Tuna may be a favorite, but it can carry unexpected risks for cats with kidney disease.

Some cats leave food in the bowl, then come running the moment you open a can of tuna.

Many cats love tuna, which is why it appears in everything from complete cat foods to canned and lickable treats. When your cat isn’t eating much, seeing them enjoy tuna can be a relief.

Before long, both meals and treats may be tuna-based.

When your cat is eating happily, it is easy to overlook heavy metals such as mercury and cadmium, along with phosphorus content.

Complete and balanced tuna-based foods can be a reasonable choice for healthy cats. But if your cat has chronic kidney disease (CKD), even familiar foods deserve another look.

A food your cat loves may not be the best choice for their kidney health.

Watch Out for Mercury and Cadmium in Tuna

Tuna can contain methylmercury, which builds up through the food chain. It can be present in the flesh, so even pale-colored tuna can contain it.

Excessive exposure to methylmercury can damage the nervous system. Cadmium can also accumulate in the kidneys and cause damage with excessive exposure over time.

In 2026, Clean Label Project published results from testing 100 cat food products from 62 brands sold in the United States.

For the same number of calories, fish- and seafood-based products contained about 18 times as much mercury and 2.5 times as much cadmium as the other products tested.

Among wet foods, fish-based products contained about 28 times as much mercury as those made with other protein sources.

These figures compare metal levels in foods; they do not mean CKD is that much more likely to get worse.

Feeding the same product every day means repeated exposure to any heavy metals it contains. If both meals and treats are tuna-based, take a closer look at how often you serve them and how much your cat eats.

Phosphorus in Tuna Can Also Put a Strain on the Kidneys

Phosphorus is an essential nutrient for cats. But as kidney function declines, the kidneys become less able to remove excess phosphorus from the body.

That is why managing phosphorus intake matters in CKD, and why kidney diets have controlled phosphorus levels.

Tuna contains phosphorus, too. Even after switching to a kidney diet, the phosphorus in treats still counts.

If your cat’s main food is tuna-based but isn’t formulated for kidney disease, consider the phosphorus in every meal. If they eat a kidney diet, remember to include the phosphorus in treats and toppers.

A spoonful of tuna at breakfast, half a treat tube at midday, and the rest in the evening. Each serving may seem small, but the phosphorus adds up over the day.

When choosing foods and treats, look at phosphorus content, too. Dry and wet foods differ in moisture and calorie content, so phosphorus per 100 kcal is a useful way to compare them.

For reference, diets formulated for cats with kidney disease typically contain about 80–130 mg of phosphorus per 100 kcal.

For example, a product with 200 mg per 100 kcal contains about 1.5–2.5 times as much phosphorus as the kidney diets in that range. If you serve it as a main food or a frequent treat, consider both its phosphorus content and how much your cat actually eats.

Consider Chicken- or Salmon-Based Foods

If you want to move away from a tuna-heavy diet, start by looking at chicken- or salmon-based products with controlled phosphorus levels.

To reduce mercury intake, consider foods based on chicken or other poultry.

If your cat loves fish, salmon-based foods may be another option. Salmon generally contains less mercury than tuna.

Chicken and salmon still contain phosphorus, though. Choose a main diet formulated for kidney care. For treats, check both phosphorus content and the amount you give each day.

Offer a small amount of the new food beside your cat’s familiar meal. Once you find a flavor and texture they enjoy, gradually increase the proportion of new food while making sure your cat keeps eating well.

If your cat starts eating less during the transition, keep offering enough of the familiar food and slow down. For cats with CKD, managing phosphorus and eating enough both matter.

Take a look at the food and treat packages you used today.

If both meals and treats are tuna-based, check the ingredients and phosphorus content when choosing the next product.

Keep mealtimes enjoyable while making food choices with your cat’s kidney health in mind.

Sources

• Heavy metals in commercial cat foods: Clean Label Project: Cat Food Study (2026)
• Phosphorus management in cats with CKD: Feline VMA: From Diet to Binders: Targeted Management of Hyperphosphatemia in Cats with CKD
• Mercury levels in fish, including tuna and salmon: FDA: Mercury Levels in Commercial Fish and Shellfish (1990–2012)

[Educational Disclaimer]

This article is for educational purposes only and is not a substitute for professional veterinary advice, diagnosis, or treatment. It is not intended to diagnose, treat, cure, or prevent any disease.

Please consult your veterinarian before making changes to your cat’s diet, supplements, or care routine.