Fish should not be overlooked as a beneficial protein source for patients adhering to a renal diet. When considering how fish can play a role in helping a patient follow dietary guidelines, it is important to consider both beneficial nutrients and those that should be limited.

A recent study by Castro-González et al published online ahead of print in the International Journal of Food Sciences and Nutrition evaluated 14 fish species (oilfish, round herring, longjaw leatherjacket, broadbill swordfish, black bullhead, gray snapper, spotted weakfish, yellowfin tuna, longjaw silverside, American harvestfish, Atlantic tripletail, Atlantic blue marlin, parrot sand bass, and spotted scorpionfish) and their nutrient compositions in relation to renal diet recommendations.

The researchers evaluated protein, phosphorus, potassium, sodium, fat, and cholesterol, and quantified content of the n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) as well as vitamin D and vitamin E.

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The study revealed the following:

  • The three fish with the highest content of EPA and DHA (in order of content) were round herring, oilfish, and longjaw leatherjacket, which contained 1667.35, 646.12, and 606.12 grams of n-3 fatty acids per 100 gram portion of fish, respectively;
  • All fish contained the recommended protein content for renal disease patients (less than 25 g/80 g portion);
  • All but three fish (broadbill swordfish, black bullhead, and spotted scorpionfish) had a phosphorus content within the recommended range of less than 10 mg/g protein;
  • All fish had a cholesterol content within the range of 44.70-68.45 mg/100 g portion;
  • The fish with a sodium content of more than 100 mg/100 g portion included round herring, parrot sand bass, and the gray snapper;
  • The fish with a potassium content greater than 300 mg/100 g portion included (in order of content) the spotted scorpionfish, yellowfin tuna, Atlantic tripletail, broadbill swordfish, and gray snapper;
  • Oilfish had the highest total fat content (12.98 g/100 g portion). Round herring and longjaw leatherjacket had the second and third highest total fat content (6.37 and 3.99 g/100 g portion;
  • Spotted scorpionfish, black bullhead, and American harvestfish had the most vitamin D per gram of protein;
  • The black bullhead, American harvestfish, and round herring had the most vitamin E per gram of protein.

The primary purpose of this paper was to consider the content of beneficial n-3 fatty acids in relation to typical nutritional restrictions that renal patients must follow. Various studies have shown that n-3 fatty acids possible confer a number of positive health benefits.

The two primary n-3 fatty acids studied, EPA and DHA, appear to have different individual effects on mitigating the pathophysiology of inflammatory pathways, and DHA is often regarded as more beneficial due to its ability to reduce collagen aggregation.

Some trials using high doses suggest that possible benefits include reductions in proteinuria and blood pressure. Population studies tend to indicate that regular intakes of actual fish instead of n-3 fatty acid supplements tends to produce favorable outcomes, such as reduced incidence of chronic kidney disease (CKD) and 50% reduced mortality risk in dialysis patients.

Based on the study’s findings, the primary goal should be to maximize intake of n-3 fatty acids while reducing the intake of restricted nutrients. Each patient’s needs must be considered on an individual basis.

Before making recommendations to patients, however, some characteristics of the following fish need to be considered:

  • Round herring has the highest content of n-3 fatty acids, but its main drawback is that it has the highest sodium content (192.9 mg/100 g portion) of any other fish in this study;
  • Oilfish, which has the highest total fat content, is a great source of calories for CKD patients, who simultaneously need to increase calories while restricted protein. It is low in potassium, sodium, and phosphorus, but interestingly, it has the lowest vitamin D content of any fish in this study;
  • Longjaw leatherjacket has the third-highest content of n-3 fatty acids among the fish studied, but it is closer to the median ranges regarding mineral content. It has the second-lowest vitamin D content of the fish in the study;
  • Atlantic tripletail has a more moderate n-3 fatty acid content (390.32 mg/100 g portion) compared with some of the other fish, but it is an adequate source that has the lowest phosphorus content of the fish studied (50.86 mg/100 g portion). It also has the highest protein content (30.06 g/100 g portion). When considering an 80 g serving as the typical serving size, this species will fall within the appropriate protein guidelines, but it could potentially be advantageous in renal disease patients who need to control phosphorus while maximizing protein.

Regional availability

This study took place in Mexico, and thus the fish examined were primarily available to that region. Because the availability of different species is geographically limited, following these same guidelines may not be possible if located in other regions such as the northern regions of the continental U.S.

Of note though, cold water fish are more commonly associated with higher concentrations of n-3 fatty acids, and thus local fish from northern climates may have similar ratios of nutrients to that of the round herring found in this study. In general, most of the fish in this study proved to be beneficial, high-quality protein sources that are within or close to the appropriate phosphorus/protein ratios.

Repeating this research for a wider range of fish species in the continental U.S. would be of great benefit for dietitians and physicians when suggesting appropriate protein sources for their patients.