The Mediterranean diet and other mostly plant-based diets that replace meat with fish for the sake of health may be creating an unwanted side effect. The knowledge that meat is linked to heart disease has created a generation that eats fish regularly, which doctors and nutritionists alike have applauded as a healthy choice.

Now, however, because of contaminants such as mercury and other pollutants that have made their way into the oceans' eco-systems, fish carries its own risk and a new study has linked regular fish consumption to an increased risk of malignant melanoma, a potentially fatal form of skin cancer. The risk of developing melanoma is 22 percent greater for those who eat the most fish in their diets, the study found.

Published in the medical journal Cancer Causes, the review study looked at the potential connection between fish consumption and risk of melanoma, using data from nearly one half a million adults. The study subjects were originally part of the Nation Cancer Institute’s NI-AARP Diet and Health Study that recruited people between 1995 and 1996. At the time, the participants’ average age was 62. The original cancer research study recorded the frequency the participants ate fried fish, tuna, and non-fried fish and how often they developed cancer.

Using this data, the research team recorded the instances of melanoma over 15 years by syncing the data with cancer registries. To properly assess the data, the researchers noted that the study accounted for physical activity levels, smoking history, family history, alcohol consumption, caffeine levels, ultraviolet radiation specific to location, and body mass indexes.

The study concluded that people who consumed an average of 1.5 ounces of fish a day (or 3 ounces every other day, which would be about the amount in a tuna salad sandwich) experienced a 22 percent higher risk of malignant melanoma – and 28 percent higher risk of developing abnormal cells in the outer skin layers – than those who didn't consume nearly as much fish (about one tenth of an ounce daily on average). The findings have serious implications for consumers who, like pescatarians, regularly eat fish for their health,

Human-Related Pollution Could Cause Fish to Become Cancerous

The study also examined the differences between the three categories of fish products that people consumed. Most significantly, those consuming 14.2 grams (0.5 ounces) of tuna showed a 20 percent higher risk of malignant melanoma and 17 percent higher risk of stage 0 melanoma than those who consumed 0.3 grams (0.01 ounces) of tuna.

Researchers also found that non-fried fish consumption substantially increased the risk of skin cancer. The study group with a median intake of 17.8 grams (0.62 ounces) showed an 18 percent higher risk of malignant melanoma and a 25 percent higher risk of stage 0 cancer. Overall, 5,034 participants from the original data pool developed malignant melanoma and 3,284 were afflicted by stage 0 cancers.

The study noted that non-fried fish and tuna consumption were linked to higher spikes in skin cancer risks. Although this study did not examine the exact reason why skin cancer and fish consumption were linked, the study author Eunyoung Cho speculates that human-related bio-contaminants could cause the fish to carry more carcinogens, presenting a higher risk to humans.

“We speculate that our findings could possibly be attributed to contaminants in fish, such as polychlorinated biphenyls, dioxins, arsenic, and mercury,” Cho, an associate professor of dermatology and epidemiology at Brown University, said in a statement. “Previous research has found that higher fish intake is associated with higher levels of these contaminants within the body and has identified associations between these contaminants and a higher risk of skin cancer.

“However, we note that our study did not investigate the concentrations of these contaminants in participants’ bodies and so further research is needed to confirm this relationship.”

Meat-Centric Diets Can Lead to Cancer

This study joins a growing body of research that links consumption of animal-based foods to higher risks of several types of cancers. This March, another study found that you can reduce your cancer risk by 14 percent by cutting meat out of your diet. The study found that even marginally lowering your meat consumption can significantly help curb your risk of cancer.

The World Health Organization has characterized red meat and processed meat as a class one carcinogen, which if consumed daily is as deadly to long-term health as smoking.

“This study adds to a growing body of research reinforcing the positive, protective effects of a vegetarian diet,” Chief Executive of the Vegetarian Society Richard McIlwain said at the time. “With cancer now affecting one in every two of us across the country, adopting a healthy vegetarian diet can clearly play a role in preventing this disease. Indeed, evidence from previous surveys suggests a balanced vegetarian diet can also reduce the risk of heart disease and diabetes, in addition to cancers.”

Bottom Line: Cutting Out Seafood Could Help Prevent Cancer

This new study links fish consumption with higher risks of developing skin cancers – malignant melanoma or stage 0 cancers. The study authors suggest that this correlation is likely due to the increase of bio-contaminants in the ocean. Luckily, the plant-based seafood market is expanding at unprecedented rates with some brands developing fully vegan salmon fillets. To start incorporating plant-based seafood, check out The Beet’s favorite vegan seafood products.

For more plant-based happenings, visit The Beet's News articles.

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