Fish oil and cod liver oil supplements are popular for their omega-3 fatty acids, which offer numerous health benefits. However, not all supplements are created equal. Understanding how to read their labels can help you choose a high-quality product. Here’s what to look for:

Production Method

Most fish oils and cod liver oils are produced using molecular distillation, a process that purifies the oil and removes contaminants. If the label or product website does not specify the production method, it’s safe to assume molecular distillation was used. This detail is crucial for understanding the product’s purity and concentration.

Fatty Acid Form

The fatty acids in molecularly distilled oils are typically in the form of ethyl esters (EE), but they can be converted back into triglycerides (TG), closer to their natural form. This process yields re-esterified triglycerides (rTG). If the label mentions “rTG,” the product contains re-esterified triglycerides; otherwise, if it’s molecularly distilled and doesn’t specify, the fatty acids are likely in ethyl ester form. Most brands do not revert to the TG form due to the additional processing required.

Vitamin Content

Molecular distillation often removes naturally occurring vitamins, so these oils are usually fortified with additional vitamins. Fish oils typically do not contain vitamins unless added during manufacturing. Be wary of products that add vitamins in vegetable oil solutions, as this is less desirable. Phrases like “vitamin D prep” on the label indicate added vitamins, not naturally occurring. If the product is molecularly distilled, any vitamins listed are likely added, and some products may not contain vitamins at all.

Vitamin Forms

The bioavailability and efficacy of vitamins can vary significantly. Vitamin D3 is generally considered more effective than vitamin D2, and similar distinctions apply to forms of vitamin A. The label should specify which forms are included. If it does not, the product may contain less effective forms of these vitamins.


Labels often indicate whether the oil is concentrated. Natural cod liver oil is approximately 25% total omega-3 fats, with about half as DHA (docosahexaenoic acid) and a third as EPA (eicosapentaenoic acid). A 1g serving of natural cod liver oil contains roughly 0.25g DHA and 0.166g EPA. Deviations from these ratios suggest the product is concentrated. For fish oil, concentration is even more likely, given the common use of molecular distillation to increase omega-3 levels.

Other Ingredients

Inspect the label for other ingredients used to bulk out the product, such as vegetable oils. These fillers can dilute the potency and diminish the health benefits of the omega-3 fatty acids.


Choosing a high-quality fish oil or cod liver oil supplement involves more than just picking any bottle off the shelf. By understanding the nuances of production methods, fatty acid forms, vitamin content and forms, concentration levels, and additional ingredients, you can make an informed decision that aligns with your health goals. Always opt for products that provide transparency about their sourcing, manufacturing, and formulation to ensure you’re getting the best possible benefit from your supplement.