Too Much Fish May Increase Risk of Melanoma Skin Cancer: Study

Researchers warn that eating large quantities of fish may increase a person’s risk of developing melanoma, a severe and particularly life-threatening form of skin cancer.

People who ate more fish overall had a 23% increased risk of developing melanoma, according to the findings of a report published this month in the medical journal Cancer Cases & Control.

Brown University researchers researchers studied nearly 500,000 participants in the National Institutes of Health (NIH) and Association of the Advancement for Retired People (AARP) Diet and Health Study, which began in the 1990s and focused on the correlation between intake of total fish and specific types of fish and risk of melanoma.

Overall, the study included more than 6.6 million person-years with an average follow-up of 15 years. A total of 8,300 cases of melanoma were diagnosed across the study with 5,000 of those being malignant.

People who ate the most fish, about 43 grams per day, had a 23% higher likelihood of developing melanoma compared to light fish eaters who ate an average of 3 grams per day. Forty-three grams is about 1.5 ounces per day.

Participants with the highest fish intake, about two servings per day, also had a 28% increased risk of developing abnormal skin cells, which can lead to skin cancer. Those who ate about three-quarters of a serving of tuna per week had a 20% increased risk of melanoma compared to those who ate none.

Researchers said the positive associations were consistent across several demographic and lifestyle factors.

“We found that higher total fish intake, tuna intake, and non-fried fish intake were positively associated with risk of both malignant melanoma and melanoma in situ,” the researchers concluded. “Future studies are needed to investigate the potential biological mechanisms underlying these associations.”

The study did have flaws. Researchers used dietary recall surveys at the start, where the participant is asked to remember how much fish they ate over the course of a year. People can often have trouble remembering what they ate yesterday, much less a year ago.

Additionally, researchers did not check to see how a person’s diet changed over time. They assumed fish intake recorded at the beginning remained steady throughout the course of the study.

Furthermore, sun exposure is the greatest risk factor for melanoma and researchers did not account for sun exposure. They attempted to account for this by using the average sun exposure times based on where participants lived, but that is not a definite answer to actual sun exposure as lifestyle factors can vary greatly.

Researchers also suggested the increased risk may be due to contaminants in fish such as mercury and arsenic, which have been linked to melanoma in other studies.

Melanoma is the deadliest form of skin cancer and can be life-threatening when it spreads to other parts of the body. If untreated, it can become deadly within six weeks. Risk factors include fair skin, history of sun burn, excessive UV light exposure either from the sun or tanning beds, living close to the equator, having unusual moles and a family history of melanoma.

Authors do not advise changing fish-eating habits based on the findings of this study, since fish has benefits for cardiovascular health and other types of cancers and is a healthier alternative compared to red meat. The researchers determined further research is needed to investigate what factors could contribute to the underlying biological mechanisms which may increase the skin cancer risk.