June 9, 2022

expert reaction to study looking at the consumption of fish and risk of melanoma

A study published in Cancer causes and Control looks at fish intake and melanoma risk.

Dr Stacey Lockyer, Senior Nutrition Scientist, British Nutrition Foundation, said:

“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, 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 found 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 published in Cancer Causes and Control 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 140 g 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 is also important for women who are pregnant or breastfeeding as it can help a baby’s nervous system to develop. Oily fish typically contains higher levels of pollutants than other types of fish and seafood and so all girls and premenopausal women should have no more than two portions of oily fish per week. 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 a week 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 and halibut and rock salmon (also known as dogfish, flake, huss, rigg or rock eel), which may contain similar levels of certain pollutants as oily fish. Those who eat a lot of fish should avoid eating these 5 types of fish, and brown crab meat, too often.

“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.”

Prof Tom Sanders, Professor Emeritus of Nutrition and Dietetics, King’s College London, said:

“This paper reports an association between moderate fish intake 1-2 serving a week and a slightly increased risk of melanoma in the USA. Melanoma is well known to be associated with exposure to ultraviolet (UV) radiation especially during childhood. While this study did try to adjust for geographical differences in UV radiation in the study, no adjustment were made for individual exposures (e.g. sun exposure and mitigating factors such as use of sunblocks). Recreational fishing is a popular sport that involves high UV exposure, especially as there is additional exposure from reflection by water. It would be expected that those who partake in recreational fishing would also eat more fish. The authors do not discuss this possibility but speculate on potential contaminants in fish which seems an unlikely cause of increased risk because any intake of contaminants would be trivial.”

Dr Michael Jones, Senior Staff Scientist in Genetics and Epidemiology at The Institute of Cancer Research, London, said:

“The results are from a large US observational study of diet and health that has resulted in numerous scientific publications.  This analysis was based on 491,367 men and women, followed on average for about 15 years.  There were over 5,000 cases of malignant melanoma – a potentially deadly form of skin cancer – and 3200 cases of a melanoma in-situ – sometimes called ‘pre-cancer’.

“An earlier report from 2011 on this group of men and women had found an association between melanoma and higher fish intake. This new report extends the previous analysis, with more follow-up to look in more detail at the association between different types of fish intake and melanoma.

“The authors found a higher intake of non-fried fish and tuna was associated with melanoma. These results were statistically significant and therefore unlikely due to chance. The participants in this cohort study completed diet questionnaires when they enrolled, before the onset of melanoma; it is therefore unlikely there was a bias in the reporting of fish intake between those who developed melanoma and those who did not.

“It is possible people who intake more non-fried fish or tuna have other lifestyle habits that increase their risk of melanoma.  The authors considered this and adjusted for some potentially confounding factors. However, as the authors acknowledge, this is an observational study (not a randomised trial) and it is possible there are (known and unknown) factors that the authors did not adjust for, or adjust for sufficiently well enough.

“The authors speculate that the association may be possibly due to contaminants in fish, but they did not measure levels of these contaminants in the participants. No one study should be considered in isolation and further research is needed to see if the findings from this study are replicated in other populations and countries, where levels of contaminants may be different.

“A general healthy balanced diet should include fish and the results from this study do not change that recommendation. Melanoma is easier to treat if it is diagnosed at an early stage, so it is important to have a check-up if you notice a change to a mole, freckle, or normal patch of skin.”

Prof Gunter Kuhnle, Professor of Nutrition and Food Science, University of Reading, said:

“A large body of research tells us that the recommended amount of fish consumption reduces the risk of cardio-vascular disease and improves cognition for many people. This study challenges the perceived wisdom of healthy fish as it claims that fish consumption increases the risk for melanoma – a type of skin cancer which can be deadly if not diagnosed early – however there are several important limitations of the study. 

“So should we stop eating fish? The study found more than 3 g/day was associated with a 20% increase in cancer risk. (For comparison – the recommendation in the UK is to eat 2 portions per week, or 40 g/day – which would put consumers in the top range of intake in this study). It is more difficult than that: melanoma is an incredibly difficult endpoint to assess, as detection can vary considerably depending on whether study participants attend regular screening or not. Furthermore, there is one very well-established risk factor, and that is sun exposure. Screening and sun-exposure depend on many factors such as education, income and occupation – and these are also factors that influence diet.

“The authors did adjust for a number of potential factors, but the adjustment for sun exposure relied on the estimate of sun exposure in the month of July – without taking into consideration other factors such as occupation or hobbies. It is very possible that high fish intake is simply an indicator of a lifestyle that leads to high sun exposure – but the authors have not explored this option.

“Public health recommendations need to be based on good evidence and need to take positive and negative consequences into consideration. Heavy metals and other contaminants of fish are clearly a cause for concern – that is why this is taken into consideration in recommendations. The current recommendation in the UK is for 2 portions of fish per week – or 40 g day – well beyond what the majority of the study population has eaten. This is based on a thorough assessment of risks and benefits, and the data presented in this study are in no way convincing enough to put this into question.”

Dr Duane Mellor, Senior Lecturer, Aston Medical School, said:

“This study looked at a large number of people to see if there was an association between fish intake and risk of melanoma (a type of skin cancer). Although the researchers tried to control their analysis to account for other risk factors such as UV exposure, diet, smoking and age, it is still likely that residual confounding factors might have led to these results. The authors suggest that there could be a link between contaminants in the fish which could increase risk of cancer, but this is likely to affect the risk of more than just skin cancers. This study does not have a clear mechanism of how fish intake could increase risk of melanoma risk, there is no clear evidence that eating risk can lead to an increased risk of developing skin cancer. 

“It is also worth noting that the diet data was collected using a food frequency questionnaire asking how often they ate foods over the past year including fish. This was used to estimate how many grams of fish they ate, giving impossibly precise median values such as 4.2 grams per day for those eating less fish and were seen 15 years later to have a lower risk of melanoma. Food intake was only measured once and then associated with risk of developing cancer; therefore the study does not consider how food intake and other behaviours may have changed over the following decade and a half.

“It is important to remember eating two portions of fish per week as recommended in government nutrition guidelines can be a way of including important nutrients such as omega-3 fatty acids as part of a healthy diet and this study should not discourage people from including fish as part of a healthy diet.”

Prof Stephen Duffy, Professor of Cancer Screening, Queen Mary University of London, said:

“These are striking results and perhaps raise more questions than they answer. The estimated effect of fish consumption on risk is small. A 1200% increase in median consumption is associated with only a 22% increase in risk of melanoma. One might speculate that there may be confounding with fishing, either as a leisure or occupational activity, which would confer higher exposure to sunlight. Also, the fact that with fried fish there is no increase in risk, and possibly even a decrease, requires some explanation. Estimation of risks associated with diet is fraught with difficulties due to the possibility of multiple association of dietary factors with other risk factors.”

‘Fish intake and risk of melanoma in the NIH‑AARP diet and health study’ by Yufei Li et al. was published in the Cancer Causes & Control at 01:00 UK time on Thursday 9^th June 2022.

DOI: 10.1007/s10552-022-01588-5

Declared interests

Dr Stacey Lockyer: “Funding to support the British Nutrition Foundation’s charitable aims and objectives comes from a range of sources including membership, donations and project grants from food producers and manufacturers, retailers and food service companies, contracts with government departments; conferences, publications and training; overseas projects; funding from grant providing bodies, trusts and other charities. Further information about the British Nutrition Foundation’s activities and funding can be found at http://www.nutrition.org.uk/aboutbnf/”

Dr Michael Jones: “No declarations or conflict of interest.”

Prof Gunter Kuhnle: “No conflicts to declare.”

Dr Duane Mellor: “No conflicts of interest to declare.”

Prof Stephen Duffy: “No interests to declare.”

No others received.