The scenario: Your friend loves sushi like most people love their children, pets, and sports teams. Their eyes bore into a spicy salmon roll with the intensity of a wild animal sizing up its prey. They appear to eat the stuff for every meal.
You’ve never seen someone stuff that much raw fish, seaweed, and rice down their gullet. It’s awe-inspiring and a bit nauseating. Doesn’t fish have mercury? And the fact that it's raw has to be upping their chance of acquiring some kind of parasite, right?
The facts: Sushi is, for the most part, perfectly good for you, what with its healthy fats and punch of protein. But then things happen like a man in Fresno pulling a five-foot tapeworm out of his rectum because he got a parasitic infection from eating sushi and you start to wonder.
There can be some nasty side effects to eating raw fish, including exposure to environmental contaminants, parasites such as Anisakiasis or Diphyllobothrium nihonkaiense, and disease-causing bacteria such as Vibrio parahaemolyticus, Vibrio vulnificus, and Mesophilic Aeromonas, which can make you shit-your-pants sick. Salmonella and Listeriosis can also be contracted from eating sushi gone bad.
What’s likely to happen: Parasitic infections are rare, largely because any sushi chef worth his salt will freeze potentially parasite-carrying fish at parasite-killing temps before serving it, per FDA regulations. Of course, probability is a factor here; the more sushi you eat the higher your chances of contracting something rare, like that Fresno man (who says he ate sushi every day) did, but it's still unlikely. Food-borne illness isn't unheard of, but according to one study of 250 sushi samples, salmonella occurred in 1.6 percent of them and listeria 1.2 percent. So not huge risks there, and most of these predicaments can be treated with drugs or will go away over time.
The more likely danger in eating a shit-ton of sushi comes when certain substances accumulate within the body. Research has shown that people who frequently eat a lot of high-mercury fish can experience damage to their central nervous systems. Mercury accumulates through the food chain, explains Roxanne Karimi, a research scientist at the School of Marine and Atmospheric Sciences at Stony Brook University. “Mercury, in the form of methylmercury, is very sticky. Once you consume it in your food it tends to stay in the body for a long time,” she says. “It builds up and that can happen all along the food chain. So the higher you go in the food chain the higher the levels of mercury are,” she says. These are fish like bigeye tuna, swordfish, marlin, tilefish, King mackerel and sea bass.
Karimi adds that the level of mercury can also vary based on the provenance of the fish. “We did a study which found that wild fish tend to have much higher mercury levels than fish of the same type that are farmed.” The study found that wild seafood had between two and 12 times higher concentrations of mercury than the farmed version, depending on the seafood item. So depending on what kind of fish, where it comes from and how much of it you’re eating, you could go over the limit of mercury and suffer some not so pleasant consequences. Also, keep in mind that children and pregnant women are more sensitive to the risks of mercury poisoning.
In addition to mercury, fish accumulates other chemicals, such as polychlorinated biphenyls (PCBs), which have been classified as probably carcinogenic in humans. According to a 2004 report, the PCB levels in farmed salmon averaged about five times the safe EPA standards—and when did you last see wild salmon on the menu at a sushi place? (That's right, it's almost always the farmed variety.)
The worst that could happen: According the Environmental Protection Agency, too much methylmercury is toxic to the human brain, kidney, liver, heart, and nervous system. At lower levels of exposure symptoms include: sleep disturbance, headache, fatigue, difficulty with memory and concentration, poor coordination and neuropathy. At high levels you can go blind, deaf and suffer “impaired consciousness.” And then there are the cancer associations with the aforementioned PCBs.
What to tell your friend: The EPA estimates the minimum lethal dose of methylmercury for a 154-lb (70-kg) person is between 44 and 132 mg/lb (20 to 60 mg/kg). So a 26-year-old female who weighs about 130 pounds, isn’t pregnant, and doesn’t have heart disease is cleared to eat up to three 4-ounce servings of salmon per week if she eats no other seafood—but only one serving of yellowfin tuna per week (that's according to this handy calculator; go ahead and try it with your own deets). That's 12 ounces of salmon, which amounts to anywhere between 12 and 24 pieces of salmon sushi, depending on their size.
And that seems like the universal consensus: the Food and Drug Administration and the EPA both recommend consuming no more than three servings—again, around 12 ounces—of the safer stuff for pregnant women, which would be well within a safe range for the rest of us. The FDA's serving suggestions are for low-mercury fish; they urge you to stay away from high-mercury fish like shark, swordfish, king mackerel, and tilefish, and big-eye tuna (which, along with yellowfin, are most commonly used in sushi in the US) to limit the amount of mercury you’re exposing yourself to.
Also: If you’re going to sushi-block your fiending friend, tell them not to be an idiot and avoid the really obviously suspect stuff. One study in Norway found that 71 percent of grocery store sushi had the bacteria Mesophilic Aeromonas spp., which is known to cause “acute diarrhea.” (And somehow it seems unlikely that US supermarkets are doing much better.)
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Correction: A previous version of this story said that Listeria occurs in 1.6 percent of sushi samples. That figure has since been corrected to 1.2 percent.