Salmon is often used as an example of a health-conscious food choice, but its health value depends greatly on its source. While wild salmon is nutritious, there are many problems with farm-raised salmon, which makes up the bulk of salmon sold in U.S. supermarkets and served in restaurants.

A key part of that lies with their diet, which in the wild is made up of marine life, including zooplankton and other fish. In an attempt to simulate their wild diet, during the 1990s virtually all farmed salmon were fed diets rich in fishmeal and fish oil.¹ This wasn’t sustainable, however, and in an effort to create feed for farmed fish that didn’t involve overfishing, vegetable ingredients were added as a replacement.

While fishmeal and fish oil once made up about 90% of farmed Norwegian salmon feed, by 2013 this dropped to about 30%.² Other research suggests that by 2016, only 10% of the fat in farmed salmon feed was marine-based.³ High levels of vegetable oils, including rapeseed, or canola, are now used instead, which has had dramatic consequences for the salmon and, likely, for those who use them as a food source.

Farmed Salmon Feed Alters Cell Metabolism

Researchers from the Norwegian Institute of Food, Fisheries and Aquaculture (Nofima) conducted a study to find out how dietary changes affect the way salmon utilize fat, particularly during fasting.⁴ In the wild, salmon regularly experience periods of fasting when they’re unable to catch food. Farmed salmon may also fast, but only due to illnesses, spawning and prior to slaughter.⁵

“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,” study author Bente Ruyter of Nofima explained. “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.”⁶

Using primary fat cells isolated from 20 live fish, the researchers added fatty acids found in fish feed to determine their effects. The fatty acids included:

• Eicosapentaenoic (EPA), an omega-3 marine fat that used to be more plentiful in the feed than it is now
• Oleic acid (OA), which is found in vegetable oil and is added in higher quantities to fish feed than it was before
• Palm acid, a saturated fatty acid found in fish oil and plant oil, which has also decreased in farmed fish feed

 

Significant differences were seen in the cultured cells depending on fatty acid, including changes in the amount of mitochondria — the cells’ powerhouses — and alterations in the release of fatty acids during fasting.

The findings also suggested that oleic acid may promote overweight and obesity in Atlantic salmon more so than EPA and palm acid, with the researchers noting, “The supplementation of OA to mature Atlantic salmon adipocytes lead to a higher production of intracellular lipid droplets.”⁷

Dietary Changes Cause Significant Cellular Alterations

Changes in commercial farmed fish diets have led to significant reductions in EPA, the omega-3 fat DHA and palm acid in salmon’s adipose tissue, along with an increase in OA. These fats yield “very different and often opposing effects on central adipocyte functions,” the researchers explained, affecting adipose tissue metabolism and physiology via a number of mechanisms, including:⁸

• Modulating the transcript level of relevant genes
• Modifying lipolytic activity
• Modulating metabolic processes, such as lipid droplet formation, the leptin system and mitochondrial dynamics

 

It was formerly believed that salmon transport fat as free fatty acids, including during sexual maturation, when nutrients from fat tissue are transferred to reproductive cells. The study revealed, however, that the fat is transported on phospholipids, cholesterol esters and triglycerides.⁹

Correlations were also seen with humans, as the salmon fat cells reacted to fasting similarly to human fat tissue.

“Many of the regulatory mechanisms associated with energy metabolism when on a fatty diet appear to be similar to those found in humans,” according to Nofima.¹⁰ The researchers suggested that more research is needed to find out how the lipid composition of fat cells affects fish physiology and health, especially during fasting.

When a fish stops eating, the ability to recruit lipids from fat cells is essential for reproduction as well as recovery from disease, making the dietary changes potentially disastrous. What’s more, it could also serve as a warning for humans. According to Nofima scientist Marta Bou Mira:¹¹

“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.”