People who eat two portions of fish a week are at an increased risk of developing malignant melanoma, a study suggests.
Britain's National Health Service recommends eating at least two portions of fish a week, including one of oily fish, with a portion weighing about 140 grams.
But now researchers in the US have warned this amount may be putting people at risk of malignant melanoma, the most deadly form of skin cancer.
Other experts said fish was an important, healthy food and there was no need to stop eating it.
In the research, experts from Brown University found that people whose typical daily intake of fish was 42.8g, equal to about 300g a week, had a 22 per cent higher risk of malignant melanoma than those whose typical daily intake was just 3.2g.
Those eating more fish also had a 28 per cent increased risk of developing abnormal cells in the outer layer of the skin only – known as Stage 0 melanoma or melanoma in situ, and also referred to as pre-cancer.
The findings were based on a study of 491,367 US adults and published in the journal Cancer Causes & Control.
Those in the study were aged 62 years on average and reported how often they ate fried fish, non-fried fish and tuna during the previous year, as well as portion sizes.
The researchers then calculated the frequency of new melanoma cases that developed over 15 years using data obtained from cancer registries.
They took into account factors that could influence the results, such as people’s weight, whether they drank alcohol or smoked, diet, family history of cancer and average UV radiation levels in their local area.
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Overall, 5,034 people (1 per cent) developed malignant melanoma during the study period and 3,284 (0.7 per cent) developed Stage 0 melanoma.
A breakdown of the results showed that total fish intake was linked to higher risks.
Meanwhile, people whose typical daily tuna intake was 14.2g had a 20 per cent higher risk of malignant melanoma, compared with those with a typical intake of 0.3g.
Eating 17.8g of non-fried fish a day was associated with an 18 per cent higher risk of malignant melanoma and a 25 per cent higher risk of Stage 0 melanoma, compared with 0.3g.
But no significant link was found between eating fried fish and skin cancer.
Author Dr Eunyoung Cho said previous studies had been inconsistent.
“Our findings have identified an association that requires further investigation," Dr 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.”
Other limits to the study included that experts did not account for some established risk factors for melanoma, such as mole count, hair colour, history of severe sunburn and whether people sunbathed or wore sun cream.
And average daily fish intake was calculated at the start of the study and may not represent how much people eat over the course of their lives.
“The authors found a higher intake of non-fried fish and tuna was associated with melanoma," said Dr Michael Jones, senior staff scientist in genetics and epidemiology at the Institute of Cancer Research, London.
"These results were statistically significant and therefore unlikely due to chance.
“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 and it is possible there are 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.”
Dr Duane Mellor, senior lecturer at Aston Medical School, said: “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 – there is no clear evidence that eating fish can lead to an increased risk of developing skin cancer.
“It is important to remember eating two portions of fish a week … 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.”
