Delay in primary haemostasis produced by a fish diet without change in local thromboxane A2

Twelve apparently healthy male volunteers were maintained for 6 weeks on a fish diet, the haemostatic effects of which were determined as standardized skin bleeding times; thromboxane A2 formation in vivo in the bleeding time blood and in vitro in clotting venous blood; platelet aggregability; and platelet phospholipids. Within 1 week the fish diet changed the fatty acid composition of platelet membrane phospholipids, with increases in omega-3 and decreases in omega-6 fatty acids; described platelet aggregability by ADP and collagen; but the bleeding time was unchanged. Not until 6 weeks on the diet did the bleeding time increase (by 37%: P less than 0.01) and it was still increased 3 weeks after the end of the diet, when the fatty acid composition of the platelets had returned to normal. The effect of the diet on decreased aggregability of platelets also failed to follow the changes in their fatty acid composition and in the bleeding time. Aggregation by ADP and by the highest dose of collagen was decreased throughout the diet and for several weeks thereafter, i.e. even when the fatty acid composition had reverted to the pre-dietary pattern and long after the normalization of the bleeding time. The diet caused no change in the in vivo appearance of thromboxane A2 in the bleeding time blood, contrasting with its effect in decreasing the in vitro formation of thromboxane A2 in clotting venous blood. These observations suggest that such fish diets do not delay haemostasis by diminishing the formation of thromboxane A2 locally nor directly by decreasing the aggregability of platelets or directly by the induced changes in their omega-3 fatty acids.