Just take a walk down the pet food aisle and you’ll see just how many cat products contain fish. Despite how common it is, feeding cats fish is divisive, with some strongly advocating to fed it only occasionally, or not to feed it at all. Let’s look at whether cats should eat fish and the risks and benefits.
Let’s consider the risks when feeding fish.
Fish is not normal prey for cats. The semi-domestication of cats is thought to have occurred several times around different regions of the world, independently of each other. But the most referenced origin story is the divergence of our housecats from the African wildcat, a cat native to Africa, West and Central Asia. For the African wildcat, it’s native habitat extends from grasslands to savannas to deserts; the concern is that fish do not make up the diet of the African wildcat and so it is therefore “unnatural” to be feeding them to our cats. But let’s think about that reasoning. Putting aside those cats that hunt, we don’t often feed our cats their most natural prey…small rodents and small birds. And while “large mammals” have been described as being part of the African wildcat’s diet, it is certainly not the majority of their diet (1). So, the beef, venison, lamb, kangaroo, and pork that we commonly feed our cats today are also not major staples of the diets of our cat’s closest wild relatives. Also, cats are remarkedly adaptable, as illustrated by the European wildcats, which can widen their prey network, including to fish, depending on their environment (2).
Fish is a common food allergen. Any protein can cause an allergy. It’s true that some food proteins could be more allergenic than others. Things such as the number of IgE-binding epitopes on the protein, its resistance to digestion, post-translational modifications (or in other words, changes the shape or conformation of the protein), all of these can play a role in increasing the likelihood of a protein becoming an allergen for any given individual (3,4). But for a food to be considered a common allergy, that requires lots of people (or in this case, cats) being exposed to it and developing a food allergy. The most commonly fed foods are going to be the ones that are common causes of allergies. In cats with allergic skin disease, beef, fish and to a lesser extent, chicken, have been named as the more common food causes (5). But no one ever talks about the allergy potential of beef for cats. And considering how many feline diets contain fish, it’s interesting it’s not squarely number one on the list!
The most commonly fed foods are going to be the ones that are common causes of allergies.
Fish contains histamine. This one is related to the previous concern. Scrombroid or histamine toxicity is an allergy-like reaction, but it’s not a true allergy. The signs are similar though, with hives, flushing, headaches, dizziness, and nausea being reported in humans. It’s caused by fish not being properly stored, allowing bacteria in the fish to flourish and release histamines. These histamines are heat-resistant, so they aren’t destroyed with cooking or canning. A study of canned cat food in Europe found histamine present in 21% of the diet tested, which increased when the food was exposed to a hot environment, simulating a sunny day (6). No increases in the amount of histamine were found when food was left at room temperature or refrigerated, so take care not to leave food out too long on a hot day. All of the diets though, had histamine concentrations well below the EU maximum for histamine, so they wouldn’t have caused in issue in the vast majority of cats. Also, another study in New Zealand found similar results, with concentrations in canned fish diets well below what is thought to be toxic (7).
Fish contains heavy metals and other toxins. The concern about fish and heavy metals is well known, and the reason why it is recommended that humans should eat a maximum of only two portions of fish per week. Commercial wet cat foods have been shown to contain higher levels of mercury compared to dry food, which may be related to the fish content (8). But it’s important to remember that mercury isn’t the only heavy metal, and the presence of heavy metals isn’t exclusive to fish. High levels of heavy metals have been reported in all types of cat food, dry and wet, commercial and homemade, and even those not containing fish at all (9,10). So, while fish does contain relatively higher levels of heavy metals, it is not the only meat that does so.
There are also other toxins such as domoic acid from algae that can accumulated in sardines and other small fish as well, but these occur less so than what’s found in shellfish. Industry and government agencies tests for these toxins in their products, so if you’re sourcing quality fish from a country that monitors, the risk is small (12). It’s also worth noting that there has never been a reported case of domoic acid toxicity in cats. However, warming waters and more chances of algae blooms may make it a bigger concern in the near future.
Fish contain chemicals that causes hyperthyroidism. Hyperthyroidism is common in older cats, and the difficult thing is that we don’t know exactly what factors leads to it. It’s unlikely going to be one factor, but instead a combination of factors. Pollutants that are thyroid hormone disrupters are hypothesised to be one of those factors, but the link between pollutants and hyperthyroidism in cats hasn't been fully established. Bisphenol A from canned food has previously been looked at, and more recently, polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs), types of forever chemicals, have been shown to be in higher concentrations in the blood of cats with hyperthyroidism than in cats without hyperthyroidism (13, 14). These compounds were used in industrial manufacturing and have since been banned from use or from being imported in several countries, including Australia (15). This has led to improvements in some regions of the world, but unfortunatley it's still widely present in others. For instance, imported fish and shellfish from Southeast Asia have been found to contain higher concentrations of these compounds compared to Australian sourced fish (16).
Fish contains thiaminases. Thiaminase is considered to be an antinutrient because it destroys thiamine, an essential B vitamin. Cats have a relatively high thiamine requirement, and a deficiency of thiamine can be fatal. Thiaminase is present in both freshwater fish and marine fish, such as carp, herring, whiting, cod, salmon, bass, halibut and skate, to name a few. Luckily, cooking denatures thiaminase, so that is an easy fix.
Now let’s consider the benefits of feeding fish.
Fish is a good source of protein. Non-oily, white fish are a great source of lean protein. Because they do not contain a lot of fat, by weight, they contain more protein for every gram. This can be helpful in cats which are really needing more protein, such as when they’re older and having signs of muscle loss, or if they are intolerant of a large amount of fat – again thinking about senior cats or those with gastrointestinal disease.
Fish can be sustainable. Eating fish could lower your cat’s carbon pawprint. Overfishing is a serious issue, however, eating fish can be done in a sustainable fashion. Some species of fish had a lower greenhouse gas (GHG) emission score than either beef, pork or chicken (17). This is because fish do not require the same amount of input as it does for land animals, and they don’t release GHGs like cattle. But not all fish have a good GHG and sustainability rating, so focus on small pelagic fish such as sardines, herring, mackerel and anchovies as they are the best.
Fish is rich in omega-3 fatty acids. This is probably the most important benefit of fish. Marine fish is rich in two specific types of omega-3 fats, namely EPA and DHA. These healthy fats have lots of great benefits, but reducing inflammation is the most well-known. In senior cats, cats with renal disease, arthritis or inflammation of the gut, enriching their diet with these fats can be really beneficial. It’s also important to note that you can also get similar effects giving algae oil instead, which is even more sustainable.
Fish is super palatable. This one is a blessing and a curse. Animal behaviourist talk about the “addiction” of cats to fish. Not every cat will love fish, but there’s a large portion that really do, and some may even refuse to eat other foods after being introduced to it. They love it, and that’s why it’s such a popular ingredient for cat food. Fish contain the umami compounds, inosinate and glutamic acid, that really ticks the taste sensation box for cats. While we don’t want cats becoming addicted to fish, this intense love is helpful in cases where your cat isn’t eating as well, or you need to boost the palatability of their diet. You can use fish as a topper or even use it to help hide medication in, which is super handy!
So where does this all leave us? There are some legitimate concerns about feeding fish to cats, such as with forever chemicals, and heavy metals. But there are certainly benefits to feeding fish as well. So, I think the best strategy is the one that works for most things in life – everything in moderation. Feeding your cat fish sardines once a week or fish that bioaccumulates heavy metals and other toxins (such as tuna, shark, mackerel, etc), only occasionally, is unlikely to cause any harm.
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