Fishy diet may be linked to increased melanoma risk

Melanoma is a serious type of skin cancer that develops in the cells that produce melanin, the pigment that gives skin its color. The causes of this type of cancer are not well understood, although exposure to UV radiation appears to increase a person’s risk of developing it. And now a new study, involving almost 500,000 adults across America, has linked an increased risk of melanoma with the consumption of certain forms of fish. 

“Melanoma is the fifth most common cancer in the USA and the risk of developing melanoma over a lifetime is one in 38 for white people, one in 1,000 for black people and one in 167 for Hispanic people. Although fish intake has increased in the USA and Europe in recent decades, the results of previous studies investigating associations between fish intake and melanoma risk have been inconsistent,” said study co-author Eunyoung Cho.

The study, conducted by researchers from Brown University, made use of the data from 491,367 adults who were recruited from across the USA to the NIH-AARP Diet and Health Study between 1995 and 1996. The aim of the current study was to investigate the relationship between daily median fish intake and melanoma risk. The participants, who were aged 62 years on average when the baseline data was collected, reported at the time how frequently they had eaten fried fish, non-fried fish and tuna during the previous year, as well as giving details of their portion sizes.

The researchers then obtained data on the incidence of melanoma amongst the participants over the following 15 years from cancer registries. They found that 5,034 participants (1.0%) developed malignant melanoma during the study period and 3,284 (0.7%) developed stage 0 melanoma, or melanoma in situ, which is characterized by the development of abnormal cells in the outer layer of the skin only. 

Once the scientists had accounted for sociodemographic factors, participants’ BMI, physical activity levels, smoking history, daily intake of alcohol, caffeine and calories, family history of cancer, and the average UV radiation levels in their local areas, they found that higher intake of fish in general, and of non-fried fish and tuna in particular, was associated with increased risks of malignant melanoma and stage 0 melanoma.  

The findings, published today in the journal Cancer Causes & Control, showed that participants whose median daily fish intake was 42.8 grams (approximately one 140 gram portion every three days) had a 22 percent higher risk of developing malignant melanoma than those whose median daily fish intake was 3.2 grams. The researchers also found that participants whose median daily intake was 42.8 grams of fish had a 28 percent increased risk of developing stage 0 melanoma when compared to those whose median daily intake was 3.2 grams of fish. 

When considering the influence of the form of fish eaten, the researchers found that participants whose median daily tuna intake was 14.2 grams (a portion every 10 days) had a 20 percent higher risk of malignant melanoma and a 17 percent higher risk of stage 0 melanoma, when compared to those whose median daily tuna intake was only 0.3 grams. 

In addition, a median intake of 17.8 grams of non-fried fish per day was associated with an 18 percent higher risk of malignant melanoma and a 25 percent higher risk of stage 0 melanoma, compared to a median intake of 0.3 grams of non-fried fish per day. 

The researchers did not identify any significant associations between consumption of fried fish and the risk of malignant melanoma or stage 0 melanoma.

“We speculate that our findings could possibly be attributed to contaminants in fish, such as polychlorinated biphenyls, dioxins, arsenic and mercury,” said Cho. “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.”

The study uses an observational method along with statistical analysis, which does not allow for conclusions about whether fish intake actually causes an increased risk of melanoma. In addition, the method did not take into consideration certain other risk factors for melanoma development, including initial mole count, hair color, history of severe sunburn and sun-related behaviors. The researchers also note that, since average daily fish intake was reported by participants for the period of a year before the start of the study, it may have changed over time. 

However, the study authors suggest that their findings do warrant further investigation, particularly of the components of fish that could contribute to the observed association between fish intake and melanoma risk and any biological mechanisms underlying this. 

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By Alison Bosman, Earth.com Staff Writer