Why Cooking Doesn't Remove Mercury
Many people believe that proper preparation, like cooking or trimming, can reduce contaminants in fish. This is true for some types of pollutants, such as PCBs and DDT, which concentrate in the fatty tissue and can be reduced by trimming the fat and skin before cooking. Mercury, however, behaves differently. As methylmercury, it is chemically bound to the proteins within the fish's muscle tissue, the very part that is consumed. This chemical bond is stable and is not broken down by standard cooking temperatures.
Studies have shown that cooking can actually increase the mercury concentration per serving. This is not because new mercury is introduced, but because the cooking process, particularly methods like frying or baking, causes the fish to lose moisture and fat. As the fish shrinks, the remaining mercury becomes more concentrated, resulting in a higher mercury level per unit of cooked weight compared to raw weight. This phenomenon has been observed in various cooking studies on different fish species.
Bioaccessibility vs. Total Mercury Content
Some research offers a nuanced perspective on the effects of cooking. While cooking does not remove the total amount of mercury, some studies have found that cooking methods can impact mercury bioaccessibility. Bioaccessibility refers to the amount of a substance that is released from the food matrix during digestion and becomes available for absorption by the body. Heat-induced protein denaturation can alter the reactivity of the protein-bound mercury, potentially reducing its bioaccessibility.
For example, one study found that while cooked fish had a higher mercury concentration due to moisture loss, the mercury's bioaccessibility was much lower compared to raw samples. The baked and fried samples in another study showed a decrease in the concentration of mercury absorbed during simulated digestion, though a large percentage remained in the undigested sediment. This suggests that while the mercury is still present, less of it might be absorbed by the body. However, the total mercury content remains unchanged by cooking, so relying on this potential reduction is not a guaranteed method for reducing exposure.
Strategies to Reduce Mercury Exposure
Since cooking is not an effective solution, the most reliable way to minimize mercury exposure is to make informed choices about the fish you eat. The Environmental Protection Agency (EPA) and Food and Drug Administration (FDA) offer clear guidelines to help consumers navigate these choices.
Best practices include:
- Choose fish lower on the food chain: Smaller, younger fish and those lower on the food chain, such as shrimp, salmon, sardines, and pollock, tend to have less mercury.
- Limit high-mercury species: Fish like shark, swordfish, king mackerel, and tilefish are top predators that accumulate high levels of mercury through bioaccumulation.
- Vary your fish choices: Eating a variety of different fish species prevents the cumulative build-up of mercury from a single source.
- Follow advisories: Pay attention to local and statewide advisories, which may have specific recommendations for fish caught in particular water bodies.
- Consider canned tuna options: Canned light tuna generally has lower mercury levels than canned white (albacore) tuna.
Mercury Levels in Common Fish
| Fish Type | Mercury Level | Recommendation | Key Considerations |
|---|---|---|---|
| Swordfish | Very High | Limit/Avoid: Contains some of the highest mercury concentrations. | Large, predatory ocean fish. |
| Shrimp | Very Low | Eat Frequently: Very low mercury levels make it a safe option. | Small seafood, lower on the food chain. |
| Canned Light Tuna | Low | Eat Frequently: A good choice, especially for those who enjoy tuna. | Generally has much less mercury than albacore tuna. |
| Canned Albacore Tuna | Moderate | Limit Consumption: Has more mercury than light tuna, so consume less frequently. | Check local advisories and FDA/EPA recommendations. |
| Salmon | Very Low | Eat Frequently: An excellent source of omega-3s with low mercury levels. | Most salmon species are safe to eat regularly. |
| Shark | Very High | Limit/Avoid: A large, long-lived predator with high mercury levels. | Top of the food chain, accumulates mercury over time. |
Cooking Can Help with Other Contaminants
While cooking does not affect mercury, it is an effective strategy for reducing exposure to other types of contaminants, such as polychlorinated biphenyls (PCBs) and dioxins. These fat-soluble pollutants accumulate in the fatty tissues of fish. To reduce these contaminants, it is recommended to:
- Trim off fat and skin: These are the areas where fat-soluble contaminants concentrate. Removing them before cooking is crucial.
- Choose low-fat cooking methods: Broiling, baking, or grilling on a rack allows fat to drip away from the fillet. The drippings should not be used for sauces.
- Do not reuse oil: If frying, the oil should be discarded after use to avoid reintroducing contaminants.
This approach is a good practice for general food safety, but it's important to remember it does not apply to mercury. The distinction highlights why addressing mercury requires a different strategy centered on selection rather than preparation. The best approach for consumers is a combination of both: choosing lower-mercury fish and using proper cleaning and cooking techniques to reduce other potential contaminants.
Conclusion
In summary, cooking fish does not remove mercury. The toxic metal, primarily in the form of methylmercury, is chemically bonded to the proteins in the fish's muscle and is not affected by heat. While cooking may increase the concentration of mercury due to moisture loss, some studies suggest it might decrease bioaccessibility, though the total amount of mercury remains the same. The most effective ways to protect yourself from excessive mercury exposure are to be mindful of your fish selection by choosing smaller, younger, and lower-mercury species, adhering to consumption advisories, and eating a varied diet. This allows you to reap the significant nutritional benefits of fish while minimizing health risks. National Resources Defense Council (NRDC) Mercury Guide provides additional resources for making safer seafood choices.
