HAVBRUK2-Stort program for havbruksforskning

Cholesterol is essential in many physiological functions. It gives structure to cell membranes and is a precursor for bile acids, steroid hormones and vitamin D. Vertebrates, like humans and fish, produce their own cholesterol and do not depend on cholesterol from food. Phytosterol, the plant equivalent of cholesterol, is in humans known for its ability to reduce the uptake and production of cholesterol and thus lowering the plasma cholesterol. In Atlantic salmon, high phytosterol concentrations in the diets have been suggested to be associated with increased levels of fat in liver and plasma. Insect protein and fat are now becoming realistic ingredients in the diet of farmed Atlantic salmon. Insects cannot synthesize cholesterol and depend on a dietary supply of sterols. Some plant-eating insects do not have access to the animal-derived cholesterol, and have developed the ability to convert plant-derived phytosterol into cholesterol. A conversion of phytosterol into cholesterol could be beneficial for insects as raw material for fish feed and hence for the robustness and health of the salmon. In this post-doc project, the larvae of three different insect species (black soldier fly and two species of seaweed fly) were grown on marine media (seaweed). The sterol composition of the larvae was affected by the media they grew on and the insects showed little or no production of cholesterol. Especially the black soldier fly larvae showed a change in fatty acid composition when we changed their growth substrate. They had increased content of marine fatty acids when they grew on a marine substrate compared to a standard plant-based substrate. Direct effects of phytosterols on the liver of salmon was investigated using a cell model. Earlier work has shown that phytosterols from the diet accumulates in the liver, where it can affect gene expression. The Atlantic salmon liver cells showed only few and small effects on lipid accumulation when they were exposed to different phytosterols. One specific phytosterol, brassicasterol, affected the expression of important markers of lipid metabolism in the cells. This is especially interesting due to brassicasterol being a characteristic sterol of rapeseed oil, the most commonly used oil in feed for aquacultured Atlantic salmon. Zebra fish were used to look at effects of different phytosterols on the uptake of cholesterol. Isotope-labelled cholesterol was used to track the cholesterol from the feed to the tissues and the water. This way we can say something about the uptake and tissue accumulation of cholesterol in the presence of different phytosterols. We saw an effect of a pharmaceutical blocker of cholesterol (ezetimibe) but no clear effects of phytosterols were seen. The concentrations of phytosterols were selected to be relevant for modern aquaculture feeds. Fat and protein meal from the black soldier fly larvae were used in a feeding trial with fresh-water stage Atlantic salmon, making up as much as 65% of the feed. The insect used in the feeding trial had growen either on a standard vegetable media or on the same media with 60% content of the brown algae, Ascophyllum nodosum. High content of layric acid (12:0), a medium-chain fatty acid, in the feed with insect ingredients, led to an increase in the same fatty acid also in the fish. Especially the feeds where insect oil replaced rapeseed oil were lower in phytosterols than the other feeds, and the feeds with insect meal replacing fish meal were the lowest in cholesterol. No clear effects on health were seen due to the sterol composition of the feeds.

In salmon aquaculture, limitations on availability of sustainable feed ingredients is a major obstacle. AquaFly is a project where insects are proposed as a feed ingredient for Atlantic salmon. The current post doc would allow for a more thorough look at some of the lipid-related aspects of this project, including the metabolism of sterols and fatty acids in the insects as well as the effects of the insect-derived sterols on the Atlantic salmon. Insects produced in cooperation with the University of Stirling (kelp fly) and Protix Biosystem B.V. (black soldier fly) will be assessed for their ability to store and produce lipids required by Atlantic salmon. Little is known about the lipid metabolism and the retention of fatty acids and sterols in insects, and this post doc will help broaden the knowledge on how to use these insects as carriers of important lipids, such as saturated fats, marine n-3 fatty acids and cholesterol. For the assessment of metabolic effects of the insect-derived sterols on the Atlantic salmon, samples from a feeding trial using insect lipids will be used. To gain further knowledge on the details of the uptake and metabolism of these sterols, model systems involving zebrafish and salmon hepatocytes will be used.