Eating fish has been linked to an increased risk of melanoma – but that doesn’t mean we should take it off the menu

You wouldn’t expect fish and melanoma to be in the same title – but they were last week. Researchers in the United States have reported a higher risk of developing melanomaa common type of life-threatening skin cancer, in people who ate a relatively large amount of fish.

The researchers speculated that their findings could be due to contaminant levels in certain species of fish, particularly oily fish. These contaminants include polychlorinated biphenyls (PCB) – synthetic chemical pollutants 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 review of the research shows the findings don’t necessarily mean we should all cut fish from our diets for fear of skin cancer.

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Beyond the title

The title comes from a published study who followed more than 490,000 adults in the United States for more than 15 years and checked cancer registry databases to see how many melanomas occurred in this same group of adults. Researchers classified melanomas as “in situ” meaning on the surface of the skin, or “malignant” meaning that they had spread deeper.

They also asked study participants how much fish they usually ate using a reliable food frequency questionnaire.

Study participants reported how often they ate fish and the size of their servings of fried fish or fish sticks, unfried fish or seafood such as flounder, cod, shrimp , clams, crabs or lobster. They also reported how much and how often they ate canned tuna, including tuna in water and oil.

The average amount of fish consumed by study participants ranged from 20 grams or less per week (equivalent to the size of half a matchbox) to around 300 grams per week.

Among the lowest fish eaters, there were 510 cases of in situ melanoma and 802 cases of malignant melanoma over the 15 years, compared to 729 and 1102 respectively in the highest fish consuming group. This means that the rates were 28% and 22% higher for in situ melanoma and malignant melanoma for those who ate the most fish compared to the least.

As for specific types of fish, there was a higher rate of melanoma in people who ate more tuna and unfried fish. Interestingly, there was no association with fried fish consumption. Although this seems counter-intuitive, this is likely due to the very low consumption of fried fish – ranging from less than one to seven grams per day (equivalent to one heaped teaspoon).

Although the researchers adjusted their analyzes for factors that could affect the results – such as physical activity, smoking, family history of cancer and alcohol consumption – the adjustment for daily exposure to UV was based solely on the average UV index of the suburbs in which they lived. means that there was no adjustment for UV exposure related to a person’s occupation. They also did not have information on melanoma risk factors such as number of moles, hair color, history of severe sunburn or individual sun-related behaviors.

Observation is not causation

This study does not prove that eating fish causes melanoma. It’s because it’s a “cohort studymeaning that people were observed over time to see whether or not they developed melanoma.

There was no intervention to feed them specific amounts of fish, which wouldn’t have been practical to do over 15 years anyway. The researchers measured a range of behaviors at the start of the study (or “baseline”), such as food intake and physical activity levels. But these may have changed over time.

Thus, results are based on observation rather than cause and effect. This does not mean, however, that observational results should be ignored.

Fish, especially oily fish like tuna, can contain contaminants such as mercury and PCBs. This could contribute to findings that eating more fish is associated with a higher rate of malignant melanoma and melanoma in situ (skin cancer).

PCBs are easily absorbed by the body, accumulate in fat stores and stay there for years.

Contaminant levels in Australian fish are closely monitored.
Unsplash/Tim Davies, CC BY

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The fish link has been studied before

The role of contaminants that may be present in some fish needs further investigation. A 2017 study on more than 20,000 Swedish women evaluated exposure to PCBs – potentially from oily 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 the women most exposed to PCBs via their diet compared to the lowest.

However, this study also reported intake of omega-3 fatty acids found in fish and identified that among women with the highest intake, the risk of melanoma was 80% lower, even after adjusting for levels. dietary exposure to PCBs. This could explain why a systematic review 2015 Case-control and cohort studies found that higher fish intakes seemed to protect people against malignant melanoma of the skin in some, but not all, studies.

Regular monitoring of contaminants in fish sold here is carried out by Food Standards Australia New Zealand (FSANZ). It places the levels of total exposure to contaminants well below Australian and European authorized levels.

For many other health reasons, including reducing your risk of heart disease or death from all causeswe should continue to eat Australian oily fish like salmon, tuna and sardines.

Eating fish regularly has many health benefits.
Unsplash/Chuttersnap, CC BY

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Don’t remove fish from the menu

Further studies in other groups are needed to assess PCBs and exposure to other contaminants including dioxins, arsenic and mercury, while also adjusting for individual factors such as sun exposure , skin type and history of sunburn. Such research could help strengthen or refute recently reported American findings.

Given the positive benefits of eating fish, especially for heart health and nutritional value, my advice to Australians would be to eat fish caught in Australian or New Zealand waters – and heed the protection advice against sun to minimize your melanoma risk.

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