You wouldn’t expect fish and melanoma to be in the same cup, but they were last week. Researchers in the United States reported a higher risk of developing melanomaa common type of potentially fatal skin cancer, in people who ate a relatively large amount of fish.
The researchers speculated that their results may be due to the levels of contaminants in some fish species, especially fatty fish. These contaminants include: polychlorinated biphenyls (PCBs) – synthetic chemical pollutants used as equipment coolants and lubricants and as paint additives. PCBs are common in the environment and can cause cancer in humans.
But a detailed look at the study shows that the findings don’t necessarily mean we should all cut fish from our diets for fear of skin cancer.
Beyond the head
The head comes from a published study that followed more than 490,000 adults in the US for more than 15 years, checking cancer registry databases to see how many melanomas were occurring within that same group of adults. Researchers classified melanomas like ‘in-situ’, meaning on the surface of the skin, or ‘malignant’, meaning they had spread more deeply.
They also asked study participants how much fish they usually ate with a trustworthy food frequency questionnaire.
People in the study reported how often they ate fish and their portion sizes of fried fish or fish fingers, non-fried 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 in oil.
The average amount of fish eaten in the study ranged from 20 grams or less per week (equivalent to the size of half a matchbox) to about 300 grams per week.
Among the lowest fish eaters, there were 510 cases of in-situ and 802 cases of malignant melanoma during the 15 years, compared with 729 and 1102, respectively, in the highest fish-eating group. This means that the rates were 28% and 22% higher for both melanoma in situ and malignant melanoma for those who ate the most fish compared to the least.
Looking at specific types of fish, there was a higher rate of melanoma in people who ate more tuna and non-fried fish. Interestingly, there was no association with the intake of fried fish. While that may seem counterintuitive, it’s likely due to the very small intake 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 influence the results — such as physical activity, smoking, family history of cancer and alcohol use — the adjustment for daily UV exposure was based only on the average UV index for the suburb in which they lived. means there was no adjustment for UV exposure related to a person’s occupation. They also had no information about the risk factors for melanoma, such as the number of moles, hair color, a history of severe sunburn, or individual sun-related behaviors.
Observation is not a causal relationship
This study does not prove that eating fish causes melanoma. This is because it is a “cohort study”, meaning that people were observed over time to see if they developed melanoma or not.
There was no intervention to feed them specific amounts of fish, which wouldn’t have been practical 15 years from now anyway. Researchers measured a range of behaviors at the start of the study (or “baseline”), such as dietary intake and physical activity levels. But these may have changed over time.
The results are thus based on observation rather than on cause and effect. However, this does not mean that observation results should be ignored.
Fish, especially fatty fish like tuna, can 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 melanoma in situ (skin cancer).
PCBs are easily absorbed into the body, accumulate in fat storage and remain there for years.
The fish link has been studied before
The role of contaminants that may be present in some fish needs further consideration. A 2017 study by more than 20,000 Swedish women evaluated PCB exposure – possibly from fatty fish – and the incidence of melanoma.
After four and a half years of follow-up, researchers reported four times the risk of malignant melanoma for those women with the highest PCB exposure through their diet compared to the lowest.
However, this study also reported the intake of omega-3 fats in fish and identified that in the women with the highest intakes, there was an 80% lower risk of melanoma, even after adjusting for levels of dietary PCB exposure. . This could explain why a 2015 systematic review case-control and cohort studies found that higher fish intake appeared 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 reflects the total exposure to pollutants far below Australian and European allowable levels.
For many other health reasons, including lowering your risk of heart disease or death from all causeswe should continue to eat Australian fatty fish such as salmon, tuna and sardines.
Don’t take fish off the menu
Further studies in other groups are needed to evaluate PCBs and exposure to other contaminants, including dioxins, arsenic, and mercury, while also adjusting for individual factors such as sun exposure, skin type, and sunburn history. Such research could help reinforce or refute the newly reported US findings.
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 to follow the advice that is safe from the sun to your risk of melanoma†
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