The incorporation of n-3 and n-6 essential fatty acids into the chick embryo from egg yolks having vastly different fatty acid compositions

The effect of egg yolk fatty acid composition on essential fatty acid utilization by the developing chick embryo was studied by feeding laying hens a fat-free diet supplemented with oils containing widely divergent contents of the essential n-6 and n-3 fatty acids. A control hen was fed a commercial feed for laying hens. The diets contained 20 to 4370 mg/100 g n-3 fatty acids and 360 to 8020 mg/100 g n-6 fatty acids. Fertile eggs were collected in pairs: one was incubated and the other served as an unincubated control. The fatty acid content of the unincu-bated egg and the newly hatched chick from each pair was compared. Some 50% of the total fatty acids in the egg yolk were incorporated into the tissues of the newly hatched chick. Regardless of diet, more yolk n-6 fatty acids were incorporated into the chick compared to saturated or monounsaturated fatty acids. The percentage of incorporation especially increased from the eggs containing relatively low amounts of n-6 fatty acids. The percentage of incorporation of n-3 fatty acids was similar to that of saturated and monounsaturated fatty acids when n-3 fatty acids were plentiful in the egg yolk, but increased significantly when n-3 fatty acids were low in the eggs. There was a generally linear relationship between essential fatty acids in the egg and in the chick, although levels of docosahexaenoic acid [DHA; 22:6(n-3)] in the brain did not respond proportionally. The developing chick preferentially removed DHA from the yolk, but did not synthesize more DHA when the amount of the DHA precursor, 18:3(n-3), in the yolk was increased. We concluded that the developing chick embryo requires 0.4-1.1% of egg energy as n-3 fatty acids and 4.8-6.2% as n-6 fatty acids, or a “dietary” ratio of n-6/n-3 of 5 to 14. This requirement may have relevance for humans as well.