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Study Summary

Several observational trials have shown that dietary fish intake several times per week (40 to 60 g of fish daily providing about 0.2 to 1.0 g of n−3 fatty acids) or omega-3 fatty acid supplementation were associated with better cardiovascular outcomes. Randomized trials have had varying results, which culminated in several recent meta-analyses investigating ~78,000 subjects with and without diabetes, concluding that omega-3 supplementation was no better than placebo in preventing cardiovascular events. Because diabetic patients are at higher risk for cardiovascular events, even a modest effect size could prove beneficial. The ASCEND study was a randomized placebo-controlled trial of ~15,000 patients followed for over 7 years who were randomized to receive either 1 g omega-3 fatty acid supplementation or an olive oil pill. There was no difference in the primary outcome (a composite of non-fatal myocardial infarction, stroke (excluding hemorrhagic), transient ischemic attack or vascular death), with 8.9% event rate in the fatty acid group compared with 9.2% in controls (rate ratio 0.97, 95% confidence interval 0.87-1.08, P = 0.55) (see accompanying Hurst’s Central Illustration).


Study Strengths: This was a well-designed, double-blinded, placebo-controlled, randomized trial that followed study subjects over a long period (7.5 years). The results are consistent with other recent randomized control trials as well as large meta-analyses.

Study Weaknesses: Adherence to the assigned regimen was only 77% and 76% in the omega-3 supplementation and control groups, respectively. Naturally occurring omega-3 — as in portions of dietary fish consumption — may have a different effect. Furthermore, the study participants were allowed to take over-the-counter omega-3 fatty acid supplementation if the daily dose was less than 1 g. Additionally, the placebo was a matched olive oil (type and amount not specified) capsule which may itself have a beneficial effect and thereby mask any benefit of omega-3 supplementation. Outcomes were only measured by survey in the majority of participants. Laboratory data was only gathered in a fraction of the subjects. Additionally, the dose of omega-3 fatty acids used in this trial was 1 g per day, which is a lower dose than is often used clinically.

Next Step/Clinical Perspective: Though omega-3 fatty acid supplementation may be associated with triglyceride reduction and appears safe, it seems to not be efficacious for reduction of cardiovascular risk in diabetic patients. Whether previous benefit seen in observational trials was due to some other dietary factors or a combination thereof in fish that is not present in supplementation alone is unclear, and perhaps this can be the subject of future trials. As the dose of omega-3 used was lower than many currently used regimens, future studies should determine if a higher dose would be more efficacious.

Trial Reference

The ASCEND Study Collaborative Group. Effects of n−3 fatty acid supplements in diabetes mellitus. N. Engl. J. Med. doi: 10.1056/NEJMoa1804989

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