company.

Whether it’s from remembering Grandma’s cod liver oil or taking those horse-pills in the cupboard, many people believe that fish oil ought to taste bad. We put up with the unpleasant smell and taste because, well, it’s hard to argue with the benefits reported in thousands of studies.

But the truth is that high quality omega-3 products shouldn’t taste like yesteryear’s salmon. If your current fish oil capsules smell fishy – or give you nauseating burps throughout the day – you most likely have a rancid product on your hands.

What Is Rancidity?

If you eat fish regularly, you probably know that fish spoils more easily than other foods. The same goes for fish oil.

The reason fish and fish oil spoil quickly is that they are rich sources of polyunsaturated omega-3 fatty acids. Known for their anti-inflammatory benefits, omega-3s are characterized by their large numbers of reactive double bonds. While this chemical structure makes the omega-3s extremely flexible and responsive in the cell, this same trait also explains why these fatty acids are so vulnerable to the effects of oxygen. 

As fish oil oxidizes, new byproducts begin to form in the oil. Known as lipid peroxides, aldehydes and ketones, these oxidation byproducts are what gives off the unpleasant smell and taste of spoiled fish.

The rate at which the omega-3 molecules oxidize depends on a number of factors, including how the fish were harvested and the oil processed. But once an oil has started to oxidize, there is no going back. At that point, it’s simply a matter of time before the oil goes rancid.

How Rancidity Affects Your Health

Aside from tasting and smelling bad, rancid fish oil is less effective than fresh fish oil. Because oxidized omega-3 molecules have a different chemical shape and reactivity than their unoxidized counterparts, they’re not able to carry out the same functions in the cell.

Many scientists also believe that consuming rancid fish oil may be harmful. At least two comparative studies to date show that consuming rancid fish oil can increase bad cholesterol levels. And in animal studies, oxidized fatty acids have been found to cause organ damage, inflammation, and atherosclerosis, among other problems.

How to Combat Rancidity

Rancid fish oil is unfortunately common. Studies from Canada, New Zealand, Norway and other nations have found that a high percentage of omega-3 supplements exceed acceptable oxidation limits.

Yet fresh fish oil exists and can provide consumers with a relatively inexpensive, safe way to improve health. To ensure you get a fresh omega-3 product, do the following:

1. Evaluate your current product.

The best way to tell whether or not your omega-3 supplement is fresh is to put it to the taste and smell test. If you have capsules, break them open. If your nose gets a whiff of a strong fishy smell, get something else.

2: Make smart purchases.

Fish oil is a perishable food substance. To a certain degree, it can be stabilized, but beware of labels that have two- or three-year expiration dates.

You wouldn’t eat two-year-old salmon stored at room temperature, would you? If your product has been sitting on the shelf for years, it’s almost certainly rancid.

3: Check your fish oil’s oxidation values.

How oxidized an omega-3 product is can be measured by its oxidation values. The two most common oxidation measurements are called the peroxide and anisidine values.

The peroxide value indicates the amount of lipid peroxides that have formed in the oil. The anisidine value measures the levels of secondary oxidation byproducts, like aldehydes.

Typically, the lower the peroxide and anisidine numbers are, the fresher the oil is.

Though these measurements aren’t perfect – especially if an oil contains flavoring – it’s an excellent, clinical way to determine freshness. If your fish oil brand doesn’t showcase their oxidation values, be sure to request them.

4: Keep it cold.

Low temperatures slow down the enzymatic time bomb that is ticking away. That’s why freezing or refrigerating fish oil helps keep it fresh for longer.

5: Use it or lose it.

When buying fish oil, look for small containers, not supersized bottles. The longer a fish oil is stored, the greater the chance it will oxidize over time.

Similarly, use up your omega-3 product in a timely manner. Often, people think they’re saving money by keeping old capsules or bottles of oil. But you wouldn’t keep old fish in your refrigerator. Think of fish oil supplements in the same way and throw old products out.

Read the full article. 


About Anne-Marie Chalmers, MD

Born and raised in the United States, Dr. Chalmers graduated from Brown University and completed her medical training at the University of Oslo in Norway. Dr. Chalmers practiced emergency, family, and preventive medicine in Norway for many years. Today, she serves as president of Omega3 Innovations.

Subscribe to our national newsletter!

* indicates required