EXPERT REACTION: Eating more fish may be linked to higher melanoma risk

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Peer-reviewed: This work was reviewed and scrutinised by relevant independent experts.

Observational study: A study in which the subject is observed to see if there is a relationship between two or more things (eg: the consumption of diet drinks and obesity). Observational studies cannot prove that one thing causes another, only that they are linked.

People: This is a study based on research using people.

Eating more fish may be linked to a greater risk of malignant melanoma, suggests a large study of US adults. The study found that people who ate an amount of fish equivalent to about half a can of tuna a day had a risk of melanoma that was 22% higher than people who ate the equivalent of a can of tuna a month. Interestingly they also found a link to melanoma risk in people who ate more non-fried fish and tuna, but not in those who ate more fried fish.  The study can't show that eating fish in any way causes melanoma, although the researchers suggest that contaminants such as mercury in fish might explain the link. The researchers also did not account for some risk factors for melanoma, such as mole count, hair colour, history of severe sunburn and sun-related behaviours in their analyses. 

Journal/conference: Cancer Causes & Control

Link to research (DOI): 10.1007/s10552-022-01588-5

Organisation/s: Brown University, USA

Funder: This work was supported in part by the Intramural Research Program of the US National Institutes of Health (NIH). The authors have not disclosed any competing interests National Cancer Institute.

Media release

From: Springer Nature

1.  Health: Higher fish consumption may be associated with increased melanoma risk

Eating higher levels of fish, including tuna and non-fried fish, appears to be associated with a greater risk of malignant melanoma, suggests a large study of US adults published in Cancer Causes & Control.

Eunyoung Cho, the corresponding author said: “Melanoma is the fifth most common cancer in the USA and the risk of developing melanoma over a lifetime is one in 38 for white people, one in 1,000 for Black people and one in 167 for Hispanic people1. Although fish intake has increased in the USA and Europe in recent decades, the results of previous studies investigating associations between fish intake and melanoma risk have been inconsistent. Our findings have identified an association that requires further investigation.”

Researchers from Brown University, USA found that, compared to those whose median daily fish intake was 3.2 grams, the risk of malignant melanoma was 22% higher among those whose median daily intake was 42.8 grams. They also found that those whose median daily intake was 42.8 grams of fish had a 28% increased risk of developing abnormal cells in the outer layer of the skin only – known as stage 0 melanoma or melanoma in situ – compared to those whose median daily intake was 3.2 grams of fish. A portion of fish is approximately 140 grams of cooked fish.

To examine the relationship between fish intake and melanoma risk, the authors analysed data collected from 491,367 adults who were recruited from across the USA to the NIH-AARP Diet and Health Study between 1995 and 1996. Participants, who were aged 62 years on average, reported how frequently they ate fried fish, non-fried fish, and tuna during the previous year as well as their portion sizes.

The researchers calculated the incidence of new melanomas that developed over a median period of 15 years using data obtained from cancer registries. They accounted for sociodemographic factors, as well as participants’ BMI, physical activity levels, smoking history, daily intake of alcohol, caffeine and calories, family history of cancer, and the average UV radiation levels in their local area. 5,034 participants (1.0%) developed malignant melanoma during the study period and 3,284 (0.7%) developed stage 0 melanoma.

The researchers found that higher intake of non-fried fish and tuna was associated with increased risks of malignant melanoma and stage 0 melanoma. Those whose median daily tuna intake was 14.2 grams had a 20% higher risk of malignant melanoma and a 17% higher risk of stage 0 melanoma, compared to those whose median daily tuna intake was 0.3 grams. A median intake of 17.8 grams of non-fried fish per day was associated with an 18% higher risk of malignant melanoma and a 25% higher risk of stage 0 melanoma, compared to a median intake of 0.3 grams of non-fried fish per day. The researchers did not identify significant associations between consumption of fried fish and the risk of malignant melanoma or stage 0 melanoma.

Eunyoung Cho said: “We speculate that our findings could possibly be attributed to contaminants in fish, such as polychlorinated biphenyls, dioxins, arsenic and mercury. Previous research has found that higher fish intake is associated with higher levels of these contaminants within the body and has identified associations between these contaminants and a higher risk of skin cancer. However, we note that our study did not investigate the concentrations of these contaminants in participants’ bodies and so further research is needed to confirm this relationship.”

The researchers caution that the observational nature of their study does not allow for conclusions about a causal relationship between fish intake and melanoma risk. They also did not account for some risk factors for melanoma, such as mole count, hair colour, history of severe sunburn and sun-related behaviours in their analyses. Additionally, as average daily fish intake was calculated at the beginning of the study, it may not be representative of participants’ lifetime diets.

The authors suggest that future research is needed to investigate the components of fish that could contribute to the observed association between fish intake and melanoma risk and any biological mechanisms underlying this. At present, they do not recommend any changes to fish consumption.

-ENDS-

References:

  1. American Cancer Society: Key Statistics for Melanoma Skin Cancer https://www.cancer.org/cancer/melanoma-skin-cancer/about/key-statistics.html

Notes to the editor:

  1. Please name the journal in any story you write. If you are writing for the web, please link to the article.
  2. Cancer Causes & Control is an international refereed journal that both reports and stimulates new avenues of investigation into the causes, control, and subsequent prevention of cancer. By drawing together related information published currently in a diverse range of biological and medical journals, it has a multidisciplinary and multinational approach.

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Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Professor Clare Collins is a NHMRC Senior Research Fellow, Director of Research in the School of Health Sciences, and Acting Director of Priority Research Centre in Physical Activity & Nutrition at The University of Newcastle

Although the results are from a cohort study, which means they are observational and hence do not imply causation, they cannot be ignored. While the researchers did adjust for a range of factors that might confound the results, such as UV exposure this was only based on the average for where they lived, they are concerning enough to call for further investigation to evaluate whether contaminants found in some fish species could be contributing to their finding that eating more fish is associated with a higher risk of both malignant melanoma and melanoma in situ. This is particularly important given the elevated risks of malignant melanoma reported in the study were between 17-28% among those with the highest median daily intakes of total fish (about 43 grams), tuna (about 14 grams) and non-fried fish (18 grams) compared to the lowest intakes (< 1 up to 3 grams per day). Interestingly there was no association with fried fish, which is counterintuitive and although it could be due to analytic reasons also needs further investigation. 

The role of contaminants that may be present in some fish needs to be considered. A 2017 study in over 20,000 women from the Swedish Mammography Cohort evaluated exposure to contaminants found fatty fish, called dietary polychlorinated biphenyls (PCBs) and incidence of melanoma. After 4.5 years of follow-up they reported a four-fold increased risk of malignant melanoma comparing highest PCB exposure to the lowest. Interestingly this study found protection associated with omega-3 fats found in fish, but only after adjustment for dietary PCB exposure. This could explain the conflicting results from a 2015 systematic review of case-control and cohort studies that found a protective role associated with higher fish intakes and risk of malignant melanoma of the skin.

Further studies in other cohorts are needed that also evaluate PCBs and exposure to other contaminants including dioxins, arsenic and mercury, while also adjusting for individual confounders such as sun exposure, skin type and history of sunburn in order to verify or refute these findings.

Given the positive aspects of eating fish such as heart health, and that that fish is a nutritious food, my advice to Australians would be to eat fish caught in Australian or New Zealand waters."

Last updated: 08 Jun 2022 4:28pm
Declared conflicts of interest:
None declared.
Matthew Browne is the CEO of Melanoma Institute Australia

With one Australian dying from melanoma every six hours, it is critical that we don’t confuse or cloud the prevention message. The scientific evidence is clear - sun exposure is the single biggest risk factor for developing melanoma. It is critical that all Australians live a sun safe life to reduce their melanoma risk.

Last updated: 08 Jun 2022 12:30pm
Declared conflicts of interest:
None declared.

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