You pick up a fish-oil bottle and see a phrase such as micro distilled, molecularly distilled or mercury tested. It sounds reassuring, but it is fair to ask what the words actually mean. What happened to the oil during processing, and what was actually checked afterwards?
Short answer: distillation is a purification step used during fish-oil processing. Molecular or short-path distillation is one established way of reducing certain unwanted compounds from fish oil. But a processing statement is not the same thing as a finished-oil testing statement, and neither should automatically be read as mercury free or contaminant free.
If you are comparing options in our fish oil collection, the most useful approach is to read purification wording and testing wording together, then check EPA and DHA strength as a separate part of the label.
What those distillation words are telling you
In plain English, distillation is used to separate parts of an oil according to how they behave under controlled heat and pressure. In fish-oil processing, molecular or short-path distillation uses a very short travel path under reduced pressure so selected compounds can be separated with carefully controlled conditions.
Peer-reviewed fish-oil research has shown that short-path distillation can substantially reduce persistent organic pollutants such as dioxins, dioxin-like PCBs and PBDEs. The result is not a single fixed number for every oil. How much is removed depends on the contaminant involved and manufacturing conditions such as temperature, feed rate and other process settings.
Those studies are evidence about particular persistent organic pollutants. They should not be stretched into proof that one distillation claim removes every possible contaminant, including mercury, from every finished fish oil.
That is why we would not treat every distillation phrase as interchangeable. Molecularly distilled fish oil and short-path-distilled fish oil refer to recognised technical processing approaches. Micro distilled is the wording we currently use for our own fish oil. Unless a manufacturer specification spells out the exact method, we do not assume that micro distilled and molecularly distilled are identical technical specifications.
Processing and testing answer different questions
This is the distinction that clears up most of the confusion.
A purification-process statement tells you something about how the oil was processed. Words such as distilled, molecularly distilled or micro distilled sit in this category.
A testing statement tells you which contaminants or quality factors the product information says were checked. A label might specifically mention mercury, other heavy metals, pesticides or other impurities.
Both pieces of information can be useful. They simply do different jobs. A good processing method does not replace testing information, and a testing statement does not tell you every detail of the purification process.
So, does fish oil contain mercury?
Mercury is a reasonable question because some seafood can contain methylmercury. It does not mean fish itself should be treated as unsafe. Fish and seafood remain normal food sources of omega-3s, and Healthify NZ recommends food first for most people, including oily fish as part of a balanced diet.
The NIH Office of Dietary Supplements notes that omega-3 supplements have generally not been found to contain methylmercury because it is removed during processing and purification. That is reassuring general evidence about fish-oil supplements. It is not a guarantee that every product on every shelf is mercury free.
When comparing mercury-tested fish oil in NZ, the more useful question is not simply whether the bottle says distilled. Look for a clear statement about whether mercury is among the impurities the product says it tests for.
What distillation does not tell you
A distillation claim can be useful, but it does not automatically tell you:
- how much EPA and DHA the product provides
- whether the finished oil has been independently tested
- whether the oil is completely contaminant free
- whether the oil is fresh or oxidised
- whether it suits your allergies, medicines or health needs
- whether it will produce better health results
Those are separate questions. Keeping them separate makes fish oil purity claims much easier to read without giving one reassuring phrase more meaning than it actually has.
Purity and freshness are not the same thing
Contaminant purity is about unwanted substances in the oil. Freshness is about what happens to the oil over time. Fish oil contains unsaturated fats, and Healthify NZ notes that these can oxidise and degrade. A purification claim should therefore not be treated as an oxidation test or a guarantee that an oil cannot go rancid.
The short-path-distillation research also shows why the distinction matters: oxidative stability can be affected in different ways depending on processing conditions. In other words, purification and oxidation are related manufacturing considerations, but they are not the same quality measure.
If your concern is fishy aftertaste, reflux or day-to-day comfort rather than purity, we cover that separately in our guide to fish oil burps and aftertaste.
Purity is not the same as EPA and DHA strength either
A clean, carefully purified oil still needs a useful EPA and DHA label if you are comparing omega-3 content. EPA and DHA are the key long-chain omega-3 fats people usually want to compare in fish oil, and the amounts can vary widely from one product to another.
Keep that comparison separate from purity. A product does not gain extra EPA or DHA simply because it has been distilled, and a higher EPA or DHA number does not tell you whether contaminants have been checked.
For the detailed label maths, see our EPA vs DHA fish-oil label guide. It walks through how to compare the figures without mixing total fish oil, serving size and named EPA or DHA amounts.
What Our Micro-Distilled and Tested Wording Actually Tells You
With our Super Fish Oil 2XP Omega 3, we want the quality wording to tell you more than simply that the oil is good quality.
- Our Super Fish Oil 2XP is made from wild-caught cold-water fish species from a sustainably managed natural resource.
- Our current product information says the oil is micro distilled.
- Our current product information says it is comprehensively tested for impurities including mercury, heavy metals and pesticides.
- One capsule provides 360 mg EPA and 240 mg DHA.
- The formula is deodorised with natural orange oil.
- The standard current direction is one capsule daily with food or as professionally prescribed.
For us, the useful part is the combination. The micro-distilled wording gives you a stated purification step. The impurity-testing wording tells you which specific checks our current product information names. Together, those details are more useful than a vague quality claim on its own.
They still should not be stretched into claims we have not made. We do not translate micro distilled into mercury free, contaminant free or stronger health results, and we do not use it to imply extra EPA or DHA.
A simple way to compare fish-oil quality claims
When you are standing in front of a label, four quick checks are usually enough to make the quality wording more useful.
- Find the process wording. Does it say purified, distilled, micro distilled, molecularly distilled or something else?
- Find the testing wording. Does it name mercury, heavy metals, pesticides or other impurities, or does it only use a broad quality phrase?
- Check EPA and DHA separately. Purity does not tell you omega-3 strength, so read the named EPA and DHA amounts.
- Check freshness and directions. Look at expiry, storage guidance, the standard serving and any cautions that matter to you.
There is another good reason to read specific claims carefully in New Zealand. Medsafe says dietary supplements are not subject to a pre-approval process, and the sponsor is responsible for ensuring the product meets the relevant quality, safety and legal requirements. That does not mean every supplement is questionable. It simply makes specific, checkable product information more useful than broad reassurance.
If you want the bigger picture on fish oil benefits, food-first omega-3 intake and suitability, our fish oil in NZ guide covers those topics without mixing them into this purity discussion.
A quick safety note
Purification does not make a supplement suitable for every person. Check with a pharmacist, GP or other qualified health professional if you have a fish or seafood allergy, take blood-thinning or antiplatelet medicines, have bleeding concerns, have planned surgery, are pregnant or breastfeeding, or have been given a clinician-directed omega-3 plan.
Common questions
What does micro-distilled fish oil mean?
Micro-distilled fish oil means the oil has gone through a stated purification step. The term tells you something about processing, but not by itself which method was used, which contaminants were tested in the finished oil, or that the oil is mercury free.
Is micro-distilled the same as molecularly distilled?
Not necessarily. Molecular or short-path distillation has a specific technical meaning in fish-oil processing. Micro distilled is the wording used for our product, but without a manufacturer specification we do not treat the two terms as technically identical.
Does molecular distillation remove mercury from fish oil?
Fish-oil processing and purification can remove methylmercury, and NIH notes that omega-3 supplements generally have not been found to contain this contaminant. A molecular-distillation claim alone is still not a product-specific guarantee, so it is sensible to look for a clear mercury testing statement as well.
Does micro-distilled mean mercury free?
No. Micro distilled describes a stated purification step. It should not be translated into mercury free unless the manufacturer has specific evidence and wording to support that claim.
What contaminants should fish oil be tested for?
Useful testing information may cover mercury, other heavy metals, pesticides and other contaminants relevant to the oil and manufacturing process. The key is to check what the product actually says it tests for rather than assuming one purification word covers everything.
Does distillation stop fish oil from going rancid?
No. Distillation is a purification process, not a freshness guarantee. Fish oil can oxidise and degrade over time, so freshness, storage and expiry remain separate quality questions.
How can I tell if a fish oil is good quality?
Look for clear processing information, specific contaminant testing statements, meaningful EPA and DHA amounts, sensible directions and freshness information such as storage and expiry guidance. In New Zealand, it is also worth remembering that dietary supplements are not pre-approved by Medsafe.
Is purity more important than EPA and DHA strength?
Neither replaces the other. Purity information helps you understand processing and contaminant checks, while EPA and DHA strength tells you how much of the key omega-3 fats the product provides. A good comparison looks at both.
What does Gold Health test its Super Fish Oil 2XP for?
Our current Super Fish Oil 2XP information says the oil is comprehensively tested for impurities including mercury, heavy metals and pesticides. It also states that the oil is micro distilled.
Next steps
If you are choosing fish oil, start by looking for clear information in three places: how the oil is processed, what impurities the product says it is tested for, and how much EPA and DHA it provides. None of those replaces the others.
You can compare the options in our Gold Health fish oil collection and choose the product whose quality information, strength and directions make sense for you.
References
- NIH Office of Dietary Supplements: Omega-3 Fatty Acids Fact Sheet for Health Professionals
- Healthify NZ: Omega-3 and fish oil supplements
- Medsafe: Regulation of Dietary Supplements
- PubMed PMID 20444484: Modelling short-path distillation to remove persistent organic pollutants in fish oil
- PubMed PMID 21070072: Effects of refining and short-path distillation on fish-oil quality and oxidative stability
