Fish & Shelfish

Mercury and Cadmium in Fish and Shellfish

Draft by: Dr. Su Fairchild MD

We have been told in the past that eating fish is good. Fish are a good source of lean protein, and many contain healthy omega 3 fats that can help reduce your bad cholesterol and be good for your health. However, this may no longer hold as true due to increasing amounts of toxicity in the oceans.

The issue of environmental toxicity
Heavy metals, PCB’s (polychlorinated biphenyl), and other environment toxins accumulate in the oceans, and fish and shellfish tend to accumulate these toxins in varying amounts.

Eating the wrong kinds of seafood too often can raise the levels of mercury, cadmium, and PCB’s in your body. Pregnant and breastfeeding women need to be extra careful because fetuses and newborns are more easily hurt by mercury and other poisons.

Mercury is associated with myocardial infarction, and any benefit from omega 3′s on heart attack risk is totally counteracted by methylmercury.

Methylmercury has a half life in blood of 50-70 days, so mercury blood levels can be useful for monitoring exposure.

Cadmium is associated with myocardial infarction, kidney damage, breast and other cancers, and is also associated with learning disabilities in children. It can be detected by urinary cadmium, but can also be stored in tissues and not be detected. It is well excreted in sweat.

Cadmium in our diet comes from shellfish, fish, liver, and kidney meats, as well as seaweed, cereals, nuts, oilseeds (sunflower seeds) and pulses, and chocolate.

Genetics and environmental toxicity
Some people are also genetically less able to clear various toxins from their bodies. These people need to be extra careful, and do well to limit exposure as best they can. I do offer such genetic testing in my office. However, everyone, no matter what their genetics, will do best by limiting toxic exposure.

People at more risk
Pregnant and nursing mothers, as well as young children, need to be very careful about their fish consumption.
Pregnant women, as well as children under 6, should not eat more than 2 servings of fish per week, and should limit themselves only to fish with the very lowest mercury content.

The problem with seafood
All fish contain varying amounts of mercury and other toxins. Some fish, however, contain higher levels of mercury, and eating them more frequently can contribute to health problems. Higher fish intake does correlate with having higher mercury levels.

Also beware of many farmed fish, especially salmon, which can have very high levels of PCB’s. Farmed salmon can be one of the most toxic-laden foods available, with regular commercial beef possibly coming in second.

Wild salmon has less toxic PCB’s than farmed salmon. Only eat wild caught salmon if possible. Any salmon at a restaurant is presumed to be farmed unless the chef can tell you the name of the wild salmon species (Pink, Coho/Silver, Chum/Calico/Keta, Sockeye/Red, Chinook/King). Note that King salmon, being bigger, tend to have more mercury.

Shellfish may have less mercury, but make up for it with more cadmium and arsenic. The arsenic in shellfish tends to be the less harmful organic arsenic, but people with genetic weaknesses in methylation may not be able to clear organic arsenic as well.

Shellfish and fish can also accumulate BPA (bisphenol A), and atrazine. Bivalve mollusks can accumulate BPA, with the accumulation and elimination rates of the mono and disulfate conjugates of BPA being slower than those of BPA.

One study tested 1,392 specimens of different species of cephalopod molluscs (broadtail squid, spider octopus, curled octopus, horned octopus, elegant cuttlefish, and pink cuttlefish) for cadmium concentrations. In all species, mean cadmium concentrations were higher in hepatopancreas (digestive gland) than in the flesh. Pink cuttlefish and spider octopus had the highest concentrations of cadmium (do not eat them) and the lowest concentrations were encountered in broadtail squid.

Which fish tend to have more mercury?
Big fish tend to have more mercury for the simple reason that big fish usually live longer and have more time to accumulate toxins in their bodies. Other factors can include the diet of the fish (carniverous versus vegetarian), and the region the fish lives in. Albacore tuna from Thailand tends to have the highest mercury content of all albacore. The Gulf of Mexico is a heavily contaminated region, and it is best to avoid any seafood from there.

Do not eat more than 2 servings a week:
Anchovies 0.017 PPM
Butterfish 0.058 PPM
Catfish 0.025 PPM (but often contaminated with other toxins)
Clam/Hamaguri 0.009 PPM
Crab 0.065 PPM
Crawfish 0.033 PPM
Croaker, Atlantic 0.065 PPM
Flatfish/Karei 0.056 PPM
Haddock (Atlantic) 0.055 PPM
Hokkigai/surf clam
Ikura/salmon roe
Mackerel (N. Atlantic) 0.050 PPM
Mullet 0.050 PPM
Oyster 0.012 PPM
Pollock 0.031 PPM
Salmon (choose wild pacific) 0.022 PPM
Sardine (but high in PCBs) 0.013 PPM
Scallop 0.003 PPM
Shad, American 0.045 PPM
Shrimp (but highest in cadmium) 0.009 PPM
Squid 0.023 PPM
Tilapia (often farmed, but are vegetarian, so are much cleaner than farmed salmon) 0.013 PPM
Whiting 0.051 PPM

Do not eat more than 1 serving a week:
Buffalofish 0.137 PPM
Carp 0.110 PPM
Cod 0.111 PPM
Herring 0.084 PPM
Lobster 0.107-0.166 PPM (North American has lower mercury)
Mackerel/Saba (Pacific) 0.088 PPM
Mahi Mahi
Perch, Ocean 0.121 PPM
Sheepshead 0.093 PPM
Skate 0.137 PPM
Tuna (Canned Chunk light)
Whitefish 0.089 PPM

Bass, Saltwater/Black/Striped 0.152 PPM
Halibut 0.241 PPM
Monkfish 0.181 PPM
Perch, Freshwater 0.150 PPM
Scorpionfish 0.233 PPM
Sea Bass 0.152 PPM
Snapper 0.166 PPM
Tilefish, Altantic 0.144 PPM
Weakfish/Sea Trout 0.235 PPM

Avoid or limit if possible:
Bass, Chilean 0.354 PPM
Bluefish 0.368 PPM
Croaker, White (Pacific) 0.287 PPM
Grouper 0.448 PPM
Stablefish 0.361 PPM
Tuna/Toro (Canned Albacore, Yellowfin) 0.354 PPM

Do not eat if possible:
Mackeral, Spanish (Gulf of Mexico) 0.454 PPM
Mackeral King 0.730 PPM
Marlin 0.485 PPM
Orange Roughy 0.571 PPM
Shark 0.979 PPM
Swordfish/Kajiki 0.995 PPM
Tilefish (Gulf of Mexico) 1.450 PPM
Tuna (Ahi)
Tuna, Bigeye 0.689 PPM

Other than avoidance
N-acetyl cysteine (NAC) can increase the urinary excretion of methylmercury, so take it with any seafood meal. NAC 600 mg twice daily is a good idea for most people, especially those living in urban areas.

Su Fairchild, MD
Integrative and Orthomolecular Medicine
Environmental Medicine

Office: 3022 Javier Road, Fairfax, VA 22031
Phone for appointments:  571-344-4673
Appointments available: Monday through Thursday, 10 am to 5 pm.

For more information on this and other subjects, please visit Dr Fairchild’s Integrative Medicine page

The information in this post is for educational purposes only, and is not a substitute for proper medical care.