Fish is celebrated for its health benefits and often recommended as part of a balanced diet. However, concerns about potential contaminants and their link to health issues, including cancer, frequently arise. This conflicting information can make it challenging to understand the true impact of fish consumption. This article explores the scientific understanding behind these contrasting perspectives, providing clarity on the risks and benefits associated with eating fish to help individuals make informed dietary choices.
The Potential Risks of Eating Fish
Fish can accumulate various environmental contaminants that may pose health risks. One such contaminant is mercury, particularly in its organic form, methylmercury, which is a neurotoxin and a potential carcinogen. This substance enters aquatic ecosystems from both natural sources and human activities, such as industrial processes and power plant emissions. Once in the water, microorganisms convert inorganic mercury into methylmercury, which then enters the food chain.
Methylmercury concentrations increase through bioaccumulation or biomagnification. Smaller organisms absorb mercury from their environment. When larger fish consume these smaller contaminated organisms, mercury becomes more concentrated in their tissues. This means larger, longer-lived predatory fish, such as shark, swordfish, king mackerel, and some tuna varieties, tend to have higher levels of methylmercury. Consuming these fish can lead to human exposure, with studies suggesting increased cancer risk.
Beyond mercury, fish can also contain polychlorinated biphenyls (PCBs) and dioxins. These persistent organic pollutants remain in the environment for extended periods. Like mercury, PCBs and dioxins are lipophilic, binding to fatty tissues and biomagnifying up the food chain. Human ingestion through contaminated food, especially fish, is a common exposure route.
Both PCBs and dioxins have been linked to various adverse health effects, including cancer in animal studies. The International Agency for Research on Cancer (IARC) has classified certain dioxin compounds and some dioxin-like PCBs as human carcinogens. These compounds can alter cellular processes and contribute to cancer promotion.
The Cancer-Protective Properties of Fish
Despite contaminant concerns, fish also contain beneficial compounds that may offer protection against cancer. Omega-3 fatty acids are abundant in oily fish like salmon, sardines, and mackerel. These polyunsaturated fats are recognized for their anti-inflammatory properties, relevant to cancer prevention. Chronic inflammation is a known factor that can contribute to tumor growth and progression.
Omega-3 fatty acids may reduce cancer risk by modulating inflammatory responses and influencing cell signaling pathways. They can inhibit the synthesis of pro-inflammatory mediators and regulate gene expression, potentially suppressing cell proliferation and promoting programmed cell death (apoptosis) in malignant cells. Some research suggests a link between fish consumption and a reduced risk of bowel and liver cancers, though more evidence is needed for other cancer types.
Fish also provides other nutrients with potential anti-cancer effects, such as selenium and Vitamin D. Selenium is an essential micronutrient that can inhibit malignant cell growth and induce programmed cell death without harming healthy ones. It has shown value in preventing certain cancers, including hepatocellular, colorectal, lung, and prostate cancers.
Vitamin D, also found in oily fish, has been researched for its role in protecting against breast cancer and other malignancies. Both selenium and Vitamin D contribute to overall cellular health and immune function, important for the body’s defense against disease. These compounds work through different mechanisms to prevent cancer.
The Impact of Cooking Methods
The way fish is prepared can introduce another layer of potential risk, independent of inherent contaminants. High-temperature cooking methods, such as grilling, pan-frying, and broiling, can lead to the formation of harmful chemicals called heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs). These compounds are formed when muscle meats, including fish, are cooked at temperatures generally above 300°F (about 150°C).
HCAs develop from the reaction of amino acids, sugars, and creatine present in muscle tissue under high heat. PAHs, conversely, form when fat and juices drip onto a heated surface or open flame, creating smoke that then adheres to the surface of the food. Both HCAs and PAHs have been found to be mutagenic in laboratory experiments, meaning they can cause changes to DNA that may increase cancer risk.
Studies in rodents have shown that high doses of HCAs and PAHs can induce various tumors, including those of the breast, colon, liver, and lung. While the doses used in these animal studies are significantly higher than typical human dietary intake, epidemiological studies have observed increased risks with frequent consumption of well-done or charred meats. Cooking methods that expose fish to smoke or charring, like direct grilling, contribute more to PAH formation.
Conversely, lower-temperature cooking methods produce fewer or none of these compounds. Baking, steaming, or poaching fish are gentler alternatives that reduce the formation of HCAs and PAHs, making them safer choices for preparing fish. Microwaving fish before grilling can also reduce HCA content by shortening the high-heat cooking time.
Making Safer Fish Choices
Balancing the potential risks and benefits of fish consumption involves making informed choices about species and preparation. The U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) provide guidelines to help consumers navigate these decisions. Their advice categorizes fish based on mercury levels to guide consumption frequency.
To minimize exposure to contaminants like mercury, choose fish from the “Best Choices” list, which includes species with lower mercury concentrations. These fish can generally be consumed two to three servings per week for adults, with children also benefiting from two servings per week from this list.
Examples of low-contaminant options include:
Salmon
Sardines
Anchovies
Atlantic mackerel
Catfish
Clams
Cod
Flounder
Haddock
Pollock
Shrimp
Tilapia
Trout
Fish categorized under “Good Choices” can be consumed about one serving per week. For instance, albacore tuna typically has higher mercury levels than light canned tuna and should be limited. Conversely, certain fish should be largely avoided due to their consistently high mercury levels. These larger, predatory species accumulate more mercury through their diet.
High-mercury fish to largely avoid include:
Shark
Swordfish
King mackerel
Marlin
Orange roughy
Tilefish
Regarding cooking methods, prioritizing lower-temperature techniques like baking, steaming, or poaching is recommended to reduce the formation of carcinogens like HCAs and PAHs. If grilling or pan-frying, minimizing charring, flipping fish frequently, and using marinades can help reduce the creation of these compounds.