Eating fish has been linked to an increase in melanoma risk – but that doesn't mean we should take it off the menu

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

The researchers speculated their results may be due to 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 commonly found in the environment and can cause cancer in humans.

But a detailed look at the research shows the findings don’t necessarily mean we should all be cutting fish from our diets for fear of skin cancer.

Read more: The doctor says my mole is a melanoma. What happens next?

Beyond the headline

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

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

People in the study reported how often they ate fish and their portion sizes of fried fish or fish sticks, 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 both water- and oil-packed tuna.

The average amount of fish those in the study ate varied from 20 grams or less per week (equivalent to the size of half a matchbox) up to about 300 grams a week.

Among the lowest fish eaters there were 510 cases of in-situ and 802 cases of malignant melanoma over the 15 years compared to 729 and 1102 respectively in the highest fish-eating group. This means 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 melanoma rate among people who ate more tuna and non-fried fish. Interestingly, there was no association with fried fish intake. While that appears counter-intuitive, it is likely due to the very small intakes of fried fish – ranging from less than one up to seven grams a day (equivalent to one heaped teaspoon).

Although the researchers adjusted their analyses for factors that could affect results – such as physical activity, smoking, family history of cancer and alcohol intake – the adjustment for daily UV exposure was only based on the average UV index for the suburb they lived in. This means there was no adjustment for UV exposure related to a person’s occupation. They also did not have information on the melanoma risk factors such mole count, hair colour, history of severe sunburn or individual sun-related behaviours.

Observation is not causation

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

There was no intervention to feed them specific amounts of fish, which would not have been practical to do over 15 years anyway. Researchers did measure a range of behaviours at the beginning of the study (or “baseline”), such as dietary intake and physical activity levels. But these could have changed over time.

So the results are based on observation rather than cause and effect. This doesn’t mean observational results should be ignored, though.

Fish, especially fatty fish such as 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 readily absorbed into the body, accumulating in fat stores and staying there for years.