Infants with birth weight < 1.5 kg are randomized to either an intervention or a control group. All infants receive fortified human milk, and a daily dose of 0.5 ml study oil per 100 ml milk. Infants in the intervention group receive oil with DHA and AA (Formulaid, Martek, USA), while the control oil contains vegetable oil without DHA or AA. Blood samples are collected at birth (cord), and at start and stop of the intervention. Plasma is analyzed for fatty acid pattern using high performance liquid chromatography.