Not all fish oil is absorbed the same way. The re-esterified triglyceride (rTG) form of fish oil has been shown to have a bioavailability roughly 50 percentage points higher than the ethyl ester (EE) form1. Understanding the difference between ethyl ester and triglyceride fish oil can help you choose a supplement that works as the label suggests.
This guide breaks down what ethyl ester, triglyceride, and re-esterified triglyceride fish oil are and what the research says about them. It also covers what to consider regarding purity and manufacturing.
EE vs. TG Fish Oil: The Quick Answer
Ethyl ester and triglyceride are two different chemical forms of fish oil. Both contain eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These two omega-3 fatty acids are what give fish oil its health benefits, supporting heart, brain, and joint health as part of a healthy diet.*
Although both forms of fish oil contain the same fatty acids, research suggests the body does not absorb them equally. Triglyceride fish oil, especially re-esterified triglyceride, generally shows better bioavailability than ethyl ester fish oil. Bioavailability refers to how much of a nutrient is absorbed and available for the body to use, as opposed to what passes through the system unused.
What Is Ethyl Ester (EE) Fish Oil?
Ethyl ester fish oil is created by chemically separating EPA and DHA from their natural glycerol structure and binding them to ethanol instead. This process, known as transesterification, is followed by molecular distillation to concentrate the oil’s omega-3 content. EE is widely used in the supplement industry because it's a more efficient, lower-cost way to produce a high-concentration fish oil.
What Is Triglyceride (TG) Fish Oil?
Triglyceride fish oil is the natural form of omega-3s found in fish, where EPA and DHA are bound to a glycerol structure. This is the same structural form that most dietary fats take in the body. Because it isn't concentrated through additional processing, natural TG fish oil generally provides a lower percentage of EPA and DHA than processed forms like EE or rTG.
What Is Re-Esterified (rTG) Fish Oil?
Re-esterified triglyceride fish oil starts out as EE. The EPA and DHA are then removed from the ethanol structure and reattached to a glycerol structure, restoring a triglyceride-like structure. The output is described as being similar to natural fish oil, since it returns the fatty acids to the same glycerol structure found in unprocessed TG fish oil, while retaining the higher concentration achieved during the EE process. This is the process used to manufacture OmegaVia omega-3 supplements. The higher manufacturing cost is worth it to better support your wellness goals.
However, the completeness of this re-esterification process can vary. Research suggests that rTG supplements aren't always fully re-esterified. One analysis found that rTG products may be composed of roughly 55% to 60% true triglycerides, with the remainder made of diglycerides and monoglycerides, and in some cases, residual ethyl esters. This variation in re-esterification quality is one reason product purity and testing matter—topics we'll get into in detail below.
Absorption: Does The Form Really Matter?
Yes, absorption matters, as it determines how much of the EPA and DHA in a fish oil supplement your body can actually use, rather than passing through unused. When it comes to omega-3 ethyl esters vs. triglycerides, absorption differs, but the size of that difference depends on which studies you look at and the conditions the research was done under.
Research on ethyl ester vs. triglyceride fish oil has produced a range of results, in part because variables like dose, study duration, and whether the supplement is taken with food all influence how much EPA and DHA actually reaches the bloodstream.
What The Research Shows
A two-week study conducted by researchers at the Department of Human Nutrition at the University of Copenhagen compared five fish oil preparations, including natural triglycerides, ethyl ester, and re-esterified triglyceride forms, in 72 participants. Researchers found a mean relative bioavailability of 124% for the rTG form compared to 73% for the EE form, a gap of roughly 50 percentage points1.
The study's authors also found rTG's bioavailability was somewhat higher than natural TG fish oil, though that particular difference was only statistically significant when compared with cod liver oil specifically1.
A different study looked at incomplete re-esterification as a possible complicating factor. Because the process of converting EE back into rTG doesn't always fully succeed, commercial rTG products can vary in how much true triglyceride they actually contain. One analysis found rTG supplements may be composed of only about 55% to 60% true triglycerides.
A follow-up trial testing two products with different rTG purity levels (above 96% true rTG versus about 70%) found that the higher-purity product led to significantly greater increases in blood EPA and DHA levels after 16 weeks. This suggests that within the rTG category itself, purity may be as important as form.
However, not all research agrees on the EE-versus-TG gap. Some studies have found smaller or non-significant differences between the two forms, depending on factors like dose, study duration, and whether the oil was taken with food. This variation is exactly why absorption research on fish oil forms can look different between studies.
Purity, Oxidation Risk, And Manufacturing Differences
The chemical form isn't the only manufacturing variable that affects fish oil quality. Purity and contaminant testing matter regardless of whether a product is EE, TG, or rTG. Because fish oil may contain trace contaminants like mercury, polychlorinated biphenyls (PCBs), and dioxins, it's a good idea to choose supplements that have undergone third-party testing to confirm purity and label accuracy.
Re-esterified triglyceride products bring additional considerations because of the variations that come with converting ethyl ester back to triglyceride form. As noted above, one analysis found that some products labeled as rTG may actually be composed of only about 55% to 60% true triglycerides, with the remainder made up of diglycerides, monoglycerides, and in some cases, residual ethyl esters. As a result, two products that carry rTG on their label can differ meaningfully in their actual composition.
Third-party testing means an independent lab, not the manufacturer, verifies that a product's actual EPA and DHA content, purity, and freshness match its label. OmegaVia is IFOS 5-star certified, for example, with every batch independently tested for purity, potency, and freshness, and separately tested for radiation by Eurofins.
Which Form Should You Choose?
If bioavailability is your main concern, a triglyceride or re-esterified triglyceride fish oil product is a reasonable starting point, based on the available research. If you opt for an rTG product, look for one that discloses its actual re-esterification percentage or third-party test results, since rTG alone doesn't guarantee a fully converted product.
Beyond form, make a point of checking purity and manufacturing. Look for third-party certifications, such as IFOS, NSF International, or USP, which confirm that a product's actual EPA and DHA content, purity, and freshness match what's on the label, regardless of whether it's EE, TG, or rTG.
Ultimately, there is no single best choice. The right option for you depends on your health needs and goals. Talk to a healthcare provider for guidance on your supplement usage, especially if you're managing a specific health condition or taking other medications.
FAQ
What's the difference between ethyl ester (EE) and triglyceride (TG) fish oil?
Ethyl ester fish oil and triglyceride fish oil both contain the omega-3 fatty acids EPA and DHA, but the way in which those fatty acids are chemically bound varies. Ethyl ester formulations bind these fatty acids to ethanol, while triglyceride formulations bind them to a glycerol structure. Re-esterified triglyceride (rTG) fish oil goes through both processes, converting EE back into a triglyceride-like structure.
Which form of fish oil absorbs better, EE or TG?
Research generally suggests that triglyceride fish oil, especially in its re-esterified form, absorbs better than ethyl ester fish oil. One study found that rTG had a bioavailability roughly 50 percentage points higher than EE1. However, the research varies, and some studies have found smaller or no significant absorption difference between the two forms.
Is ethyl ester fish oil safe to take?
Ethyl ester fish oil is widely used in supplements, and the FDA has also recognized the ethyl ester form as a prescription drug ingredient. For dietary supplements specifically, look for third-party testing to confirm quality, since supplements don't go through the same FDA review process as prescription products.
Why do some fish oil brands use the ethyl ester form?
Ethyl ester fish oil is created through a process that makes it easier and more cost-effective to concentrate omega-3 content. This allows manufacturers to produce a higher-potency product more efficiently than natural triglyceride oil, which is typically less concentrated.
Does the FDA recognize a difference between EE and TG fish oil?
The FDA has recognized omega-3-acid ethyl esters as a distinct drug ingredient. However, note that the FDA only regulates prescription products, not non-prescription dietary supplements. This is why third-party testing is so important when selecting supplements.
* These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Sources
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Dyerberg, J., Madsen, P., et al. (September 2010). Bioavailability of marine n-3 fatty acid formulations. Prostaglandins, Leukotrienes and Essential Fatty Acids. Retrieved August 27, 2026, from https://pubmed.ncbi.nlm.nih.gov/20638827/
