this review. For an actual study, see Watson et al. (2009), American Journal of Cardiology.
Frankly, if fish oil didn't reduce platelet aggregation, you probably wouldn't get the heart-protective effects. There are two situations, though, in which the blood-thinning effects of omega-3 fatty acids can be important. One is if you are deficient in vitamin K1, a vitamin crucial to forming blood clotting protein. Omega-3 fatty acids in the presence of vitamin K1 deficiency can cause an excess risk of bleeding. (So by all means, eat plenty of K1-rich foods like broccoli, lettuces, green peppers, and other green-colored foods.) The second situation is the intentional vitamin K1 "deficiency" induced by drugs like Coumadin® (warfarin). Omega-3 fatty acids modestly reduce production of several clotting proteins (e.g., factors VII and X) that overlap with Coumadin's action, so blood may thin more with omerga-3s than without omega-3s.
The bottom line is that if you are taking Plavix, Coumadin, or any similar medication, you should consult with your doctor on the use of fish oil; it's possible you may need to take less of the prescription drugs while taking fish oil.
For most of us, the clotting-reduction effects of fish oils are a benefit, rather than a risk. It helps offset the clot-promoting effects of smoking, physical inactivity, fast foods, etc. (And yes, you should definitely modify your lifestyle appropriately, but until then, a little fish oil can't hurt.) As one doctor puts it:
Rather than causing blood thinning, I prefer to think that omega-3 fatty acids from fish oil restore protection from abnormal clotting. Taking omega-3 fatty acids from fish oil simply restores a normal level of omega-3 fatty acids in the blood sufficient to strike a healthy balance between blood "thinning" and healthy blood clotting.
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