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.