Two of my favourites fish oil and krill oil.
Let's Look At Krill Oil
Krill oil is a mixture of fatty acids high in EPA and DHA (fish oil fatty acids) in the form of phospholipids, mostly as phosphatidylcholine; it appears to be better absorbed than fish oil, may be more cardioprotective, and has some unique (unexplored) fat burning effects.
Krill Oil is an oil that is derived from krill; it contains the same two fatty acids that fish oil contains (Eicosapentaenoic Acid, or EPA, and Docosahexaenoic Acid, DHA). However, a large portion of the EPA and DHA in krill is in the form of a phospholipid, with a phosphate group on the end of the fatty acid. This results in higher bioavailability (rate of absorption) of krill oil, and thus the same effects of fish oil can be seen with Krill Oil but at a lower dose.
How to Take Krill Oil
Supplementation of Krill oil tends to be in the range of 1-3g daily (overall oil weight), which has been used in the clinical trials of krill oil supplementation.
If supplementing in accordance with the omega-3 content, the omega-3 content that is supplemented from krill oil should be equal to approximately 2/3rds that used with basic fish oil supplementation to account for the increased absorption. If one were to normally supplement 1000mg EPA plus DHA, then 660mg of EPA and DHA from krill oil would be equivalent.
Let's Look At Fish Oil
Fish oil is a general health supplement, and is taken as a source of omega-3 fats. It has large effects on biomarkers such as triglycerides, but does not seem to affect clinical endpoints, such as cardiovascular events.
Fish oil is a common term used to refer to two kinds of omega-3 fatty acids: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These omega-3 fats are usually found in fish, animal products and phytoplankton. Fish oil is recommended as a source of these omega-3 fats as they are the cheapest and most common source of them.
Fish oil provides a variety of benefits when supplemented, particularly when the ratio of omega-3 and omega-6 fatty acids in the body is almost equal (1:1). The average diet (red meat, eggs, and so forth) are high in omega-6 fatty acids, which is why fish oil is recommended (to balance the ratio).
A ratio of roughly 1:1 is associated with healthier blood vessels, a lower lipid count and a reduced risk for plaque buildup. Fish oil can also decrease the risk of diabetes and several forms of cancer, including breast cancer.
Fish oil works primarily through eicosanoids, which are signalling molecules. A proper ratio of omega 3:6 fatty acids will influence which eicosanoids are released in response to stress.
It should be noted that fish oil can also reduce triglycerides in people with high triglyceride levels. However, it can also increase cholesterol, so care should be taken before supplementing fish oil for this purpose.
Three large systematic reviews of observational studies and RCTs concluded that fish oil could alleviate clinical depression,especially when used to complement standard antidepressant therapies.
Some researchers have stressed, however, that the evidence remains weak, maybe due to the different studies using different designs and methodologies, including different combinations and doses of omega-3 fatty acids, and that clinical implications should therefore be tempered.
Essential fatty acids (EFAs) are polyunsaturated fatty acids (PUFAs) your body needs and cannot produce. There are only two kinds of EFAs: linoleic acid (LA) and alpha-linolenic acid (ALA). Neither is very active, so your body transforms the former notably into arachidonic acid (AA) and the latter into eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). LA and AA are omega-6 fatty acids, while ALA, EPA, and DHA are omega-3 fatty acids. EPA and DHA make up most of the PUFAs in fish oil.
Several mechanisms of action have been posited to explain the antidepressive effect of PUFAs in general, omega-3 fatty acids, EPA specifically, and DHA specifically, but on a practical level what stands out is that EPA seems to reduce depression more than DHA does.
EPA and DHA are mostly found in seafoods, notably fatty fish (such as salmon, tuna, and sardines). The omega-3 fatty acid in most plant foods is ALA, little of which your body converts to EPA and DHA. For vegetarians and vegans, the only rich source of EPA and DHA is algal oil in supplemental form.
Sources of Omega-3s
Plant oils that contain ALA include flaxseed (linseed), soybean, and canola oils. Chia seeds and walnuts also contain ALA.
The omega-3 content of fish varies widely. Cold-water fatty fish, such as salmon, mackerel, tuna, herring, and sardines, contain high amounts of LC omega-3s, whereas fish with a lower fat content, such as bass, tilapia and cod as well as shellfish contain lower levels.
The omega-3 content of fish also depends on the composition of the food that the fish consumes. Farmed fish usually have higher levels of EPA and DHA than wild-caught fish, but it depends on the food they are fed. An analysis of the fatty acid composition of farm-raised Atlantic salmon from Scotland showed that the EPA and DHA content significantly decreased between 2006 and 2015 due to the replacement of traditional marine ingredients in fish feed with other ingredients.
Beef is very low in omega-3s, but beef from grass-fed cows contains somewhat higher levels of omega-3s, mainly as ALA, than that from grain-fed cows.
Some foods, such as certain brands of eggs, yogurt, juices, milk, and soy beverages, are fortified with DHA and other omega-3s. Since 2002, manufacturers have added DHA and arachidonic acid (the two most prevalent LC PUFAs in the brain) to most infant formulas available in the United States.
Several food sources of ALA, DHA, and/or EPA are listed in table below. The U.S. Food and Drug Administration (FDA) has established a Daily Value (DV) of 65 g for total fat but not for omega-3s. Thus, Table below presents the amounts of omega-3 fatty acids in grams per serving only and not the percent of the DV.
*Except as noted, the USDA database does not specify whether fish are farmed or wild caught. **The USDA database does not specify whether beef is grass fed or grain fed.
The above information is taken from the folks at examine.com and US National Institutes Of Health