Fish Oils: Ethyl Ester versus Triglyceride forms

Low strength natural fish oils are in triglyceride (TG) form, as are most natural oils. To purify the oils by removing impurities and less desirable fatty acids and to concentrate the EPA and DHA fatty acid components, molecular distillation is used.  This process allows the oil to vaporize at low temperatures under pressure and is also known as vacuum distillation. The resulting vapor is collected and cooled, stratifying into distinct layers of fatty acids. This allows a purer esterified oil (called an ethyl ester form, or EE) with higher levels of EPA-DHA than crude fish oils and removal of triglycerides and cholesterol as well as other undesirable components. 

Some companies like to add triglycerides back in; this dilutes the fish oil concentrate by about 20% and the process also introduces mono- and di-glycerides, which don’t exist in natural fish oils to a significant degree but are thought responsible for higher absorption levels of this reconverted triglyceride form (rTG) oil over natural triglyceride and ethyl ester forms. If no triglycerides are added back, the body’s own store of triglycerides will be used to bond to the fatty acids after absorption, potentially lowering serum triglycerides more than an rTG oil. 

Absorption from our guts is always in the form of free fatty acids, so the oil’s original triglycerides are typically discarded during digestion and reattached later from body stores. This lowers the potential of the TG or rTG oils to lower triglycerides already in our bodies since they introduce additional triglycerides, which may be absorbed either into the body (adding more triglycerides into the liver) or into fiber or other materials that carry cholesterol out through the stool (potentially reducing the removal of cholesterol by competitive absorption). 

The prescription fish oils are in the ethyl ester form, not triglyceride form, because of the higher purity and higher strength of EE fish oils over TG or rTG forms resulting in better study outcomes on cardiovascular health measures. 

Krill oil is distinct because of its phospholipid matrix that greatly enhances omega-3 absorption more than the EE, TG, or rTG forms of fish oil.

What is pharmaceutical grade (USP)?

The US Pharmacopeia (USP) publishes official monographs for certain substances. These monographs include specific assay methods and product specifications to assure identity and potency. Material that is tested by these methods to meet those specifications is then eligible to be called pharmaceutical grade, or USP. The key concept is "standards and verification" http://www.usp.org/aboutUSP/ Many natural ingredients do not have published USP monographs, so cannot be called USP grade (as there are no USP specifications or standards to test against). Also, the testing is often a bit imprecise, so a test result for purity may be acceptable within a narrow range (from 98% - 101% of label potency, for example) and still be considered good. Another standard monograph is that found in the Food Chemicals Codex, desribing FCC food grade materials. A product with an FCC monograph can be tested to meet the food grade specifications that are typically less rigorous than those in a pharmaceutical monograph, but do provide standards for purity and identity. http://www.usp.org/fcc/ Some substances have both USP and FCC monographs and can claim both grades. For example, the amino acid l-arginine has monographs in both compendia, as do some other aminos.