Don’t expect the fish and melanoma to be in the same headline, but they were last week. U.S. researchers reported an increased risk of developing melanoma, a common type of life-threatening skin cancer, in people who ate a relatively large amount of fish.
The researchers speculated that their results may be due to contaminant levels in some fish species, especially fatty fish. These contaminants include polychlorinated biphenyls (PCBs), synthetic chemical contaminants used as equipment coolants and lubricants, and as paint additives. PCBs are commonly found in the environment and can cause cancer in humans.
But a detailed view of the research shows that the findings do not necessarily mean that we should all cut fish from our diets for fear of skin cancer.
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Beyond the headline
The headline comes from a published study that tracked more than 490,000 adults in the U.S. for more than 15 years and checked the cancer registry databases to see how many melanomas occurred within that same group of adults. The researchers classified melanomas as “in situ” meaning on the surface of the skin or “malignant” meaning that they had spread more deeply.
They also asked study participants about the amount of fish they usually eat using a reliable food frequency questionnaire.
The people in the study reported how often they ate fish and the size of their portions of fried fish or fish sticks, uncooked fish or seafood such as dish, cod, prawns, clams, crabs or lobster. They also reported how much and how often they ate canned tuna, including tuna packaged with water and oil.
The average amount of fish eaten by those in the study ranged from 20 grams or less per week (equivalent to the size of half a box of matches) to about 300 grams per week.
Among the smallest fish eaters, there were 510 cases of melanoma in situ and 802 cases of malignant melanoma during the 15 years, compared with 729 and 1102, respectively, in the largest group of fish. This means that rates were 28% and 22% higher for both in situ melanoma and malignant melanoma for those who ate more fish compared to less.
In terms of specific types of fish, there was a higher rate of melanoma among people who ate more tuna and uncooked fish. Interestingly, there was no association with the intake of fried fish. Although it seems contrary to intuition, it is likely due to the very small intake of fried fish, which ranges from less than one to seven grams per day (equivalent to a full teaspoon).
Although the researchers adjusted their analyzes for factors that could affect outcomes, such as physical activity, smoking, a family history of cancer, and alcohol intake, the adjustment for daily exposure to UV was only based on the average UV index of the suburb where they lived. means that there were no adjustments for UV exposure related to a person’s occupation. They also lacked information on melanoma risk factors, such as polka dot count, hair color, a history of severe sunburn, or individual sun-related behaviors.
Observation is not causation
This study does not show that eating fish causes melanoma. This is because it is a “cohort study,” that is, people were observed over time to see if they were developing melanoma or not.
There was no intervention to feed them with specific amounts of fish, which in any case would not have been practical to do for 15 years. The researchers measured a number of behaviors at the start of the study (or “baseline”), such as dietary intake and physical activity levels. But those could have changed over time.
Therefore, the results are based on observation rather than cause and effect. This is not to say that observational results should be ignored, however.
Fish, especially fatty fish such as tuna, may contain contaminants such as mercury and PCBs. This could contribute to the findings that eating more fish is associated with a higher rate of both malignant melanoma and in situ melanoma (skin cancer).
PCBs are easily absorbed into the body, build up in fat stores, and remain there for years.
Contaminant levels in Australian fish are closely monitored. Unsplash / Tim Davies, CC BY
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The link between fish has been studied before
More attention should be paid to the role of contaminants that may be present in some fish. A 2017 study of more than 20,000 Swedish women assessed PCB exposure, potentially from fatty fish, and the incidence of melanoma.
After four and a half years of follow-up, the researchers reported four times the risk of malignant melanoma for women with the highest PCB exposure through their diet compared to the lowest.
However, this study also reported the intake of omega-3 fats found in fish and identified that among women with higher intake there was an 80% lower risk of melanoma, even after adjust dietary PCB exposure levels. This could explain why a systematic 2015 review of case-control and cohort studies found that higher fish intake appeared to protect people from malignant skin melanoma in some studies, but not all.
Food Standards Australia New Zealand (FSANZ) regularly monitors fish contaminants sold here. It places total levels of exposure to pollutants well below the permitted levels in Australia and Europe.
For many other health reasons, including reducing the risk of heart disease or death from all causes, we should continue to eat fatty fish from Australia such as salmon, tuna and sardines.
Eating fish regularly has many health benefits. Unsplash / Chuttersnap, CC BY
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Do not remove fish from the menu
Further studies are needed in other groups to assess PCBs and exposure to other pollutants, such as dioxins, arsenic, and mercury, while adjusting for individual factors such as sun exposure, skin type, and skin history. sunburn. This research could help to reinforce or refute the recently reported findings in the US.
Given the positive benefits of eating fish, including for heart health and nutritional value, my advice to Australians would be to eat fish caught in Australian or New Zealand waters, and follow the sun protection tips for minimize the risk of melanoma.
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