Is Your Fish Oil Doing More Harm Than Good

Omega-3 fish oil is one of the most widely used supplements for general health and wellness. The polyunsaturated fats EPA and DHA found in the supplement are associated with a wide range of benefits for cardiovascular and metabolic health.

The issue with fish oil, and many polyunsaturated fats in general, is their stability. When exposed to light and heat, they are more likely to go rancid compared to other fats due to their structure.

The byproducts of this oxidation are called hydroperoxides and aldehydes (this will be important to note later). Like anything else we consume, there are thresholds for these byproducts. At a certain level, the fish oil is deemed to lose its "freshness" and considered to be of lower quality. It's important to note that these aren't necessarily safety thresholds but quality thresholds.

When you consider the transport of fish oil supplements in trucks that are not temperature controlled, along with exposure to light and oxygen, many products on the market are found to be oxidized when subjected to testing.

Before you panic about the fish oil in your cabinet, here's an important confounding factor:

Remember the aldehydes we talked about? The lemon and citrus flavors that you often see in fish oil products naturally contain aldehydes. The testing used to measure aldehydes in fish oil is unable to distinguish between the flavoring and oxidation. It's conceivable that a fish oil product that tested high in aldehydes was due to the flavoring, and not a quality issue of the fish oil itself.

A separate, but related question is how harmful oxidized fish oil is in general.

A study from the British Journal of Nutrition separated healthy non-smokers into three groups: one consuming fresh fish oil, one consuming oxidized fish oil, and a control group consuming sunflower oil.

They each consumed 8 grams of oil daily, a more than sufficient amount for 7 weeks. The results may surprise you.

There were no significant differences in oxidative stress or inflammatory markers between the fish oil groups. Both saw similar increases in EPA and DHA concentrations in plasma as well. So despite consuming the lesser quality fish oil, it did not affect the participants' ability to absorb it. Omega-6 to omega-3 ratio improved equally in both groups.

Now before you go leaving your bottle of fish oil in the trunk of your car in the summer, the study did have limitations.

7 weeks is a relatively short period of time, so long-term health effects remain unknown. The oxidized fish oil was also artificially oxidized. Fish oil products that sit on a shelf for months may degrade in a different way.

The oxidation level was also fixed at a certain value, anything higher may cause harm. Finally, the participants were all healthy and relatively young (18-50), so other populations may be affected.

The evidence we have suggests that moderately oxidized fish oil is unlikely to cause short-term harm in otherwise healthy people. That said, quality still matters. Look for products that are third-party tested, stored properly, and ideally come in dark, opaque bottles to block light. Overall this study should quell some fears overall, but there are still questions to be answered due to its limitations. Until further studies emerge, you are probably good to go with your fish oil supplementation.

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This story was originally published April 20, 2026 at 12:39 PM.