Often lauded as a superfood, fish has its clear nutritional benefits, providing the body with vital fatty acids and vitamins.
However, too much fish could well be a bad thing. According to a new study, eating two portions per week – as recommended by the NHS – has been linked to an increased risk of skin cancer, the most deadly of its kind.
In the new research, experts from Brown University found that people whose typical daily intake of fish was 42.8g (equivalent to about 300g per week) had a 22 per cent higher risk of malignant melanoma than those whose typical daily fish 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 (also sometimes 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.
Author Eunyoung Cho said the research has “identified an association that requires further investigation.
“We speculate that our findings could possibly be attributed to contaminants in fish, such as polychlorinated biphenyls, dioxins, arsenic and mercury.”
Other experts said fish was an important healthy food and there was no need to stop eating it.
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 per 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.”
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 their 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 smoked or drank alcohol, diet, family history of cancer and average UV radiation levels in their local area (to take account of exposure to the sun – a known risk factor for skin cancer).
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 per 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 eating just 0.3g.
However, no significant link was found between eating fried fish and skin cancer.
Also, average daily fish intake was calculated at the beginning of the study and may not represent how much people eat over the course of their lives.
Dr Michael Jones, senior staff scientist in genetics and epidemiology at the Institute of Cancer Research, London, said: “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.
“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.
“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.
“A general healthy balanced diet should include fish and the results from this study do not change that recommendation.”
