And the reason is a long-standing assumption that omega-3s thin the blood, making bleeding easier and clotting harder.
This example comes from the American Heart Association (AHA) web page on fish and supplemental omega-3 fatty acids:
“Patients taking more than 3 grams of omega-3 fatty acids from capsules should do so only under a physician's care. High intakes could cause excessive bleeding in some people.”
But when a respected expert in omega-3s and cardiovascular health scrutinized the medical literature, he could find no scientific justification for these concerns.
His experience was like that of the physicians who recently undertook to find the source of the common medical advice to drink eight full glasses of water (one gallon) daily.
After an exhaustive search of the medical literature, they concluded that this watery nostrum – repeated for decades by physicians and health writers, ad nauseum – is a medical myth that lacks any credible basis.
Bleeding beliefs hold no water, either
Like the doctors on the trail of the elusive 1/2 gallon-a-day water-needs claim, William Harris, Ph.D. recently sought evidence for the oft-repeated claim that omega-3s can promote bleeding.
There are plausible biological reasons – related to the influences that omega-3s exert on key metabolic agents called eicosanoids – to propose that high doses of omega-3s (or omega-3s combined with blood-thinning drugs) might promote bleeding.
We met William S. Harris, Ph.D., at the 2005 Seafood & Health conference in Washington, D.C.: a gathering sponsored by the U.S. government and attended by the world's top experts in fish-related health fields.
Dr. Harris is a former professor of medicine at the University of Missouri-Kansas City and former co-director of the Lipid
and Diabetes Research Center at Saint Luke's Hospital in Kansas City. (He now serves as Director of Nutrition and Metabolic Diseases Research at the University of South Dakota Health Research Foundation.)
As someone who's worked closely with cardiac surgeons and has access to virtually all scientific journals, Bill Harris was well-equipped to find any and all relevant evidence.
He started by asking this question: “What is the evidence that taking long-chain omega-3 fatty acids in doses of 1-4 grams per day causes clinically significant bleeding?”
To answer his own question, Dr. Harris started by examining studies in which these amounts and even higher doses were given to patients who underwent major cardiovascular surgery. Some of these heart surgery patients had also been taking blood thinning drugs such as warfarin.
The evidence was clear: “In these studies, the risk for clinically significant bleeding was virtually nonexistent.”
Harris also cited a prior review of the published research regarding omega-3s and blood thinning or clotting in humans.
In addition to finding no significant bleeding associated with patients taking omega-3 supplements in cardiovascular studies, pregnant women taking as much as 2.7 grams of omega-3s (as fish oil) per day did not suffer any increased blood loss at delivery, while dialysis patients taking high-dose fish oil experienced no increased bleeding.
Dr. Harris came to these conclusions: “Thus, the experience has been virtually unanimous: omega-3 fatty acid supplements do not increase the risk for clinically significant bleeding, even in patients also being treated with anti-platelet or anti-thrombotic [blood-thinning] medications.
He was careful to note that possible interactions between omega-3s and newer anti-platelet drugs (e.g., clopidogrel) have not been examined directly.
But as Harris wrote, he was “confident” – given the large amount of evidence in hand already – that omega-3 fatty acids do not increase risk for adverse bleeding.
And as he said, “…in considering the risks and benefits of omega-3 fatty acids for cardiovascular risk reduction, the latter continue to outweigh the former.”
However, a few people will always experience uncommon reactions to any given drug or food factor. And with regard to blood chemistry, some people may have very rare adverse responses to omega-3s.
If you are taking fish oil and/or blood-thinning drugs (e.g., aspirin, coumadin, warfarin, clopidogrel) and notice any signs of abnormal bleeding, consult a physician immediately.