A new Nofima study of fat metabolism (also called lipid metabolism) in salmon cells shows what happens in the cells during fasting. The study shows that salmon fat cells release lipids in different ways based on the type of fat they have stored in their bodies.

Over the past three decades, the feed of farmed salmon has gone from consisting of marine to plant-based raw materials. According to a previous Nofima report, in the year 2000, approximately 30% of the fat in salmon feed came from the ocean, while in 2016, only ten percent was marine-based.

Scientists were wondering how the change of diet affects the utilization of fat. Does the fat source affect the salmon’s ability to mobilize energy from fat depots when it is denied access to food? 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 employed consists of following 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”, said Bente Ruyter, who leads the Nofima research group that has cultivated the cells.

Gaining a lot of knowledge through cell trials

Ruyter explained that the so-called primary cells are isolated directly from live fish. These cells retain much of the function from live fish, as opposed to cell lines held over several generations. They are demanding to work with, but they are more relevant regarding what is happening in the body. Scientists 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 during spawning, illnesses and before slaughter. “During these periods, it is vital for salmon to regulate fat metabolism in the body, so it is interesting to study how these processes change with altered diets”, said 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 EPA (feeds contain less amount now than before), oleic acid (a plant-based fatty acid that is in higher proportions in current feeds) and palm acid (that can be found in both fish oil and plant oil).

In this salmon fat cell (in principle similar to a human cell), the stores of fat are seen as small droplets inside the cell. Photo credits: Elisabeth Ytteborg/Nofima.

More similar to human metabolism than was first thought

An entirely new finding from the study is that salmon’s fat tissue reacts to fasting in the same manner as the human fat tissue. Prevalent types of fatty acid in the cells affect how much leptin hormone the cell secretes. This is something that scientists weren’t aware of until now. Leptin is a hormone that regulates appetite. 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.

The study has produced several findings:

• The types of fatty acids contained in the diet affect the number of mitochondria in the cells and the release of fatty acids during fasting.
• Many of the regulatory mechanisms associated with energy metabolism in a fatty diet appear to be similar to those found in humans.

Gained recognition

The scientific publication in which the findings are presented was highlighted as being the best 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,” said Nofima scientist, Marta Bou Mira.

Bou Mira is the primary author of the article and has written it together with research colleagues at Nofima, Zhejiang University in China, the Oxford Centre for Diabetes at the University of Oxford, UK, and AquaGen in Norway. Download the study here.