PROVIDENCE, R.I. [Brown University] — Eating higher amounts of fish, including tuna and non-fried fish, appears to be associated with a greater risk of malignant melanoma, according to a large study of U.S. adults published in the journal Cancer Causes and Control.

“This study is important because it’s very large and it’s prospective by design, meaning that fish intake was assessed before the development of cancer,” said author Eunyoung Cho, an associate professor of dermatology and epidemiology at Brown University. “Although fish intake has increased in the U.S. and Europe in recent decades, the results of previous studies investigating associations between fish intake and melanoma risk have been inconsistent — our findings have identified an association that requires further investigation.”

The researchers found that compared to those whose median daily fish intake was 3.2 grams (.11 ounces), those whose median daily intake was 42.8 grams (1.5 ounces) had a 22% higher risk of malignant melanoma and a 28% increased risk of developing abnormal cells in the outer layer of the skin only — known as stage 0 cancer or melanoma in situ. A serving size of cooked fish is approximately 140-170 grams (5-6 ounces); a can of tuna is 142 grams (5 ounces).

Melanoma is the fifth most common cancer in the U.S., and the risk of developing it over a lifetime is one in 38 for white people, one in 1,000 for Black people and one in 167 for Hispanic people, according to the American Cancer Society.

To examine the relationship between fish intake and melanoma risk, the authors analyzed data collected from 491,367 adults who were recruited from across the U.S. to the National Cancer Institute’s NIH-AARP Diet and Health Study between 1995 and 1996. Participants, who were 62 years old on average, reported how frequently they ate fried fish, non-fried fish and tuna during the previous year, as well as their portion sizes.

The researchers calculated the incidence of new melanomas that developed over a median period of 15 years using data obtained from cancer registries. They accounted for sociodemographic factors as well as participants’ body mass index; physical activity levels; smoking history; family history of cancer; daily intake of alcohol, caffeine and calories; and the average ultraviolet radiation levels in each participant’s local area.

During the study period, 5,034 participants (1%) developed malignant melanoma and 3,284 (0.7%) developed stage 0 melanoma. The researchers found that higher intake of non-fried fish and tuna was associated with increased risks of malignant melanoma and stage 0 melanoma:

• Compared to those whose median daily tuna intake was 0.3 grams (.01 ounces), those whose median daily tuna intake was 14.2 grams (0.5 ounces) had a 20% higher risk of malignant melanoma and a 17% higher risk of stage 0 melanoma.
• Compared to a median intake of 0.3 grams of non-fried fish per day, a median intake of 17.8 grams (0.62 ounces) of non-fried fish per day was associated with an 18% higher risk of malignant melanoma and a 25% higher risk of stage 0 melanoma.
• The researchers did not identify significant associations between consumption of fried fish and the risk of malignant melanoma or stage 0 melanoma.

The researchers disclosed some limitations: The analyses did not account for some risk factors for melanoma such as mole count, hair color or history of severe sunburn and sun-related behaviors. Additionally, because average daily fish intake was calculated at the beginning of the study, it may not be representative of participants’ lifetime diets.

The researchers also cautioned that the observational nature of their study did not allow for conclusions about a causal relationship between fish intake and melanoma risk.

Cho, who studies the connection between diet and skin cancer, has been involved with previous research that showed an association between higher mercury levels and skin cancer.

“Mercury consumption in the U.S. is mostly from fish,” Cho said. “So if mercury is related to skin cancer, than it stands to reason that fish intake may be related, too.”

Cho said that bio-contaminants like mercury in the fish, and not the fish itself, likely play a role in the cancer association.

“We speculate that our findings could possibly be attributed to contaminants in fish, such as polychlorinated biphenyls, dioxins, arsenic and mercury,” Cho said. “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 authors say that future research is needed to investigate the components of fish — especially the contaminants — that could contribute to the observed association between fish intake and melanoma risk, as well as to understand the underlying biological mechanisms of this association. At present, they do not recommend any changes to fish consumption.

Yufei Li, who earned a master of public health from Brown University in 2021, was a first author on the paper. Other Brown contributors included Abrar Qureshi, Terrence Vance and Tongzhang Zheng.