Refined fish oil has been distilled or purified in some manner. Molecular distillation is the best choice, because it does not require the use of heat. Heating the oils reduces the nutritional value and increases oxidation. Both of those are things that we do not want to do.
There are some things that everyone should look for when they are comparing supplements. All of the ingredients should be natural. The oils should not contain vitamin A, because it can be toxic at high doses. If you feel the need for a vitamin A supplement, choose the vegetable form, which is beta-carotene. The body converts it to vitamin A on an as-needed basis.
When comparing fish oil supplements, there are specific things to look for. The nutritional value varies greatly. The label should reflect the total amount of omega3 fatty acids that are found in each capsule, along with the percentage of DHA, EPA and DPA.
Refined fish oil usually comes in 1000mg capsules. Of that 1000mg, at least 50% should be omega3 fatty acids. Of that 50%, at least 28% should be DHA, at least 12% should be EPA and at least 5% should be DPA. Research has shown that each of those is important and results are best when all are present.
If the supplement contains omega6 fatty acids, it should not account for more than 40% of the total or 40mg. We get enough omega6s in our diet, as it is. They are the primary component of most vegetable oils. Our goal should be to increase our omega-3 intake, without increasing omega6. It may even be necessary to reduce the omega6s in your diet, depending on your current state of health.
There should be some standards concerning the purity of refined fish oil, but there are very few regulations in place. Companies can voluntarily adhere to the International Fish Oil Standards or those set by the Council for Responsible Nutrition. The better companies have even higher standards, because the founders and employees care about their own health. They have designed their supplements with that in mind.
One of the standards that many companies fail to adhere to concerns oxidation. Oxidation starts to occur as soon as the fish are caught. So, some amount is inevitable. However, the amount should be very small, because as oxidation increases, free radical count increases.
Companies that make the best refined fish oil post a copy of their certificate of analysis on their websites. This analysis will show the amount of oxidation that was present before the oils were encapsulated. Once encapsulated, no further oxidation occurs, as long as the supplements are used in a timely manner.
Total oxidation value should not exceed 17, as measured by an independent laboratory. If the company that you are thinking of buying from does not provide information concerning oxidation, pick another company, there are plenty to choose from. High levels of oxidation cause those fishy tasting burps that some people experience.
So, take your time when you are comparing refined fish oil supplements. It pays to be selective.