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Why Eating Farmed Salmon May Make You Fat

A study provides more evidence that a diet high in farmed salmon contaminated by persistent organic pollutants - POPs - contributes to weight gain and increases the risk of diabetes.

The results are consistent with a growing body of research on people, linking POPs exposure to type 2 diabetes. Mice fed contaminated salmon gained twice as much weight and developed more severe insulin resistance measures than mice that ate no salmon but the same amount of fat.

When the researchers fed the mice salmon whose POPs levels had been reduced by around 50 percent, the health effects were also reduced.

The researchers could not say whether these effects are due entirely to POPs or other factors inherent in the fatty fish diet. Further work should test whether a complete avoidance of POPs further reduces weight gain and insulin resistance.

According to another study funded by the Pew Charitable Trusts and published in the journal Science, eating more than a meal of farm-raised salmon a month, could also increase the risk of getting cancer later in life.

"We are certainly not telling people not to eat fish," said David Carpenter of the University at Albany, N.Y., "...We're telling them to eat less farmed salmon."

The average dioxin level for farm-raised salmon was 11 times higher than in wild salmon - 1.88 parts per billion compared with 0.17 ppb. For PCBs, the average was 36.6 ppb in farm-raised salmon and 4.75 in wild.

Animals absorb those pollutants through the environment, storing them in fat that people then eat. High levels are believed to increase the risk of certain cancers and, in pregnant or breast-feeding women, harm the developing brains of fetuses and infants.

What Did They Do?

In a new experiment using mice, scientists tested the effects of eating farmed salmon on weight gain and metabolism changes including insulin resistance, an indicator of diabetes. The researchers also tested whether POPs played a role in these factors.

The scientists fed groups of 8-week-old male mice different diets for eight weeks. These included a control diet, a very high-fat diet (72 percent fat) without farmed Atlantic salmon, a very high-fat diet with commercially-farmed Atlantic salmon and a lower fat/high carbohydrate Western-style diet (29 percent fat) with or without salmon.

Another group of mice was fed salmon whose POPs levels had been reduced by approximately 50 percent.

The researchers measured weight gain, body fat to track obesity and insulin resistance and glucose tolerance, both of which indicate increased risk of developing diabetes. They also measured levels of POPs in tissues and fat, including organochlorine pesticides, dioxins, furans and different types of polychlorinated biphenyls.

What Did They Find?

They found the mice fed the high-fat diet with farmed salmon gained roughly twice as much weight as mice fed the high-fat salmon-free diet, even though the diets contained the same total amount of fat. A big difference is the type of fat in each diet. The salmon diet had lower omega-6 fatty acids and higher omega-3 fatty acids than the diets without salmon.

The higher weight gain was due to greater absorption of fat into the body.

The high-fat, contaminated-salmon diet also worsened insulin resistance and glucose intolerance, indicating a diabetic state in the mice.

Interestingly, the authors found similar patterns when they tested the effects of salmon in the lower-fat, Western diet. Though the effects were smaller, the mice fed salmon gained more weight and had increased insulin resistance.

Finally, mice fed the salmon diet with reduced POPs had slightly less weight gain, lower body fat and decreased indicators of type 2 diabetes compared to mice fed salmon with higher POPs levels.

What Does it Mean?

The study with mice provides more evidence that eating a prolonged diet with contaminated fatty fish may increase obesity and raise the risk of insulin resistence that may lead to diabetes. While the exact reasons for the changes aren't clear, pollutants in the fish contributed some to the increased risk of metabolic disease seen in the study, the researchers report.

The results show that eating the POPs-contaminated farmed salmon led to greater weight gain and higher predisposition to type 2 diabetes compared to a salmon-free diet with the same amount of total fat.

The findings raise a number of questions, including whether this pattern is true for all fish -- farmed or not -- or seafood in general and if it carries over to other fatty, foods, such as meat and dairy, which may also contain POPs. The researchers could not say whether these effects are due entirely to POPs or other factors inherent in the fish diet. Further research should test whether a complete avoidance of POPs further reduces weight gain and insulin resistance.

Prior research has tried to understand the influence of fish intake on type 2 diabetes and other metabolic diseases. Yet, the interplay between the two remains ambiguous. Overall, recent long-term human studies from the United States and Europe suggest a connection.

This experimental study is one of the first to test the idea in animals. The study supports those past human studies because it finds similar results.

Other lab and animal studies by the same research group have found POPs can interfere with the beneficial effects of salmon oil and impair insulin actions. This study suggests the same but provides a new twist. It is unique because whole fish fillets -- not just fish oil alone -- were used.

POPs levels in the fish's fatty tissues were similar to what has been measured in the people. This agrees with past research showing that environmentally relevant levels of one type of POPs -- polycyclic byphenyls (PCBs) -- can affect insulin resistance.

Since salmon are a complex mix of many different nutrients and contaminants, it will require more intensive study to uncover the mechanism behind these patterns. For right now, the researchers suggest that limiting "daily and long-term exposure to POPs may therefore represent a novel and attractive approach to slow down the uncontrolled rise of metabolic diseases."

April McCarthy is a community journalist playing an active role reporting and analyzing world events to advance our health and eco-friendly initiatives.

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