High Rate of Melanoma Does Eating Fish Increase the Risk of Developing Skin Cancer?

A new study in Cancer Causes and Control suggests a link between eating fish and skin cancer. However, Harvard Medical School warns that it’s too soon to make definitive conclusions about the relationship between fish in our diets and melanoma.

If you’re trying to stick to a healthy diet, fish is a good choice, right? After all, fish is high in protein, low in saturated fat, and a good source of omega-3 fatty acids, vitamin D, and many other nutrients. Eating more fish can mean eating less of foods with harmful fats and higher calorie counts. Indeed, nutritionists commonly recommend more seafood (and fewer cheeseburgers) to improve your diet, and nutrition guidelines promote fish as part of a healthy diet.

So, it seems surprising that a new study in Cancer Causes and Control suggests a link between eating fish and skin cancer, particularly since the biggest known risk factor for melanoma is not dietary ­–– it’s sun exposure. Having five or more sunburns in your life doubles your risk of developing melanoma.

In the new study, researchers found a higher risk of melanoma among people who ate the most fish. This study is among the largest and most well-designed to examine this link. Nearly 500,000 people in six US states completed a dietary questionnaire in 1995 or 1996. The average age of participants was 61 and 60 percent were male. More than 90 percent were white, four percent were Black, and two percent were Hispanic.

Over the following 15 years, the researchers tallied how many people developed melanoma, and found that:

• The rate of melanoma was 22 percent higher among people reporting eating the most fish (about 2.6 servings per week) compared with those who ate the least (0.2 servings a week, or about one serving every five weeks). Similar trends were noted for intake of tuna.

• The risk of precancerous skin changes (called melanoma in situ) rose similarly among those in the group that ate the most fish.

• Interestingly, researchers found no increased risk of melanoma among those eating the most fried fish. This is surprising because, if eating fish increases the risk of melanoma as the study suggests, it’s not clear why frying the fish would eliminate the risk.

Does this Mean Eating Fish Causes Melanoma?

No, it doesn’t. It’s too soon to make definitive conclusions about the relationship between fish in our diets and melanoma. The study had important limitations, including

• Type of study. Observational studies like this one can detect a possible link between diet and cancer but cannot prove it.

• Reliance on self-reported survey data. People self-reported how many servings of fish they ate each week, which may not be accurate. Also, researchers assumed that fish consumption reported on the initial survey persisted for 15 years, which may not have been the case.

• Accounting for other factors. Many factors affect risk for melanoma, such as varied sun exposure depending on where participants lived. The analysis did account for some key factors, yet the study didn’t collect information about sun exposure, past sunburns, or use of sunscreen — all important in melanoma risk. Nor did researchers ask about skin type or number of moles; fair skin or higher numbers of moles raise risk for melanoma.

• Contaminants. Mercury or arsenic in fish may be to blame for its link to melanoma. This study did not record contaminants, but previous studies link mercury exposure with the risk of skin cancers, including melanoma.

• Lack of diversity. It’s not clear if the findings apply broadly to people in different racial and ethnic groups, because nine in 10 study participants were white.

Are Some Fish Safer to Eat than Others?

The study did not explore this question. However, if contaminants like mercury in fish are responsible for increasing the risk of melanoma, the FDA offers advice on which fish are safer to eat, particularly for children and those who are pregnant or breastfeeding.

Yet even if fish is confirmed as a contributor to the risk of melanoma, other positive effects of fish consumption (such as cardiovascular benefit) may far outweigh this risk.

The Bottom Line

The researchers responsible for this study are not recommending a change in how much fish people eat. More study is required to confirm the findings, investigate which types of fish affect melanoma risk, and determine whether certain contaminants in fish are responsible for any added risk.

In the meantime, fish with lower mercury levels (such as salmon and clams) remain better dietary choices than the high-fat, highly processed foods typical of many Western diets.

* Robert H. Shmerling, MD, Senior Faculty Editor, Harvard Health Publishing

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