Fish Oils: ethyl ester or triglyceride or...?

Most fish oils today are molecularly distilled, changing them from their natural triglyceride forms to ethyl ester (esterified) forms.

Anything stronger than a natural strength fish oil (up to about 30% EPA+DHA combined) is typically concentrated by molecular (vacuum) distillation to allow vaporizing at low temperatures, in the process converting to the ethyl ester form. This allows separation of the fatty acid constituents in order to add back some of the desired omega-3 fatty acids in order to concentrate them up to 60% or higher levels of the oil. This is accomplished by removing undesired fractions that were separated by the distillation, such as cholesterol, triglycerides, and various other fatty acids.

Because of the warming oceans, the naturally occurring amount of omega-3 fatty acids in most fish have declined from historically about 30% to somewhat less. Due to this change, most fish oil supplements containing 180 mg. of EPA and 120 mg. of DHA (300 mg. combined in a 1,000 mg. fish oil capsule) now need to spike the potencies by adding additional EPA and DHA fractionated from the original oils by molecular distillation. It is now unusual to find even a low strength fish oil capsule that is not at least partially molecularly distilled into ethyl ester forms. 

Some of these distilled oils are then partially reconverted to a triglyceride form in a process called reconversion that involves adding back 20% or more triglycerides to the esterified fatty acids to try to reattach the triglycerides to the fatty acids with enzymes. This imperfectly produces a highly processed combination of both triglyceride and mono-and-diglyceride forms of omega-3 with many of the original oil’s natural constituents deliberately removed (cholesterol, omega-6, omega-9, stearates, et al). This newly engineered combination is called a reconverted triglyceride form (rTG) containing typically at least 60% triglyceride form fatty acids that’s distinct from the original triglyceride (TG) form, but is as far from the original triglyceride form as can be produced by intentional chemical manipulation. Numerous brands offer this rTG form and inaccurately call it a natural triglyceride form, when it is in fact far removed from that oil.

This is done because of a prevailing and largely disproven belief that the natural triglyceride form is best. That's certainly untrue for cardiology and the form has recently been shown to be largely irrelevant to absorption and efficacy in general.