Understanding Your Renal Diet: What Fish Is Not Good for Kidney Disease?

October 1, 2025 •

4 min read

According to the National Institutes of Health, over 37 million adults in the United States may have chronic kidney disease (CKD), and managing diet is a cornerstone of treatment. This includes understanding exactly what fish is not good for kidney disease, as certain types and preparation methods can negatively impact kidney function.

Quick Summary

For those with kidney disease, some fish and seafood options are not recommended due to high levels of phosphorus, potassium, or mercury, as well as excess sodium from processing. Making informed choices about fresh, low-mercury fish and controlling portion sizes are crucial aspects of a kidney-friendly diet.

Key Points

High Phosphorus and Potassium: Fish high in these minerals, like some fatty fish and canned varieties with bones, can be detrimental to weakened kidneys.
Mercury Contamination: Large predatory fish, including shark, swordfish, and some tuna, contain high levels of mercury that can harm the kidneys over time.
Processed Fish and Sodium: Processed, smoked, and canned fish are often packed with high amounts of sodium, which can elevate blood pressure and strain the kidneys.
Healthy Alternatives: Opt for fresh, lean fish like cod, sea bass, tilapia, and catfish, which are generally lower in problematic minerals.
Preparation Matters: Avoid frying and heavily salting fish; instead, choose healthier cooking methods like baking, grilling, or steaming.
Portion Control is Key: Even healthy fish should be consumed in moderation, with dietitians often recommending controlled, smaller servings.

Navigating the Renal Diet: Why Fish Choices Matter

For many, fish is a staple of a healthy diet, celebrated for its lean protein and beneficial omega-3 fatty acids. However, for individuals managing chronic kidney disease (CKD), the same nutritional profile that benefits some can pose risks. Damaged kidneys struggle to filter excess minerals like phosphorus and potassium, and a buildup can lead to serious health complications. Additionally, certain fish accumulate high levels of mercury, and processing methods often add excessive sodium, further straining a compromised renal system. Therefore, selecting the right kind of fish and preparing it appropriately is a critical part of a successful renal diet.

High-Phosphorus and High-Potassium Fish to Limit

Phosphorus is a mineral found in many foods, including fish. Healthy kidneys regulate phosphorus levels, but for those with CKD, an excess can weaken bones and harden blood vessels. Similarly, high potassium can cause heart problems if not properly managed. While many fish contain these minerals, some are particularly high. It's not always about outright avoiding these fish, but rather limiting portions and frequency, especially for those on dialysis or in later stages of CKD.

Fatty Fish: While celebrated for their omega-3s, some fatty fish like salmon and mackerel are naturally higher in phosphorus and potassium and require careful portion control.
Canned Fish with Bones: Sardines and some types of canned salmon are often packed with their bones, which significantly increases the phosphorus content. This form should be avoided.
Herring: A nutrient-dense fish, but also known to be higher in potassium and phosphorus than many other options.

Large, Predatory Fish with High Mercury Content

Mercury is a neurotoxin that can cause kidney damage in high doses, and larger predatory fish tend to accumulate higher levels through a process called bioaccumulation. While most people can safely consume fish in moderation, individuals with CKD should be cautious, as their kidneys are already compromised. Limiting or avoiding these species is a prudent step.

Shark and Swordfish: As apex predators, these fish have some of the highest mercury concentrations.
King Mackerel: Unlike the Atlantic variety, King mackerel is known for its high mercury levels and should be limited.
Bigeye and Fresh Tuna: Larger tuna species, such as bigeye, carry more mercury than smaller, canned varieties like light tuna.

The Hidden Dangers of Processed Fish

Processed and pre-prepared fish products are a major source of excess sodium, a mineral that causes fluid retention and high blood pressure, putting an extra burden on the kidneys. For a renal patient, minimizing sodium intake is a primary dietary goal.

Smoked or Salted Fish: Items like lox and pickled herring contain large amounts of added salt for preservation and flavor.
Canned Fish: Canned tuna and other fish, unless explicitly labeled "low-sodium," are typically high in salt. Rinsing canned fish can help reduce sodium but is not a complete solution.
Fried Fish: The breading and frying process adds significant sodium and unhealthy fats, which can increase inflammation.

Deep-Sea Fish and Emerging TMAO Research

Recent research has brought attention to Trimethylamine N-oxide (TMAO), a compound formed in the body from precursors found in fish and other meats. High TMAO levels have been linked to an increased risk of cardiovascular disease and may worsen CKD progression. Deep-sea fish, which live under high pressure, contain naturally higher levels of TMAO. Therefore, patients with CKD might be more susceptible to the effects of TMAO due to reduced kidney excretion, making deep-sea species a potential concern.

Comparison of Kidney-Friendly vs. Problematic Fish


Feature	Kidney-Friendly Fish	Fish to Limit or Avoid
Phosphorus	Lower amounts (e.g., cod, catfish, sea bass)	Higher amounts, especially in canned fish with bones (sardines) or fatty fish like salmon
Potassium	Lower amounts (e.g., cod, tilapia)	Higher amounts (e.g., mackerel, some salmon)
Mercury	Lower amounts (e.g., smaller, wild-caught fish)	Higher amounts (e.g., large predatory fish like shark, swordfish, king mackerel)
Sodium	Fresh, unprocessed fish	Smoked, salted, or canned fish (unless low-sodium)
Preparation	Baked, grilled, steamed	Fried, heavily salted

Making Smart Choices: Safe Fish for a Renal Diet

To ensure fish remains a healthy part of your diet, focus on fresh, unprocessed, and lower-mercury options. Portion control is paramount—dietitians often recommend one to two servings (around 3 ounces) per week.

Cod, Sea Bass, and Haddock: These are lean, low-phosphorus, and low-potassium options when fresh.
Tilapia and Catfish: Excellent sources of low-sodium, low-phosphorus protein.
Light Tuna: If choosing canned, opt for low-sodium, light tuna and consider rinsing it before consumption to further reduce sodium.
Shellfish (in moderation): While some shellfish is high in cholesterol, options like shrimp can be included in a renal diet in smaller, controlled portions, especially for those on dialysis who need extra protein.

A Final Word on Individualized Nutrition

Managing a renal diet is a highly individualized process. What's right for one person with CKD may not be suitable for another, depending on the stage of kidney disease, dialysis status, and other health concerns. It is always best to consult with a doctor or a registered renal dietitian to determine the best dietary plan for your specific needs. They can provide personalized recommendations on which fish to prioritize and which to strictly avoid, ensuring you get the benefits of a healthy protein source without putting unnecessary stress on your kidneys.

Important: Always consult with a healthcare professional before making significant changes to your diet, especially when managing a chronic condition like kidney disease. Find reliable resources and information on renal health at the National Kidney Foundation.

Conclusion

While fish is a celebrated health food, not all species are appropriate for individuals with chronic kidney disease. Limiting fish that are high in phosphorus, potassium, mercury, or sodium is essential for protecting kidney function and managing overall health. By choosing fresh, lean, and smaller fish, preparing them healthfully, and practicing strict portion control, those with CKD can continue to enjoy the nutritional benefits of seafood safely. Always seek professional guidance to tailor your diet to your specific health needs.

Frequently Asked Questions

Mackerel is a fatty fish rich in heart-healthy omega-3s, which are beneficial for many with CKD. However, it is also relatively high in phosphorus and potassium, requiring careful portion management. The specific type matters, too; Atlantic mackerel has lower mercury than King mackerel.

Canned fish is often very high in sodium due to the canning process. If you choose to eat canned tuna, opt for low-sodium varieties and consider rinsing it under water to reduce the sodium content. Canned fish with bones, like some sardines, should be avoided due to extremely high phosphorus levels.

Fish generally considered to have lower phosphorus include lean, white-fleshed fish such as cod, catfish, sea bass, and flounder. These are excellent choices for individuals on a renal diet.

Dialysis patients typically have higher protein needs because the treatment removes protein from the body. Fish can be a great source of lean protein for these patients. However, they must still manage phosphorus and potassium intake, so a renal dietitian's guidance is essential.

Fish oil supplements contain omega-3 fatty acids, which have shown potential anti-inflammatory benefits for some CKD patients. However, patients should always consult their doctor or dietitian before taking supplements, as individual needs and the overall diet must be considered.

Deep-sea fish have higher levels of Trimethylamine N-oxide (TMAO), a compound that, in high circulating levels, has been associated with increased cardiovascular risk and accelerated CKD progression. Since compromised kidneys struggle to excrete TMAO, this poses a greater risk for CKD patients.

Most renal dietitians recommend a standard serving size of about 3 ounces of fish. The National Kidney Foundation suggests one to two servings per week for maximum benefit while minimizing risks from mineral content or contaminants.