Greenland shark meat is toxic because it contains extremely high concentrations of two nitrogen-based compounds: urea and trimethylamine N-oxide (TMAO). These chemicals accumulate in the shark’s muscle tissue as a survival adaptation to deep, cold Arctic waters. When eaten fresh, the meat causes symptoms similar to severe drunkenness, including nausea, vomiting, and neurological effects sometimes described as “shark drunk.” The toxicity isn’t a defense mechanism. It’s a byproduct of how the shark’s body manages pressure and salt balance in one of Earth’s harshest environments.
How Deep-Sea Survival Creates Toxic Flesh
All sharks and rays retain urea in their tissues to balance the salt concentration of seawater, a process called osmoregulation. Freshwater fish simply excrete urea as waste, but elasmobranchs (sharks, skates, and rays) keep it circulating in their blood and muscles. Greenland sharks take this to an extreme. They live at depths ranging from near the surface to over 2,000 meters, in water temperatures barely above freezing, and their bodies stockpile unusually large amounts of both urea and TMAO to cope.
TMAO serves a specific purpose at depth: it protects proteins from being crushed out of shape by hydrostatic pressure. As ocean depth increases, pressure rises dramatically, and proteins in a fish’s cells can unfold and stop working. TMAO counteracts this by pushing water molecules toward protein surfaces, essentially keeping proteins tightly folded and functional. Research on deep-sea cartilaginous fish shows that TMAO concentrations increase almost linearly with depth, reaching levels around 250 to 289 millimoles per kilogram in species living below 1,900 meters. Meanwhile, urea concentrations actually decrease at greater depths, dropping from roughly 290 to 370 millimoles per kilogram in shallow species down to 170 to 189 in the deepest groups.
The result is that Greenland shark muscle tissue is saturated with these compounds in concentrations far higher than you’d find in most other fish. The ratio of urea to TMAO shifts depending on depth, but the total load of both chemicals remains substantial regardless of where the shark is caught.
What Makes TMAO Dangerous to Humans
TMAO itself isn’t the main problem. The real danger comes from what happens to it inside your body. When you eat fresh Greenland shark meat, bacteria in your gut convert TMAO into trimethylamine, or TMA, through a form of anaerobic respiration. Specific groups of gut bacteria, primarily Proteobacteria like Escherichia and Klebsiella species, use an enzyme called TMAO reductase to strip oxygen from the molecule, turning it into TMA.
TMA is the compound responsible for the toxic effects. It’s a potent organic base that gets absorbed from your intestine into your bloodstream. In small amounts, your liver can convert TMA back into TMAO and clear it safely. But the sheer volume of TMAO in Greenland shark meat overwhelms this system. The flood of TMA entering the blood produces effects on the nervous system that Icelandic and Inuit sources have long compared to extreme intoxication: disorientation, stumbling, convulsions, and in severe cases, intestinal distress that can last for days. Sled dogs fed fresh Greenland shark meat have reportedly been unable to stand.
Urea adds to the problem. It breaks down into ammonia, another toxic compound, especially as the meat sits after the shark dies. The combination of high TMA and ammonia makes fresh Greenland shark meat one of the most reliably toxic fish products in the world.
How Fermentation Neutralizes the Toxins
Despite all of this, people have been eating Greenland shark for centuries. The traditional Icelandic preparation, hákarl, uses a long fermentation and drying process to break down the dangerous compounds before the meat reaches anyone’s plate.
The process works like this: shark meat is buried in a shallow pit, often covered with gravel and sand, and left to ferment for six to twelve weeks. During this time, bacteria in the meat convert TMAO into TMA and dimethylamine (DMA), and the urea breaks down into ammonia. Research tracking the chemical changes during hákarl production found that TMAO levels dropped below detectable levels within just five weeks. The pH of the meat rises from about 6 to 9 as these alkaline breakdown products accumulate, and specific bacterial genera, including Pseudomonas and Tissierella, drive the conversion.
After the pit fermentation, the meat is hung to dry for several more months. This drying phase allows much of the TMA and ammonia to evaporate off. What remains is meat with a powerful smell (often compared to strong cleaning products) but dramatically reduced toxicity. The fermentation essentially does outside the body what gut bacteria would do inside it, but in a controlled way that lets the volatile toxic byproducts dissipate into the air rather than into your bloodstream.
Why Greenland Sharks Are Uniquely Problematic
Other shark species also contain urea and TMAO, so why is Greenland shark singled out as especially toxic? The answer comes down to concentration. Most shallow-water sharks carry enough urea that their meat tastes slightly of ammonia if not handled properly, but the levels aren’t high enough to cause serious poisoning. Greenland sharks combine the baseline urea load common to all elasmobranchs with the elevated TMAO levels that come from living in deep, near-freezing water. They also grow extraordinarily slowly and live for centuries (some individuals may exceed 400 years), meaning their tissues have had an unusually long time to accumulate these osmolytes.
Their size compounds the issue. Greenland sharks can reach over five meters in length, producing large volumes of meat that all carry the same high chemical load. Historically, this made them a valuable food source in Arctic communities where calories were scarce, but only if the community had already developed the knowledge to process the meat safely. Cultures without that tradition learned quickly that fresh Greenland shark meat was not worth the risk.