Is fish all it's cracked up to be?

As a nutritionist, I am always recommending oily fish as a source of Omega-3 essential fatty acids. and a range of other vitamins and minerals. 

Sprats, mackerel, sardines....they're always at the top of my list for necessary additions to an 80-10-10 diet. 

I rarely suggest salmon oil due to it being so unstable, quickly going rancid and causing more problems within the body than it prevents. (I do however, recommend it if a therapeutic dose of Omega-3 is required with brand, storage and usage precautions).

However, I have stopped recommending dog owners add fish to their dogs meals. Instead, I recommend it is fed at a different time. Why? Thiaminase.

When we talk about balanced canine nutrition, most people think of protein levels, calcium-to-phosphorus ratios, omega-3 fatty acids, etc.  But there’s one nutrient that often flies under the radar—thiamine, also known as vitamin B1. 

And one enzyme found in certain fish—thiaminase—can deplete this essential vitamin.

Thiamine is a water-soluble B-vitamin required for nearly every cell in your dog’s body. Because the body cannot store large amounts, dogs must receive thiamine regularly from their diet.

Energy metabolism: Converts carbohydrates into usable energy.

Nervous system support: Key for neurotransmitter production and nerve impulse  conduction.

Healthy digestion: Supports stomach acid production.

Cardiac function: The heart relies heavily on thiamine for energy.

Thiaminase is an enzyme that breaks down thiamine, rendering it inactive and unusable by the body. Some foods naturally contain thiaminase and, when fed in significant quantities, it can contribute to thiamine deficiency.

Cooking destroys thiaminase, so cooked fish rarely pose a problem. The concern is primarily with raw. Yet raw fish is the type I recommended as Omega 3s are destroyed at high temperatures. 

Fish species vary widely in their thiaminase content.

Fish known to contain thiaminase include: 

Fish that do not contain thiaminase 

Thiaminase does not stay active in your dog’s body. It only works during digestion and, therefore, it can only break down thiamine present in that specific meal

The danger comes from regular exposure. Each raw thiaminase-containing meal reduces available thiamine, so repeated feeding can eventually lead to deficiency.

Once digestion is complete, thiaminase is broken down like any other dietary protein and poses no further risk.

Definitely not! In my opinion, it is still an essential component of a dog's diet. As stated earlier, it is a brilliant source of Omega-3s and these are destroyed by cooking so raw fish is a must!  However, it important that thiaminase is understood so it doesn’t affect our dogs’ health.

Avoid feeding thiaminase-containing fish raw on a regular basis. Feeding occasional isn’t going to harm, but if raw fish is fed in every meal, it will start to cause nutritional problems. 

If you need to feed thiaminase containing fish on a regular basis (maybe during an elimination diet), then I would definitely recommend cooking to break down the enzyme. Preferably, choose a non-thiaminase species (but be careful, some fish like salmon can have high content of heavy metals which pose their own risk!).

For dogs who eat large amounts of fish (e.g., in homemade diets), ensure the recipe includes adequate thiamine supplementation.

The best way to deal with the issue, is to feed fish  separately from meals. As previously mentioned, thiaminase doesn’t stay active for long and only affects the current meal. Feeding oily fish at other times ensures your dog gets plenty of Omega 3s while limiting the effects of thiaminase. So feed as an extra in-between meals rather than with the meal.

Raw fish can be an excellent source of omega-3 fatty acids, high-quality protein, and minerals. But chronic exposure to thiaminase can deplete your dog’s thiamine levels over time.

Balancing these factors ensures dogs receive the benefits of fish without risking deficiency.

Thiaminase isn’t something to fear—just something to understand. With proper fish selection and feeding practices, you can safely incorporate fish into your dog’s diet while protecting their essential thiamine status.