Histamine is a chemical compound produced naturally in the body, acting as a signaling molecule involved in immune responses, digestion, and neurotransmission. When immune cells detect a threat, they release histamine, causing symptoms like flushing and swelling. Histamine is also present in many foods, and consuming high levels can cause adverse reactions in some individuals. This raises the question of whether shrimp, a common seafood, contains enough histamine to be a concern.
How Histamine Forms in Seafood
Histamine is not naturally present in large amounts in live seafood; it is created after the organism dies. This formation is a spoilage process driven by certain types of bacteria that possess an enzyme called histidine decarboxylase.
The enzyme acts on the naturally occurring amino acid histidine, found in the muscle tissue of fish and shellfish. Histidine decarboxylase converts this amino acid into histamine, a type of biogenic amine. This reaction is the basis of scombroid poisoning, which can occur in many types of seafood.
The speed and extent of histamine production depend on time and temperature. Warmer temperatures allow the bacteria to multiply rapidly, accelerating the conversion process. If seafood is not immediately and consistently chilled after harvest, histamine levels can quickly increase to problematic concentrations.
Are Shrimp High in Histamine?
Shrimp are not inherently high in histamine when caught and handled properly. Freshly harvested shrimp typically contain very low levels, comparable to other non-fermented foods. However, shrimp are categorized as a high-risk food because they are highly susceptible to rapid spoilage.
Their delicate nature means their histamine content is highly variable, unlike foods like aged cheese or sauerkraut, which are consistently high due to fermentation. The main danger is the potential for rapid accumulation if the cold chain is broken. This makes them a high-risk item for histamine-related illness if left unrefrigerated for even a short time.
The free histidine content in shrimp is generally lower than in dark-fleshed fish like tuna or mackerel, which are classic high-histamine culprits. Despite this, the rapid growth of histamine-producing bacteria can quickly elevate concentrations. Flash-frozen shrimp, processed immediately after being caught, generally retain the lowest and safest histamine levels for consumption.
Histamine Intolerance Versus Allergic Reaction
It is important to distinguish between histamine intolerance and a true IgE-mediated shellfish allergy. A true allergy is an immune system response where the body mistakenly identifies a specific protein in the shrimp flesh as a threat. This triggers the release of IgE antibodies and massive amounts of the body’s own histamine, leading to potentially life-threatening symptoms like anaphylaxis.
Histamine intolerance is a pseudo-allergic reaction caused by consuming an excessive amount of histamine from food. This occurs when the body lacks sufficient levels or activity of the enzyme Diamine Oxidase (DAO), which is the primary enzyme responsible for breaking down ingested histamine in the gut. Another enzyme, Histamine N-methyltransferase (HNMT), also helps metabolize histamine inside cells.
When the dietary histamine load is too high for the available DAO to handle, the excess histamine is absorbed into the bloodstream. This buildup can cause a range of symptoms that mimic an allergy, including flushing, headaches, hives, gastrointestinal distress, and heart palpitations. This reaction is a toxic response to the chemical overload, not an immune response to the shrimp protein itself.
Minimizing Histamine Content When Buying and Preparing
Since histamine formation is a function of bacterial growth, controlling temperature is the only effective way to keep levels low. When buying shrimp, the safest choice is to select products that were flash-frozen immediately after harvest. If purchasing fresh shrimp, ensure they are displayed on a thick bed of flaked ice and are firm, not slimy or discolored.
After purchasing, rapid cooling is essential. Place shrimp in the coldest part of your refrigerator and plan to cook them within 24 hours. Never thaw shrimp at room temperature, as this provides an ideal environment for bacterial histamine production. Thaw them slowly overnight in the refrigerator or quickly under cold running water immediately before use.
A crucial point is that once histamine has formed, it is heat-stable and cannot be destroyed by cooking. Applying heat will kill the bacteria, but the histamine molecule remains in the food. Therefore, prevention through strict cold storage is the only way to minimize the risk of high histamine content.