Fish oil is a rich source of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), fatty acids that incorporate into platelet cell membranes after ingestion
As EPA replaces arachidonic acid in platelet cell membranes, plasma thromboxane B2 decreases, inhibiting platelet aggregation.
A randomised controlled trial, published in 2007 in the Lancet, reported that EPA supplementation significantly increased adverse bleeding events in hypercholesterolaemic patients.
Because of the potential for fish oil to increase bleeding risk, it has been suggested that patients should discontinue fish oil supplementation before surgery or other invasive procedures. In a systematic review published in 2017, Begtrup et al. investigated the effects of fish oil supplementation on haemostasis in healthy subjects and in those undergoing surgery.
The review included a total of 52 publications: 32 in healthy subjects and 20 in patients undergoing surgery. Studies included literature published since 1960 and excluded small studies with fewer than 20 subjects. Doses and durations of fish oil supplementation varied among studies, and the review did not include a meta-analysis.
The majority of studies in healthy subjects reported reduced platelet aggregation after fish oil intake.
Four out of five studies reported that fish oil reduced thromboxane B2 levels.
Bleeding time was significantly increased in three out of six studies after fish oil intake. One study showed that platelet aggregation normalised after 2 months of discontinuing fish oil, and another showed that normalisation took 12 weeks.
In studies of patients undergoing surgery, clinical bleeding was assessed in 16 studies.
The majority of these studies were randomised controlled trials, and none of them reported an increase in intraoperative or postoperative bleeding in patients exposed to fish oil before surgery when compared with controls.
In contrast, two studies reported a reduced need for blood transfusions in patients exposed to fish oil. There was also no evidence that postoperative fish oil supplementation increased post-operative bleeding.
Taken together, this systematic review of studies concludes that fish oil supplementation reduces primary haemostasis in healthy individuals but does consistently affect secondary haemostasis and does not increase bleeding risk during or after surgery.
The authors conclude that discontinuation of fish oil before surgery should not be recommended.