Put on sunscreen. Wear a hat. And … don’t eat too much fish?

A recent paper suggests that eating more fish may be associated with a greater risk of malignant melanoma. But the large, observational study comes with a lot of caveats, and the senior author notes that previous research has linked higher mercury levels and skin cancer.

“Mercury consumption in the US is mostly from fish,” says Eunyoung Cho, ScD, an associate professor of dermatology and of epidemiology. “So if mercury is related to skin cancer, then it stands to reason that fish intake may be related, too.”

Nonetheless, Cho says this new study is important due to its large size and prospective design, as participants’ fish consumption was assessed before they got cancer. “The results of previous studies investigating associations between fish intake and melanoma risk have been inconsistent,” she says. “Our findings have identified an association that requires further investigation.”

The researchers analyzed data collected from nearly half a million US adults who were recruited to the National Cancer Institute’s NIH-AARP Diet and Health Study. Participants reported how frequently they ate fish as well as their portion sizes. Their average age was 62.

The analysis, published in Cancer Causes and Control, reports that compared to those whose median daily fish intake was 0.11 ounces, those who ate 1.5 ounces per day had a 22 percent higher risk of malignant melanoma and a 28 percent increased risk of melanoma in situ, the earliest form of the disease. (A typical serving size for most fish is 5 to 6 ounces.)

The study accounts for sociodemographic and physiological factors as well as medical history and the average ultraviolet radiation levels in each participant’s local area. However, the researchers wrote, it didn’t adjust for some risk factors for melanoma such as mole count, hair color, or history of severe sunburn and sun-related behaviors. And because average daily fish intake was calculated at the beginning of the study, it may not represent participants’ lifetime diets.

Furthermore, as an observational study, it couldn’t point to a causal relationship between fish intake and melanoma risk. Cho, who studies the connection between diet and skin cancer, says that bio-contaminants in the fish—like mercury, PCBs, dioxins, and arsenic—likely play a role in the cancer association, rather
than the fish itself.

“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,” she says. But this study didn’t examine the concentrations of these pollutants in participants’ bodies, Cho adds, so researchers will need to keep fishing around before they can draw stronger conclusions.