Dietary lipid content influences the activity of lipogenic enzymes in the liver and on whole body δ¹³c values of Nile tilapia, Oreochromis niloticus (L.)

Abstract

The use of stable isotope techniques for the reconstruction of diets has increased over the last decade. However, isotopic ratios in an animal are not only affected by the composition of the feed, but also by the amount of feed consumed. An uncertainty of up to 1 ‰ for both δ¹³C and δ¹⁵N values has been observed when the feeding level is unknown. This may have substantial effects on the results of back-calculation. As the feeding level of animals is unknown in nature, an additional indicator for their nutritional status is needed. High feeding levels and a consequent surfeit of dietary energy lead to the synthesis of lipids. In order to test whether the level of lipogenesis could be used as an indicator, Nile tilapia (Oreochromis niloticus) were fed four isonitrogenous and isoenergetic wheat-based semi-synthetic diets with different lipid contents (2.0 %, 4.5 %, 9.5 % and 13.3 %) for eight weeks. Body composition, gross energy content and δ¹³C values in the lipids and the lipid-free material were determined in diets and fish bodies. The livers of three fish per feeding group were assayed for the activity of two lipogenic enzymes, ATP-citrate lyase and malic enzyme.

There was a strong negative correlation between δ¹³C values in the lipids of the individual fish and the apparent lipid conversion. The activities of lipogenic enzymes decreased with rising lipid content in the diet. The δ¹³C values in the lipids decreased significantly with increasing specific activity for both enzymes. In this experiment where lipogenesis was influenced by the composition of the diet, it was possible to determine the exact value for the trophic shift in relation to the enzyme activities. Further experiments to investigate the use of enzyme activities in situations where the feeding level of an animal is unknown are recommended.

Revised version of a paper presented at the 26^th Annual Meeting of the German Association for Stable Isotope Research (GASIR) October, 6 to 8, 2003, Cologne, Germany

Keywords:

• Carbon-13
• Diet
• Enzyme activities
• Fish
• Lipogenesis
• Natural Variations
• Trophic shift

Acknowledgements

This study was partly funded by DFG grant to Dr Ulfert Focken (FO 267/8–1). The authors wish to thank R. Langel, Georg August University of Göttingen, for analysis of the isotopic ratios, Dr Hartmut Richter for critical reading of the manuscript and Beatrix Fischer for support in the laboratory. Special thanks to Dr Borrebaek for provision of protocols for enzyme assays.

Notes

Revised version of a paper presented at the 26^th Annual Meeting of the German Association for Stable Isotope Research (GASIR) October, 6 to 8, 2003, Cologne, Germany