Are modifications in tissue fatty acid profiles following a change in diet the result of dilution? Test of a simple dilution model

The fatty acid compositions of fish tissue lipids usually reflect those of the dietary lipids and tissue fatty acid profiles can be modified by altering the sources of fats and oils used in formulating fish feeds. Few attempts have been made to predict the way in which the profiles change, and the time required for the fatty acid profile to stabilise following a dietary change. The simplest model to describe the change in fatty acid composition is one in which the initial fatty acid content becomes diluted as the fish grow and deposit increasing amounts of lipid. The dilution model was tested using data relating to the influence of feed oils (vegetable vs. marine fish) and dietary fat concentrations on growth, body composition, and fatty acid profiles of the tissues (fillet, viscera and carcass) of Atlantic salmon parr and post-smolt. Fish fed feeds containing vegetable (rapeseed and linseed 7:3 v/ v) oils at high (34%) or low (22%) lipid concentration during freshwater rearing (mass increase from ca. 19 to 130 g) were switched to feeds containing fish (sand-eel) oil (31% or 18.5% lipid) following parr-smolt transformation. The changes in the proportions of the three tested fatty acids (18:1 isomers, 18:2n-6 and 18:3n-3) in the total lipids of the salmon tissues during 98 days of on-growing in seawater (mass increase from ca. 130 to 380 g) conformed closely to predictions made on the basis of the simple dilution model. Practical applications, and possible limitations, of the dilution model are discussed. (C) 2004 Elsevier B.V. All rights reserved.