The high levels of EPA in fish oil is a feedback inhibitor of the enzyme delta-5 desaturase. As with lactose intolerance, constant exposure to EPA could easily down regulate the enzyme. This is the same enzyme that produces arachidonic acid (AA), meaning less pro-inflammatory eicosanoids would be generated. The bottom line would be less heart disease due to a decreased AA/EPA ratio. However, in clinical studies on non-Inuit subjects, the same benefits are seen so how you get to a low AA/EPA ratio is not as important as getting there in the first place.