A significant issue in deciding intake levels for DHA & EPA (and the ratio between the two) is that the body can convert EPA to DHA, but not DHA to EPA. In addition, high levels of DHA compared to EPA in the blood are associated with depression. However, the EPA to DHA conversion efficiency and effectiveness likely varies in the population, suggesting that only taking EPA supplements and relying on the body’s ability to convert to DHA is not necessarily a solution either.
While EPA has well-known anti-inflammatory and other benefits, studies suggest that DHA also has a wide range of health benefits. One article on wellnessresources.com recommends DHA to support weight loss, and refers to studies linking DHA to cardiovascular health, cognitive function, memory, mood, learning, vision quality (including protection against macular degeneration), bone health, fertility, cancer prevention, and inflammation reduction. [http://www.wellnessresources.com/weight/articles/maximizing_dha_intake_in_support_of_weight_loss/]
As I have noted previously, I experienced noticeable fatigue when I started taking a high DHA:EPA ratio Omega-3 supplement, and my normal energy levels returned when I switched to a high EPA:DHA supplement. However, I have not yet tried using an EPA only, or ultra high EPA ratio supplement.