Thiaminase in Raw Fish


Thiamine, or vitamin B1, is an essential nutrient. This means it is required for the body to function properly, and cannot be made or synthesized by the body, and therefore must be obtained in the diet.


Thiamine participates in energy metabolism. Without thiamine, the body can become deprived of this energy, which results in weakness, weight loss, loss of appetite, and fatigue. Eventually, thiamine deficiency will affect the cardiovascular and central nervous systems, resulting in abnormal heartbeat, neurological symptoms, seizures, paralysis, and even death [source, source]. Although thiamine deficiency can be easily reversed by ensuring adequate thiamine is added to the diet, neurological damage and negative effects on learning and behavior can persist [source].

Since it is a water soluble vitamin, thiamine is only stored in the body in small amounts and is easily excreted in the urine. Thus, it is vital that dogs and cats obtain enough dietary thiamine on a regular basis. Cats require 2-4 times as much thiamine as dogs do [source].

Thiamine is present in meats, especially heart, liver, kidney, and pork. It is very sensitive to food processing techniques that involve heat, sulfur-based preservatives, or additives that alter the pH of the food such as thickening agents. Thus, commercial pet food manufacturers must add supplemental thiamine back into the food after processing.

Raw diets, having experienced minimal food processing and no cooking, do not have this issue and generally do not have to be supplemented with thiamine as long as the diet contains heart, liver, and/or kidney in sufficient amounts. However, thawing and refreezing may decrease the amount of available thiamine in the meat [source]. Most raw diets are low in carbohydrates, which may decrease the amount of dietary thiamine required by the body [source, source].


Thiaminase is an enzyme that breaks molecules of thiamine in half, rendering it useless and unable to perform the functions it is required for in the body. Even if the diet initially contained enough thiamine, the body can suffer from thiamine deficiency due to thiaminase activity.

Thiaminase is found in many species of fish and shellfish. Both freshwater and marine species can contain thiaminase.

Species of fish & shellfish that contain thiaminase

Here is a list of some fish and shellfish species that do contain thiaminase:

  • Atlantic menhaden (Brevoortia tyrannus)
  • Atlantic herring (Clupea harengus)
  • Baltic herring (Clupea harengus membras)
  • Broad-striped anchovy (Anchoa hepsetus)
  • Brown bullhead (Amelurus nebulosus)
  • Californian anchovy (Engraulis mordax)
  • Capelin (Mallotus villosus)
  • Channel catfish (Ictalurus punctatus)
  • Common carp (Cyprinus carpio)
  • Chub mackerel / Pacific mackerel (Scomber japonicus)
  • Clams (family Veneridae)
  • Gulf menhaden (Brevoortia patronus)
  • Lake whitefish (Coregonus clupeaformis)
  • Lobster (Homarus americanus)
  • Mediterranean mussel (Mytilus galloprovincialis)
  • Rainbow smelt (Osmerus mordax)
  • Round whitefish (Prosopium cylindraceum)
  • Ruby snapper (Etelis carbunculus)
  • Scaled sardine (Harengula jaguana)
  • Scallops (Placopecten grandis)
  • Skipjack tuna (Katsuwonus pelamis)
  • White bass (Morone chrysops)
  • Yellowfin tuna (Neothunnus macropterus)

Here is a list of some species that don’t contain thiaminase:

  • Atlantic cod (Gadus morhua)
  • Atlantic hake / silver hake (Merluccius bilinearis)
  • Atlantic halibut (Hippoglossus hippoglossus)
  • Atlantic mackerel (Scomber scombrus)
  • Atlantic salmon (Salmo salar)
  • Black crappie (Pomoxis nigromaculatus)
  • Black sea bass (Centropristis striata)
  • Bluegill (Lepomis macrochirus)
  • Brown trout (Salmo trutta)
  • Cisco/lake herring (Coregonus artedi)
  • Coho salmon (Oncorhynchus kisutch)
  • Haddock (Melanogrammus aeglefinus)
  • Hake (Urophycis spp)
  • Hardhead catfish (Ariopsis felis)
  • Lake trout (Salvelinus namaycush)
  • Largemouth bass (Micropterus salmoides)
  • Lemon sole (Microstomus kitt)
  • Mullet (Mugilidae spp)
  • Northern pike (Esox lucius)
  • Pollock/Pollack (Pollachius pollachius, Pollachius virens)
  • Pond smelt (Hypomesus olidus)
  • Rainbow trout (Oncorhynchus mykiss)
  • Redfish / red perch / rose fish (Sebastes norvegicus)
  • Rock bass (Ambloplites rupestris)
  • Silver seatrout (Cynoscion nothus)
  • Smallmouth bass (Micropterus dolomieu)
  • Southern kingfish / king whiting (Menticirrhus americanus)
  • Tilapia (Oreochromis spp)
  • Winter flounder (Pseudopleuronectes americanus)
  • Yellowtail flounder (Limanda ferruginea)
  • Yellow perch (Perca flavescens)
  • Yellow pike / walleye (Sander vitreus)

Take note that whether or not a fish contains thiaminase can differ even amongst different species of the same type or family of fish: for example, Pacific mackerel does contain thiaminase while Atlantic mackerel does not.

For a more inclusive list, see Nutrient Requirements of Warmwater Fishes and Shellfishes, page 60, table 22 (National Research Council, 1983).

Reducing the risk of thiamin deficiency due to thiaminase

Cooking reduces thiaminase activity, but freezing has no effect. The longer a food that contains thiaminase is stored, the more thiamine is rendered unavailable. The greatest content of thiaminase is found in the viscera, particularly the liver, spleen, intestines, and heart of the fish [source].

Thiamine is destroyed after prolonged exposure to thiaminase; therefore, feeding fresh fish that has not been stored for any significant length of time will reduce the risk of thiamine deficiency (but would also carry the risk of parasites if the fish is not frozen before feeding). Feeding thiaminase-containing fish separately from other foods, such as in separate meals or on different days, or storing the fish in separate containers than other foods, will also reduce the risk of thiamine deficiency since the thiaminase in the fish will not have had time to break down the thiamine in the rest of the diet.

To prevent thiamine deficiency, you should do one or more of the following:

  • only feed raw thiaminase-containing fish as a small portion of the overall diet,
  • don’t store raw thiaminase-containing fish in the same containers as other foods,
  • don’t include raw thiaminase-containing fish in every meal,
  • feed raw thiaminase-containing fish in separate meals or on separate days from other foods,
  • cook thiaminase-containing fish before feeding, or
  • avoid feeding raw thiaminase-containing fish completely.

4 thoughts on “Thiaminase in Raw Fish

  1. Thanks a lot, this helped me for my talk to a group of aquarists about nutrition! Most of their diets in aquariums have to be raw and nutritionally balanced!

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