Several news articles have been published today (9 June 2022) suggesting that eating 2 portions of fish per week is linked to skin cancer, based on the results described within a paper by Li et al. published in the journal Cancer Causes and Control. Here we take an in depth look at the facts behind these headlines.
Most skin cancer (both melanoma and non-melanoma skin cancer) is caused by ultraviolet (UV) light (which can be from the sun or tanning devices) damaging the DNA in skin cells. Other established causes of skin cancer include medicines used to suppress the immune system after organ transplantation, infection with human papilloma virus (which can cause one particular type of skin cancer called squamous cell carcinoma, especially in people whose immune systems are compromised), occupational exposure to specific chemicals used in the plastic and chemical industries (polychlorinated biphenyls, PCBs) and rare genetic mutations. Having a family history of skin cancer increases your risk and skin cancer is more common in lighter-skinned populations. On review of the evidence considering diet and lifestyle factors and skin cancer in 2019, the World Cancer Research Fund (WCRF) Expert Panel reported that there is strong evidence that drinking water contaminated with arsenic increases the risk of skin cancer. There is some evidence that drinking coffee might decrease the risk of malignant melanoma in women and might decrease the risk of basal cell carcinoma in men and women and consuming alcoholic drinks might increase the risk of malignant melanoma and basal cell carcinoma.
The recent paper by Li and colleagues describes analysis of data from a large number of subjects followed over a long period of time. The authors report a positive association between higher intakes of total fish, tuna and non-fried fish and risk of malignant melanoma and melanoma in situ after adjusting for age, sex and other factors. Individuals with a history of cancer or cancer diagnosis at baseline were excluded. The authors propose that contaminants present in fish (which include arsenic and PCBs) may explain their findings, although levels of contaminants in the subjects’ bodies were not measured and so this proposed explanation cannot be supported by the evidence presented in this study. It is important to note that this study was observational and so a cause-and-effect relationship cannot be established. The study also has several other important limitations; firstly, diet was only assessed once at baseline and fish intakes may have changed over time. In addition, while UV light exposure was estimated based on where subjects lived at baseline, individual sun-related behaviours and history of sun burn were not assessed and information on other specific risk factors for skin cancer such as mole count and hair colour were also not collected as part of the study. Further, well designed studies are needed to verify the results. With regards to the relationship between fish and oily fish intake and skin cancer, in 2019 the WCRF Expert Panel reported that the evidence was so limited that no firm conclusion could be made.
A healthy, balanced diet should include at least 2 x 140g portions of fish a week, one of which should be an oily type such as salmon and sardines. Oily fish contains long-chain omega-3 fatty acids which can help to prevent heart disease and are also important for women who are pregnant or breastfeeding as these fatty acids can help a baby's nervous system to develop. Consumption of fish in the UK is currently well below the recommendation, at 154g of total fish per week and 56g of oily fish per week among adults aged 19-64 years. Oily fish typically contains higher levels of contaminants than other types of fish and seafood and so all girls and pre-menopausal women should have no more than two portions of oily fish per week. This is because these contaminants may build up in the body and affect the future development of a baby in the womb. Due to the amount of mercury they contain, those who are trying for a baby or are pregnant should not have any more than 4 cans of tuna or no more than 2 tuna steaks a week and should avoid eating shark, swordfish and marlin (all other adults, including breastfeeding women, should eat no more than 1 portion per week of these three types of fish). Advice on white fish is that you can safely eat as many portions as you like, except for sea bream, sea bass, turbot, halibut and rock salmon (also known as dogfish, flake, huss, rigg or rock eel), which may contain similar levels of certain contaminants as oily fish. Those who eat a lot of fish should avoid eating these five types of fish, and brown crab meat, too often. Information on choosing more sustainable fish is available here.
Government advice for preventing skin cancer is to avoid getting sunburned, using sun cream and dressing sensibly in the sun and avoiding sunbeds and sunlamps. The NHS website offers advice on how to look out for the early signs of skin cancer with respect to monitoring the appearance of moles on your body.
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