EPA and DHA are not interchangeable: one lowers LDL, one raises it

Fish oil capsules typically blend two omega-3 fatty acids, EPA and DHA, and most advice treats them as a single ingredient. Yet trials using pure preparations of each have hinted the two might act differently on the cluster of risk factors known as metabolic syndrome, with inconsistent results between studies. If EPA and DHA genuinely pull cholesterol in different directions, the composition of a supplement matters as much as the dose. This analysis set out to clarify whether the two omega-3s have distinct effects on metabolic syndrome features in humans.

The researchers searched CNKI, PubMed, Embase and Scopus up to February 2021 for randomised controlled trials that had compared pure preparations of EPA and DHA. Thirty-three trials met the inclusion criteria. Mean changes in the features of metabolic syndrome, including blood lipids and insulin, were pooled as weighted mean differences using a random effects model, allowing the two fatty acids to be assessed both individually and against each other.

Both fatty acids lowered serum triglycerides, the effect fish oil is best known for. Beyond that they diverged. EPA lowered total cholesterol by 0.24 mmol/L, triglycerides by 0.77 mmol/L and LDL cholesterol by 0.13 mmol/L, while DHA raised total cholesterol by 0.14 mmol/L, LDL cholesterol by 0.26 mmol/L and HDL cholesterol by 0.07 mmol/L. DHA also increased insulin levels compared with EPA, particularly in groups with a mean age under 60 and in trials where supplementation ran for under 3 months.

The finding helps explain why earlier trials of mixed fish oil produced inconsistent lipid results: the answer may depend on the EPA to DHA ratio of the product tested. It supports a growing case that the two omega-3s deserve separate study rather than being lumped together. Open questions remain about what the LDL rise with DHA means in practice, given that DHA also lifted HDL, and about how the insulin differences play out over longer supplementation. The health significance of these opposing shifts is not settled by this analysis.

If you take an omega-3 supplement, this is a reason to look at the ingredients list rather than the front of the bottle, because EPA-dominant and DHA-dominant products moved cholesterol in opposite directions across these trials. It is not evidence that either fatty acid is harmful or that fish oil should be abandoned; both still lowered triglycerides. It simply means the generic phrase fish oil hides a real difference that pooled trial data can now see.