Don’t expect fish and skin cancer to be at the same address – but they were last week. Researchers in the United States have reported a higher risk of melanoma, a common type of fatal skin cancer, in people who eat a relatively large amount of fish.
The researchers speculated that their findings may be due to the levels of pollutants in some types of fish – particularly fatty fish. These contaminants include polychlorinated biphenyls (PCBs) – synthetic chemical pollutants used as equipment refrigerants, lubricants, and as paint additives. PCBs are commonly found in the environment and can cause cancer in humans.
But a detailed look at the research shows that the findings don’t necessarily mean we should all eliminate fish from our diets for fear of developing skin cancer.
Read more: Doctor says mole is skin cancer. what happened after that?
Beyond the title
The headline comes from a published study that followed more than 490,000 adults in the United States for more than 15 years and examined cancer registry databases for how many melanomas occurred in the same group of adults. Researchers have classified melanomas as meaning “in situ” on the surface of the skin, or “malignant” meaning that they have spread deeper.
They also asked study participants how much fish they typically ate using a trusted food frequency questionnaire.
The subjects in the study reported how often they ate fish and their serving sizes of fried fish or fish sticks, and non-fried fish or seafood such as flounder, cod, shrimp, oysters, crabs, and crabs. They also reported how much and how often they ate canned tuna, including tuna packed in water and oil.
The average amount of fish eaten by study participants ranged from 20 grams or less per week (the size of half a matchbox) to about 300 grams per week.
Among the lowest fish eaters, there were 510 in situ and 802 cases of malignant melanoma over 15 years compared to 729 and 1,102, respectively, in the highest fish-eating group. This means that rates were 28% and 22% higher for both melanoma in situ and malignant melanoma for those who ate the most fish compared to the others.
Looking at certain types of fish, there was a higher rate of skin cancer among people who ate more tuna and non-fried fish. Interestingly, there was no association with eating fried fish. While this sounds counter-intuitive, it’s likely due to eating very small fried fish – ranging from less than one to seven grams per day (the equivalent of one full 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 intake — the adjustment for daily UV exposure was based only on the average UV index of the suburb in which they lived. It means that there was no adjustment for UV exposure associated with a person’s occupation. They also had no information about skin cancer risk factors such as the number of moles, hair color, history of severe sunburn, or individual behaviors related to the sun.
Observation is not causation
This study does not prove that eating fish causes skin cancer. This is because it is a “cohort study,” which means that people are observed over time to see if they have developed skin cancer.
There was no intervention to feed them specific amounts of fish, which was not practical for 15 years anyway. The researchers measured a range of behaviors at the start of the study (or “baseline”), such as dietary intake and levels of physical activity. But these things can change over time.
So the results are based on observation and not on cause and effect. This does not mean that monitoring results should be ignored.
Fish, especially fatty 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 both malignant melanoma and melanoma in situ (melanoma).
PCBs are easily absorbed into the body, accumulate in fat stores and remain there for years.
Read more: 80% of cancers affect the skin. What happens if I have one?
The association of fish has been studied by
The role of contaminants that may be present in some fish needs further consideration. A 2017 study of more than 20,000 Swedish women evaluated exposure to PCBs – likely from fatty fish – and incidences of skin cancer.
After four and a half years of follow-up, the researchers reported a four times higher risk of malignant melanoma in women exposed to higher PCBs through their diet than in lower women.
However, this study also reported the intake of omega-3 fats found in fish, and determined that among the women with the highest intake, there was an 80% lower risk of developing skin cancer, even after adjusting for PCB exposure levels. This could explain why a 2015 systematic review of case and cohort studies found that a higher intake of fish appeared to protect people from malignant melanoma 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 puts overall exposure levels to pollutants well below the levels allowed in Australia and Europe.
For many other health reasons, including a reduced risk of heart disease or death from all causes, we should continue to eat Australian fatty fish like salmon, tuna and sardines.
Read more: 4 plant-based foods to eat every week (and why the science says they’re good for you)
Don’t take fish off the menu
Further studies in other groups are needed to assess PCBs and exposure to other pollutants including dioxins, arsenic and mercury, while adjusting for individual factors such as sun exposure, skin type and sunburn history. Research such as this could help strengthen or refute the newly reported US findings.
Given the positive benefits of eating fish, including heart health and nutritional value, my advice to Australians is to eat fish caught in Australian or New Zealand waters – and heed the sun-safe advice to reduce your risk of skin cancer.
Read more: Does eating dairy products increase the risk of prostate cancer?