CAUTION: Promptly, if you have fish oil bottles of liquid or softgels, they NEED to be under refrigeration. ref Fish oil omegas are very large molecules and can suffer oxidation and decomposition. This causes the fish oil to go rancid. ref Thus, not only do fish oils have to be tested for heavy metal and chemical toxins, they also need protection against rancidity. ref Vitamin E is often added since extra anti-oxidants may be needed as fish oils use them up at a fast rate attempting to prevent oxidation while in the bottle.
In spite of this increased oxidation during storage, once consumed, omega 3 oils tend to enhance body antioxidant capacity and lower inflammatory responses. This is a function of EPA controlling the fatty acid cascade of both omega 3 and 6 oils. Omega 3 are mainly anti-inflammatory while omega 6 have both anti- as well as pro- inflammatory roles. There is an ideal fatty acid ratio needed between omega 3 and 6. But, antioxidant elements still need to be present in the body to protect all oils during transit.
An aspect about fish oil digestion reveals an interesting twist. One Company is using this aspect as a medicial selling point advantage. But first, you have to understand the fatty acid digestion process to appreciate this story. 
In fish, fatty acids such as omega 3, (there are other omegas with different numbers, i.e. 6 and 9, plus many varieties within each group), form a structure called a triglyceride, or fat. Triglycerides are simply the preferred form to distribute fats in the body. Three of any omega type fatty acids attach to a glycerol backbone to form a triglyceride. Imagine this as the shape of a capital letter E. The omega fatty acids attach at either the top space, sn-1, the middle, sn-2 or the bottom space, sn-3. If an omega 3 as EPA or DHA attaches in space sn-2, the molecule has greater biological value to the body. This is one of the reasons Krill oil omega fatty acids are preferred, they have a greater percentage of omega 3 as EPA and DHA naturally in the sn-2 position, plus a phospholipid in one of the other positions. Phospholipids enjoy an advantage for greater absorption process.
Why is this sn-2 position preferred?
The triglyceride structure is too large to be absorbed intact, so in the digestive tract, the fatty acids in the top and bottom spaces usually are removed. The number 2 space fatty acid might keep part of the glycerol backbone attached while the sn-1 and sn-3 often do not. If they do not have the glycerol backbone still attached, the fatty acids become free fatty acids. This point has significance to the medicinal advantage in the upcoming story. Free fatty acids are more available to be used for energy production then the ones still attached to glycerol.
INTERESTING: Check out this reference for which position is dominate in human milk fats, triglycerides.
Next, the separated parts of the triglycerides are absorbed into the lymph system channels next to the intestinal tract. While the fatty acids and glycerol slowly travel to the liver, the body reassembles them again into triglycerides. But, there are at least 18-20 different types of omega fats that can re-attach to any of the three spaces. Thus, the new re-formed structures may not be exactly the same as they existed in the fish. Remember the middle sn-2 fatty acids often keep the glycerol backbone and will always become a new triglyceride, while free fatty acids may not re-combine to a glycerol backbone but get used for energy production. This means the sn-2 position fatty acids have a greater chance to eventually get incorporated into cell membranes, especially the nerve cells in the brain.
Omega 3 fatty acids EPA and DHA make up about 30% of omega fatty acids in fish oils. These Fish oils list on their labels, 1000 mg of total omega fatty acids, often with EPA at 180 mg and DHA at 120 mg. Since the EPA and DHA are considered the most important, scientists found ways to concentrate these two fatty acids. One method often used is molecular distillation. This gets the omega 3 as EPA and DHA up to 60%. BUT, a structural change has to take place for this to happen. The glycerol backbone is removed and an ethyl ester is formed, an ethanol alcohol backbone takes the place of glycerol. There is a process that can later be used to re-establish a glycerol backbone, but this adds quite an extra cost. A new company has engineered another further processing method to get the omega 3 EPA and DHA content percentage up to 90% by using at least two separate molecular distillation processes, each one just gets out EPA or DHA by themselves. These are also ethyl esters with ethanol alcohol backbones.
THERE ARE 4 SUPPLEMENT FORMS OF OMEGA 3 FISH OILS
1. NATURAL TRIGLYCERIDE FORM AS NATURAL IN FISH (ABOUT 30% AS EPA AND DHA)
2. ETHYL ESTERS THAT REMOVE GLYCEROL BACKBONE AND ADD INSTEAD ETHANOL ALCOHOL (USUALLY MORE CONCENTRATED THAN 30%, BUT LOWEST BIOLOGICAL VALUE)
3. PHOSPHOLIPID BOUND OMEGA COMPLEX (BEST ABSORBED DUE TO PHOSPHOLIPID GREATER DISPERSION AND HIGHEST BIOLOGICAL VALUE TO BUILD CELLULAR MEMBRANES) (NATURAL FORM FOUND IN KRILL and some varieties of fish)**
4. One fish oil company, Nordic Naturals, co ref, uses a process after the number 2 process has separately increased percentages of EPA and DHA to remove the new ethanol backbone and re-introduce the glycerol backbone. This is the most processed fish oil.
When ethyl ester (EE) fish fatty acids are compared against the natural triglyceride form for absorption, almost all the triglyceride form gets into the body within 12 hours, while the ethyl ester form acts like a delayed release product and doesn't show up until 12-24 hours. This is due to the digestion process. Remember the triglycerides get re-formed once inside the lymph system channel because they still have some of the glycerols attached. The EE form does not have any glycerol. The ethanol alcohol gets quickly absorbed by the body when the EE forms break apart in the first step of digestion. Once inside the lymph system, the free fatty acids with the ethyl esters have to wait until the body can build glycerol for them to attach to and become triglycerides and then move to the liver before packaging with cholesterol and phospholipids and get sent out into blood vessels. This takes time and explains why there is a delay before they show up in the bloodstream.
This delay is being used as a positive by the company with a patented 90% omega EE fish oil. The theory is that since most heart attacks occur in early morning hours, taking this delayed release EE form with dinner will get the omega 3 levels to arrive in the blood at the needed time for most heart attacks. One of the positive actions combating artery damage attributed to omega 3 is that they might prevent rupture of artery membranes through a platelet-wall cell action that helps prevent exposing plaque which could break off and cause a heart attack or stroke. There may be some value to this, but overtime, omega 3 fatty acids as EPA and DHA levels build up in the body anyway. Studies are needed to verify, but this is a very difficult concept to measure, and might be un-ethical to put people in jeopardy when positive actions are already known.
ANOTHER KRILL ADVANTAGE
Not mentioned in the above digestion process is that another form exists for omega fatty acids that is superior to just triglycerides. That form is phospholipid bound omegas. Phospholipids attach to space three on the triglyceride and add another digestion advantage for omega 3 fatty acids. Lower milligrams of EPA and DHA but higher absorption and biological value. Krill have this unique phospholipid advantage. Most other fish oils have less phosphlipids to begin with and unfortunately in the manufacturing process, any phospholipids are removed. There are also two new products, one from salmon fishheads (Vectomegatm) and the other (PhosphOmegatm) from a small North Sea Sandeel. The Sandeel is more sustainable and a less endangered fish than Krill which is a major link in the Sea life food chain. Phospholipids play a major role in cell membrane integrity, especially nerve cells.
** Fish oil differences exist not only in potency, but also in biological value of the essential fatty acids to build healthy cell membranes. If you just use milligram size as the criteria of choice, you might end up with a lower biological value fish oil plus reduced absorption percentages to boot.
How to find fresh and uncontaminated fish oil? Be sure to buy smaller sizes and refrigerate quickly, leaving desiccants and cotton in bottle to control moisture from refrigeration. The products with the least processing might also be better, as long as they are tested for heavy metals and chemicals. Not many companies mention rancidity, but this is a major factor. Check back for more details. Read next article.
Possible CONTROVERSY for Fish Oil Omega 3
Scientists like to break apart a food and study the pieces. Fish contains many types of omega 3 fats such as EPA and DHA, considered to be the important two. EPA is most abundant with lower amounts of DHA. EPA plays a major role over all fat processing while DHA works predominately for brain health. This puts EPA in control of the process to stop fat production when enough of the fat metabolism elements the body needs are available. And this process works very well when omega 6 and omega 3 fats are within a certain dietary balance. That balance point is at or below four omega 6 to one omega 3.
EPA is also able to turn into DHA if not enough is available for brain support. Inflammatory elements are formed from vegaetable oils omega 6 fats. Unfortunately, the current dietary balance ratio for omega 6 to 3 is well over ten to one. At this ratio, the body has a difficult time remaining healthy and the PGE2 inflammation generating elements are no longer held in check by EPA.
THE CONTROVERSIAL ISSUE
The controversial issue discovered from certain studies looked into the influence of EPA and DHA on cardiovascular disease. It was found that EPA at higher levels reduced CVD events while DHA at higher levels, increased CVD events. But, remember fish has both together, so did mother nature make a mistake? Or is the ratio between EPA/DHA in fish beneficial for overall health? ref More research may be needed to settle. Maybe the forms of EPA and DHA in supplements may reveal part of the story.
A Fishy Dilemma
Observational studies reveal that people who eat more fish have a lower cardiovascular disease rate. But, there are studies of native Alaskans, who consume a lot of fish, that show dramatically higher rates of heart attacks, cardiovascular disease, and strokes than people who eat little fish. There is obviously more to the story. This next reference looks into the situation with some insights. ref
FYI: This is why the recommended fish oils are the low potency ones. These have usually have 180 mg of EPA and 120 of DHA from 1000 mg of fish oil. They are the least processed. Make sure to get ones with some vitamin E to help preserve.