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In the news: Is a high fish intake linked to skin cancer?

15 June 2022

Recent news stories reported that eating two portions of fish a week increases the risk of skin cancer. While eating fish has long been recognized as part of a healthy diet, providing a valuable source of high-quality protein, minerals, vitamins, and omega-3 fatty acids, should you reconsider this long-held belief? Here are a few things to keep in mind when reading the headlines.

The study behind the headlines

The study behind the news was carried out by researchers from Brown University in the United States (US) with the aim to clarify the associations between total fish intake, as well as specific types of fish intake (tuna, fried fish, and non-fried fish), and the risk of malignant melanoma.1 Malignant melanoma is a type of skin cancer that develops from the cells (called melanocytes) that control the pigment of the skin. It is the most serious form of skin cancer. The current study used data from the NIH-AARP Diet and Health Study which is a large long-term study involving 3.5 million US citizens aimed at improving the understanding of the relationship between diet and health.

The study started between 1995-1996 with participants filling in a so-called food frequency questionnaire about their usual eating habits over the past year. The researchers then followed these participants up over a period of, on average, 15 years and combined data from the National Death Index and cancer registries to assess their vital status and cancer diagnoses.

After the years of follow-up, the researchers analysed the data from 500,000 adults who had complete data. They found that increasing total fish intake was linked with higher risks of melanoma. For example, participants who had the highest intakes of fish (about 43 grams per day or just over 2 portions per week) had a 22% higher risk of developing malignant melanoma compared to those with the lowest intakes of fish (about 3 grams per day). The researchers found similar associations with specific fish types. Participants with the highest intakes of tuna (about 14 grams per day) and non-fried fish (about 18 grams per day) had both around a 20% higher risk of developing malignant melanoma compared to those with the lowest intakes of tuna (about 0 grams per day) and non-fried fish (about 0 grams per day). However, for fried fish intake an opposite direction of association was found in the highest intake categories, meaning that those who had the highest fried fish intake (about 9 grams per day) had a lower risk of developing malignant melanoma compared to those with the lowest fried fish intake (about 0 grams per day).

The authors suggest that future research should explore the components which are potentially responsible for the association and biological mechanism between fish intake and melanoma.

What to keep in mind when reading the study’s conclusions?

  • The study cannot prove that increased fish intake causes melanoma.

Observational studies can’t prove direct cause and effect. The study has taken into account multiple health and lifestyle factors such as age, ethnicity, family history of cancer, physical activity, smoking intake, alcohol intake, and UV exposure from the sun, that could potentially influence the melanoma outcome. However, the researchers caution that even so, it’s not entirely possible to ensure that such factors had no impact on the final results. Besides, other unmeasured dietary or lifestyle factors may be involved in the intricate link with skin cancer.

  • Estimates of fish intake may be inaccurate.

While food frequency questionnaires are one of the best methods available to assess dietary intake, they still have limitations. Typical intake, portion size, and preparation may be difficult to estimate with accuracy and include subjective interpretations that vary between individuals. Neither does the food frequency questionnaire represent lifetime dietary patterns. Also, the analysis didn’t assess relevant dietary details such as how fish was prepared (if it was battered or deep fried), nor made a distinction between categories of oily and non-oily fish.       

  • The baseline risk of melanoma remains quite low.

The article communicates relative risks, not absolute risks. The relative risk displays the increased risk of melanoma in the high fish eaters group compared to the low fish eaters’ group. However, the estimated lifetime risk of developing malignant melanoma (i.e., the absolute risk) is around 1-2% of the total participants in the study. With the added 22% relative risk increase in the high fish eating group, this would mean that the absolute risk would increase to 1.2-2.4%, thus remaining fairly small. Any risk from a high fish consumption remains smaller than the leading risk factors for melanoma, mainly excessive sun exposure.

  • Any risk from ‘two portions of fish a week’ may be outweighed by the benefits.

The media reported the link with two portions a week because the National Health Service of England (NHS) (among other organisations) recommend consumption of at least two portions of fish per week (including one of oily fish) with a portion weighing around 140g. The observed 22% increased risk among participants with the highest daily fish intake equal to just over two portions per week. However, fish, particularly oily, has been associated with many health benefits including cardiovascular benefits. These benefits may outweigh any small risk increase.

  • The findings may not apply to all.

This was a large study but included only adult US citizens of predominantly white ethnicity, who were 50-70 years of age between 1995 to 1996. Even for a comparable white population, lifestyle changes, including sun exposure and dietary habits, may have changed considerably over the past decades making the results potentially less applicable today.

  • The increased melanoma risk may be explained by contaminants in fish.

The authors caution that contaminants in fish such as mercury, and not the fish itself, likely play a role in the cancer association. Previous studies have found associations between these contaminants and a higher fish intake and a higher risk of skin cancer. However, as the current study didn’t measure the concentration of contaminants in participants’ bodies, further research is needed to confirm this relationship. Therefore, at this moment in time, the authors do not recommend any changes to fish consumption. The European Food Safety Authority (EFSA) also indicates that omega-3 fatty acids in fish may counteract the negative effects of mercury exposure.2

What do authorities say?

  • The World Cancer Research Fund (WCRF) produced guidance on diet and skin cancer in 2019. They summarised that there was ‘limited evidence’, meaning that they could not conclude any link between fish or oily fish intake and skin cancer.3
  • The WCRF highlights that most cases of skin cancer could be prevented by avoiding overexposure to UV rays.4 They also advise consuming two 140g portions of fish a week, including one of oily fish, given that omega-3 fatty acids found in oily fish reduce the risk of cardiovascular disease.5
  • The European Commission presents the recommendations on fish intake from different European countries. The specific recommendations are variable, but most countries advise intake twice a week, one of which being oily.6   
  • The NHS advises that a healthy, balanced diet should include at least two 140g portions of fish a week, including one of oily fish. The NHS also mentions the potential risks of excess consumption of certain types of fish and shellfish for pregnant/breastfeeding women and young children because of the risk of mercury and pollutants.7

Sources:

  1. Li, Y., Liao, L. M., Sinha, R., Zheng, T., Vance, T. M., Qureshi, A. A., & Cho, E. (2022). Fish intake and risk of melanoma in the NIH-AARP diet and health study. Cancer causes & control : CCC, 33(7), 921–928. https://doi.org/10.1007/s10552-022-01588-5
  2. EFSA Panel on Contaminants in the Food Chain (CONTAM). (2012). Scientific Opinion on the risk for public health related to the presence of mercury and methylmercury in food. Efsa Journal, 10(12), 2985.
  3. World Cancer Research Fund. (2019). Diet, nutrition, physical activity and skin cancer. Retrieved from https://www.wcrf.org/wp-content/uploads/2021/02/skin-cancer.pdf
  4. World Cancer Research Fund. (n.d.). Sun, UV rays and cancer risk. Retrieved from https://www.wcrf-uk.org/preventing-cancer/what-can-increase-your-risk-of-cancer/sun-uv-rays-and-cancer-risk/
  5. World Cancer Research Fund. (2016). How to find a place in your heart for oily fish. Retrieved from https://www.wcrf-uk.org/our-blog/how-to-find-a-plaice-in-your-heart-for-oily-fish/
  6. European Commission. (n.d.). Food-Based Dietary Guidelines in Europes – table 9. Retrieved from https://knowledge4policy.ec.europa.eu/health-promotion-knowledge-gateway/food-based-dietary-guidelines-europe-table-9_en
  7. NHS. (n.d.). Fish and shellfish. Retrieved from https://www.nhs.uk/live-well/eat-well/food-types/fish-and-shellfish-nutrition/