Can salmon studies shed light on human metabolism?

So concludes a new study into salmon cells, which shows that their fat tissue reacts to fasting in a very similar manner as the fat tissue in humans – making them a possible model for use in research into human metabolism and obesity.

Background

Over the course of three decades, the feed of farmed salmon has gone from consisting mostly of marine raw materials to consisting mostly of plant-based raw materials. According to a previous report, approximately 30 percent of the fat in salmon feed came from the ocean 20 years ago while only 10 percent was marine-based in 2016.

Scientists at Nofima were curious about what the change of diet does to the utilization of fat – and whether the fat source affects the salmon’s ability to mobilise energy from fat depots when it is denied access to food.

The scientists found out a lot about this by studying the reaction of cultured fat cells that were enriched with various fatty acids. A new method that they employed was to follow the secretion of lipids in the cells with radiolabelled trace elements.

“This is something that is not easy to study in fish that are alive, but we can conduct a more detailed study of the energy metabolism in cultured cells,” says Bente Ruyter, who leads the Nofima research group that has cultivated the cells.

Cell trials

Ruyte explains that the cells are isolated directly from live fish and retain much of their function, as opposed to cell lines held over several generations. They are demanding to work with, but they are more relevant regarding what is actually happening in the body. The scientists have mixed cells from 20 fish and placed the mixture into culture flasks. They have then studied what happens to the conversion of fat in the cells during feeding and fasting.

Fasting is common in wild salmon when they experience long periods without being able to catch food. In farmed salmon, fasting is common in connection with spawning, during illnesses and before they are slaughtered.

“During these periods, it is vital for salmon to regulate fat metabolism in the body well, which is why it is interesting to study how these processes change with altered diets,” says Ruyter.

As part of the trial, scientists enriched fat cells with selected fatty acids that are either more or less present in the feed compared to what was previously the case. The fatty acids with which the cultured cells were enriched were:

• The polyunsaturated marine fatty acid called EPA, which the feed contains less of than before.
• A typical plant-based fatty acid (oleic acid), which the feed contains more of than before.
• A saturated fatty acid (palm acid), which can be found in both fish oil and plant oil.

They discovered that during sexual maturation, when salmon need to transfer nutrients from fat tissue to reproductive cells, the fat is transported in the form of phospholipids, cholesterol esters and triglycerides in the bloodstream. Previously, it was thought that they were transported around as free fatty acids.

Recognition

The findings were singled for praise in the latest issue of the International Journal of Molecular Sciences (IJMS).

“In this journal, most research focuses on humans, but I think salmon is increasingly being considered as a possible model for humans. We have conducted basic research on fish that has never been done before, and the combination between adipose tissue models and an increased understanding of obesity-related issues most likely caught people’s eye,” says Nofima scientist Marta Bou Mira.

She is the primary author of the article, which she wrote with research colleagues at Nofima, Zhejiang University, the Oxford Centre for Diabetes and AquaGen.

Publication link: “Lipid Deposition and Mobilisation in Atlantic Salmon Adipocytes”.