So when you see a piece of fish labeled sushi- or sashimi-grade, that means that the seller has judged it safe to eat raw. The claim is only as trustworthy as the fish market that makes it. Regulations regarding fish sold for raw consumption vary from state to state, although every state points to FDA guidelines as the gold standard; the key difference between the states is whether those guidelines are enforced.

While those recommendations are primarily focused on limiting pathogenic bacterial growth more on that below , they do include rigorous specifications for killing parasites. Sushi restaurants and fish markets use what's called a "super freezer," which is exactly what it sounds like: a freezer that maintains super-cold temperatures. This FDA table details the species-specific risks of live parasites in fish. But the information here is a little misleading, and meant to serve more as an agency warning about the perils of mislabeling fish than as a practical guide to which fish must be frozen prior to raw consumption.

Exempted from the FDA's freezing requirements are, as Herron mentions, large species of tuna—deemed safe based on the frequency with which they are eaten in raw form and the infrequency of related, documented parasitic infection—as well as aquacultured fish, like salmon, given verification that the feed it's raised on is parasite-free. To meet FDA guidelines, every other type of fish must be frozen to those temperatures, even if the table does not indicate that it carries a parasite risk, because it "may have a parasite hazard that has not been identified if these fish are not customarily consumed raw or undercooked.

It's a paradox: The FDA will not deem a fish free from parasite hazards, and thus safe to eat raw without freezing, unless that fish is eaten raw, without being frozen, frequently enough to present sufficient evidence of its safety.

To Luke Davin, the general manager of Osakana, this standard means that "deviating from [the FDA's] 'freeze it all' approach puts the burden of testing and proof on the processor. The easiest solution, then, is simply to freeze everything. Despite the FDA's blanket recommendations for the elimination of parasites, which is the main goal of its freezing guidelines, very few infections from eating raw fish have been documented in American medical literature.

In the US, eating raw fish that hasn't been frozen is rare enough that the agency's "Bad Bug Book" uses Japan as a reference point, since the practice is far more prevalent there. But even in Japan, where freezing of fish meant for sashimi is not required, reported infection rates are vanishingly small compared to the total population.

Because some infections are asymptomatic, and many are thought to go unreported, the risk of infection may be greater than statistics suggest. On a less scientific level, worms—particularly parasitic worms—inhabit dark recesses of our collective imagination. The idea that eating a piece of seemingly pristine, delicious fish carries a risk of infestation by alien-like organisms is enough to give anyone—including health authorities equipped with all the relevant, fear-assuaging data—the heebie jeebies.

Both Haraguchi and Herron point out that, in certain cultures, fish has long been served raw despite never having been frozen at all, and neither was bothered by the idea of eating fish under those circumstances though they stress that the fish they sell for raw consumption has been frozen according to FDA guidelines.

Of parasites, Haraguchi says, "It's natural. Parasites are as natural as seeing a ladybug in farmers market vegetables. I'm fine, you're fine. And everybody was doing that before these regulations came out, and everyone was okay.

But there are caveats: Freshwater fish and some anadromous fish—fish, like salmon, that divide their life cycles between fresh and salt water—are susceptible to broad fish tapeworms, which are widely considered more harmful than other parasitic worms.

Few experts recommend eating fish in the cod family—particularly Atlantic cod, but also Pacific cod, haddock, and pollack—since they're highly susceptible to infection by a range of parasites. According to Haraguchi, there's another reason fish in the cod family are not eaten raw: "There's so much moisture [in the flesh], it doesn't taste good.

The parasites that infect most marine fish are nematodes, or roundworms, from the genus Anisakis. It's preferable to avoid eating them, of course, but conventional wisdom says that the stray live anisakid in your gut will, at worst, provoke some discomfort—nausea and possible stomach pain, similar in kind to a bout of food poisoning.

Judy Sakanari, a parasitologist in the Department of Pharmaceutical Chemistry at the University of California, San Francisco, believes that this view downplays the risks of ingesting anisakids.

Sakanari stresses that understanding the life cycle of the parasite is necessary to a full appreciation of the risks involved. All parasites seek to reach their end-host organisms. For tapeworms, these are bears and other fish-eating mammals; for anisakids, they're marine mammals, such as whales, seals, and dolphins.

If all goes as the parasite gods intend, fish carrying infectious worm larvae will be consumed by an end-host organism.

But if those fish are snatched up in a trawler or caught on a line, the ideal parasitic life cycle is interrupted. As soon as the fish's body temperature begins to rise to that of the end-host mammal, the parasite larvae in its gut will attempt to find a way out, leading them to burrow into the fish's flesh.

This is one reason why it's always best to keep ungutted fish cold: Any parasite larvae in the fish gut will remain immobile as long as the temperature is sufficiently low. That impulse to find a more hospitable environment, Sakanari says, is what makes anisakids particularly worrisome for humans.

The human body is sufficiently different from that of whales and elephant seals—typical anisakid end hosts—that it forces the worms to wander around inside of it. As they do so, they probe along the intestinal wall, trying to penetrate it and sometimes getting stuck in the process, which can necessitate resection. Interestingly, because humans are a natural end host for tapeworms, Sakanari says that tapeworm infection, as disgusting as it might sound, would be preferable to larval anisakid infection.

The pathologies associated with the adult fish tapeworm infection are by and large less severe, and can be treated with a simple anthelmintic. Sakanari notes that preparations like ceviche, in which fish are submerged in an acidic bath, do nothing to kill off anisakids, since they thrive in highly acidic environments. Candling—in which a strong light is shined through thin fish fillets placed on a glass, in order to spot parasites to be removed—is also not foolproof: Sakanari describes an experiment in which she and her colleagues examined a piece of rockfish using this method and determined that it was free of parasites.

After cooking, they then flaked the fish fillet and examined it, and found that they had in fact missed several worms. Even experts can fail to completely deworm a fillet. The upshot of all this: The only real way to be sure that you've eliminated any parasites in the flesh is by using temperature. That's the bottom line," Sakanari says. Is it worth the risk to eat raw fish that hasn't been properly frozen?

Haraguchi and Herron agree that parasites in raw fish are less of a concern than bacterial contamination. By "temp-abuse," or temperature abuse, Herron means that the fish could be kept at unsafe temperatures for a long enough period of time to encourage the growth of pathogenic bacteria. Bacterial strains of all kinds are worrisome to health authorities, but some are specific to certain kinds of fish. Histamine is not eradicated by cooking or freezing, so it's a particular concern for fish purveyors.

In addition, fish processors and markets must limit the introduction of pathogens, which means that those who work with the fish must work clean—in clean facilities, with clean tools and clean hands—and minimize their contact with the fish flesh. Fish sellers have a vested interest in keeping their product as pristine as possible, to maximize their chances of selling it before it goes bad. Knowledgeable sushi chefs will easily detect anisakis larvae, which is quite visible in raw fish.

Looking for the cheapest sushi may not be your best bet in this case. Reputable restaurants with trained staff will also know how to properly handle fish. The FDA has recommendations for the transportation and storage of sushi fish. It should also be flash frozen prior to preparation. Flash freezing fish at a low temperature kills parasites and prevents infection. Some fish are simply a no-no for eating raw or sushi style; that includes freshwater fish like pike, yellow perch, and brook trout.

Never eat these fish as sushi — they must be thoroughly cooked to degrees Fahrenheit before consumption. Tuna is often considered a safer option for sushi. The average person is not trained to handle fish correctly. The excess histamine does not get destroyed during heating and can cause allergic reactions and even anaphylaxis.

Sushi is most definitely not a health food, at least not given our current food supply. Hundreds or thousands of years ago, when our waters were clean and unpolluted, sushi would have been a health food.

Fish is rich in omega-3 fatty acids which is good for your heart and the brain but given the number of potential risks from eating fish, it is probably best to buy a high-quality fish oil supplement that has been rigorously tested to ensure that it is free of mercury and other toxins.

The best way by far to mitigate health risks is to not eat sushi. If you are not willing to give it up, there are a few things to keep in mind:. The fish should be flash frozen solid and stored in a commercial freezer for at least 15 hours to kill whatever parasites might be there.

There really isn't a "best" fish when it comes to sushi because each fish has its own set of issues. To avoid mercury, choose smaller fish and salmon. To avoid parasites, tuna might be best because it rarely acquires parasites due to its life cycle.

But tuna is high in mercury so it would not be a good choice for other reasons. Even wild salmon might have similar issues depending on where it comes from. Two of my favorite fish that have the lowest amounts of toxins and are great sources of omega-3 fatty acids and protein are not eaten as sushi.

They are anchovies and sardines. Is Sushi Safe To Eat?