Eating fish linked to skin cancer risk in new study – but seafood itself may not be the issue

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  • Increased consumption of fish has been linked to a higher risk of skin cancer, an observational study has found.
  • However, the issue may be with toxins in fish, rather than the fish itself.
  • The experts behind the study don't recommend people change their fish-eating habits until further investigation is completed.

Around three billion people across the globe rely on seafood as a primary source of protein. But the findings of a large, new study are certainly food for thought: eating high amounts of tuna and other non-fried fish could put you at risk for melanoma, a serious and common form of skin cancer. 

"Melanoma is the fifth most common cancer in the [US] and the risk of developing melanoma over a lifetime is one in 38 for White people, one in 1 000 for Black people, and one in 167 for Hispanic people," co-author of the study and Brown University dermatologist and epidemiologist, Dr Eunyoung Cho said in a news release.

The long-term study followed nearly half a million people who were recruited between 1995 and 1996 (average 62 years old) to examine the relationship between fish intake and melanoma risk. Participants reported how frequently they ate fried fish, non-fried fish, and tuna during the previous year, as well as their portion sizes.

The role of toxins 

The team then calculated the incidence of new melanomas that developed over a median period of 15 years. The risk of melanoma was 22% higher in those who ate around 43 grams of fish a day compared to those who ate the median amount (around three grams per day).

(A serving size of cooked fish is approximately 140 to 170 grams, while a can of tuna is 142 grams.)

But the researchers suspect this link may be due to toxins in the fish, rather than the fish itself. Said Cho:

We speculate that our findings could possibly be attributed to contaminants in fish, such as polychlorinated biphenyls, dioxins, arsenic and mercury.

The nature of the study was observational, which means the results reflect a trend and not a cause. Still, they cannot be ignored, said University of Newcastle dietitian Clare Collins, who was not involved in the study. "The role of contaminants that may be present in some fish needs to be considered."

What's in our fish?

Plenty of existing studies have already determined that many toxins in our environment can cause cancer and build up through the food web. For example, arsenic, cadmium, lead, and mercury in large doses can have negative effects in humans, such as liver disease and kidney problems, explains the US Environmental Protection Agency (EPA). 

The agency adds that some metals can bioaccumulate in organisms and even magnify in the food web, such as mercury. Many industries play a role in metal extraction and releasing it into the environment, including the construction, agricultural, mining, and chemical industries.

Netflix’s Seaspiracy documentary also warned that the flesh of fish contains high levels of contamination, such as toxic heavy metals, dioxins, and plastic compounds.

"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," said Cho. 

No need to change habits

Cho studies the connection between diet and skin cancer and has been involved with previous research showing a link between higher mercury levels and skin cancer.

“Mercury consumption in the US is mostly from fish,” she said. “So if mercury is related to skin cancer, then it stands to reason that fish intake may be related, too.”

However, she noted that their study didn’t investigate the concentrations of these contaminants in participants’ bodies, so further research will be needed to confirm this relationship. Until then, they do not recommend people make any changes to their fish consumption habits.

Cho told The New York Times that she “wouldn’t discourage people from eating fish just because of [their] finding”, and mentioned that fish consumption is associated with a lower risk of cardiovascular disease and perhaps even other cancers.

One should also bear in mind that the single biggest cause of skin cancer is over-exposure to the sun’s harmful ultraviolet (UV) rays.

The study appears in Cancer Causes & Control.

READ | Want to live a long, healthy life? Eat more nuts, whole grains, and legumes – and less red meat

READ | Sunscreen's secret bonus: It could help keep you cool

READ | Three ways to prevent skin cancer during winter

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