Farmed salmon, mice, diabetes and obesity

Two recent studies show that lab mice are at a higher risk of developing insulin resistance and obesity when consuming high quantities of farmed salmon high in Persistent Organic Pollutants, and at a higher risk of developing insulin resistance and fatty livers when consuming farmed salmon raised on a soybean oil-rich diet.

A while ago a reader forwarded us an interesting study for our library titled “Chronic Consumption of Farmed Salmon Containing Persistent Organic Pollutants Causes Insulin Resistance and Obesity in Mice” (2011).

Just a few days ago, some of the same authors published a follow-up study titled “Intake of Farmed Atlantic Salmon Fed Soybean Oil Increases Insulin Resistance and Hepatic Lipid Accumulation in Mice” (2013),  prompting us to take a look at the two studies together in a post. Both are now available in our library.

The research is very interesting.

POPs and fat mice

In the first study, mice were fed a variety of diets including “Very High Fat” and “Western Diet,” with salmon and without. The mice used were a strain highly susceptible to diet-induced obesity and type-2 diabetes.

The mice fed a very high fat diet and a western diet including salmon had higher levels of insulin resistance, obesity and glucose intolerance than mice eating diets without salmon.

Of the mice eating salmon in their diets, mice eating salmon with no Persistent Organic Pollutants had better insulin sensitivity and glucose tolerance than mice eating salmon with trace amounts of POPs. They also did not gain as much weight as mice eating salmon with trace amounts of POPs.

Soybean oil and fatty livers

The second study, published a couple years later by some of the same authors, looked at the results in mice fed salmon raised on different diets. The mice were the same strain as the previous study, highly susceptible to diet-induced obesity and type-2 diabetes.

It fed mice salmon raised on a feed with most of the fish oil removed, and replaced with either soybean oil, olive oil or rapeseed (canola) oil. There were a variety of different effects.

Overall, replacing fish oil (the main source of omega-3 fatty acids in salmon feed, which is then passed on to people who eat the salmon) with vegetable oil decreased the omega-3 content and increased the levels of omega-6 fatty acids. However, researchers observed that replacing fish oil with vegetable oil in the salmon feed did not contribute to obesity development in mice eating the salmon raised on that feed.

As well, mice eating salmon grown on a diet including rapeseed oil had improved glucose tolerance. Mice eating salmon grown on a diet including soybean oil had exaggerated insulin resistance and fatty livers, linked to the high levels of linoleic acid in soybean oil.

So what?

These two studies perfectly illustrate the challenges facing salmon feed manufacturers. Salmon feed has traditionally used a lot of fish meal and fish oil, rendered from small “forage” fish such as anchoveta and sardines. However, as aquaculture grows, there are only so many wild fish in the ocean. Salmon farmers need to find alternative sources of the oils that make salmon rich in omega-3 fatty acids.

On one hand, the fish oils are still the best source of omega-3s. On the other, they may be contaminated with POPs and may pose hitherto-unknown health risks to human beings, if the first study is accurate. They are also finite. A growing aquaculture industry worldwide needs alternative omega-3 sources. So far, the ones we’ve got might not be ideal.

But research continues. Farmers continue to reduce their feed conversion ratios, using just slightly more than one kilo of wild fish to grow one kilo of farmed salmon, which has a healthy 2:1 omega-3 to omega-6 ratio and is low in POPs.

That’s right, Pinky. Farmed salmon is still safe to eat… unless you’re one of those CS7BL/6J laboratory mice.

And yes, despite what some might glean from these studies, farmed salmon is still safe and healthy to eat, unless you are an inbred, genetically-modified laboratory mouse highly susceptible to developing tumours and diabetes from just about everything they eat.

These studies make no claims about human health, but do suggest they could help improve nutritional strategies for the prevention and therapy of insulin resistance. They also suggest that the high levels of linoleic acid in vegetable oils may be a matter of concern warranting investigation.

This is all good stuff.

Through more research, and studies such as these two, we will learn how to improve our diet as human beings, and how to farm even more sustainably in the future.

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