The aim of the study was to investigate whether it was possible a) to increase the relative incorporation of n - 3 very long chain polyunsaturated fatty acids (VLCPUFA) in a low VLCPUFA diet by feeding trout structured triacylglycerols and b) to reduce fat accumulation by feeding trout a diet containing DAG. A feeding experiment where groups of rainbow trout were fed six diets containing different types of oils for 61 days was performed. The lipid fraction of the six diets was as follows: 1) Fish oil and rapeseed oil (FO diet), 2) Specific structured lipid and rapeseed oil (SL diet), 3) Randomised structured lipids and rapeseed oil (RL diet), 4) Medium chain triglyceride and fish oil (MCT diet), 5) Diacylglycerol and fish oil (DAG diet), 6) Fish oil (FOmax diet). Five of the diets (1-5) contained mixed oils blended to contain the same amount of EPA and DHA. Three of these diets (2,3 and 4) contained medium chain fatty acids incorporated in TAG to be positioned either intentionally as specific (SL), by chance as randomised (RL) or added as medium chain TAG (MCT). Diet 1 contained fish oil (FO) in order to investigate the effect of MCFA and diet 4 contained diacylglycerol (DAG). Diet 6) was a reference diet containing pure fish oil (FOmax). After the feeding period, FO and FOmax fed fish were significantly larger than SL fed fish. Digestibility, measured by adding yttrium oxide as inert marker was significantly lowest for the MCT diet. Fish on the RL and MCT diets had significantly higher protein contents than fish fed FO and FOmax diets. The total fatty acid compositions of the fillet, liver, carcass and viscera were similar and reflected closely that of the diet, also in the sn-2 position. In conclusion, addition of MCT to the diet increased protein content of the fish. There was no additional effect of incorporating the medium chain fatty acids in specific positions and no weight reducing effect of adding DAG to the diets. (C) 2005 Elsevier B.V. All rights reserved.