Although it is well-established that the fatty acid profile of breast milk will reflect the dietary fatty acids, the response time with which this occurs is not known. We hypothesized that fatty acids from a given meal would be transferred acutely from chylomicrons into breast milk. To test this hypothesis, the following experiment was performed. Fourteen lactating women drank 700 Calorie breakfast formulas containing six different test fats: 40 grams of cocoa butter, coconut, safflower or canola oil, 20 grams of menhaden oil or 7 grams of herring oil. Each fat contained a specific fatty acid whose appearance was tracked in the milk. After consuming the breakfast formula, subjects collected mid-feeding milk samples at 0, 6, 10, 14 and 24 hours, and one morning sample on days two through seven. Fatty acids specifically tracked in milk samples were: C12:0 (coconut oil), C18:0 (cocoa butter), C18:2n-6 (safflower oil), C18:3n-3 (canola oil), C22:ln-ll (herring oil), and C20:5n-3 and C22:6n-3 (menhaden oil). There was a significant increase in each of these fatty acids in human milk (p<0.001). Elevation of these fatty acids was first observed at 6 hours. Maximum increases of these fatty acids occurred 10 h after safflower oil (177% of baseline), 14 hours after cocoa butter (154%), coconut oil (216%), canola oil (206%) and menhaden oil (C20:5n-3 [1157%]), and 24 hours after the herring oil (2621%) and menhaden oil (C22:6n-3 [506%]). Compared to baseline, these fatty acids were significantly elevated (p<0.05) from 10 to 24 hours. However, after menhaden oil, C20:5n-3 was significantly elevated for 3 days and C22:6n-3 for 2 days. These data support the hypothesis that there is an acute transfer of dietary fatty acids from chylomicrons into human milk. / Graduation date: 1996