Laboratory Animal Diets Formulated with Fish Meal

Carrie Schultz, Ph.D.

Kristi Thompson, Ph.D.

Liz Koutsos, Ph.D.

The benefits of supplementing diets with fish meal have been demonstrated in a number of species. Immunity and overall health are among some of the potential positive effects.
Fish meal has historically been used as a protein source in manufactured diets for animals ranging from beef cattle to poultry to laboratory animals. Fish meal is a concentrated source of high quality protein composed of highly digestible essential amino acids. Fish meal has one of the best overall amino acid profiles of any single protein source. In addition, fish meal is also an excellent source of omega 3 fatty acids, like DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), which have been shown to improve health by preventing cardiovascular disease, lowering serum triglycerides, potentially stabilizing atherosclerotic plaques, improving tolerance to a variety of stressors, and playing important roles in the development and maintenance of neural and retinal tissues and cognitive function.^1,5,12,20,22
In recent years, fish meal has been removed from some laboratory diets. Animal proteins generally cost significantly more than plant proteins; thus, pressure to decrease ingredient costs has driven the removal of animal-derived proteins from some formulations. Concern that fish meal is contaminated with mercury and nitrosamines has led some researchers to believe experimental results could be confounded by diets. However, as detailed below, fish meal and fish oil used in animal diet production contain very low levels of this metal or N-nitroso compounds. Further, research conducted in the last twenty years has proven that nearly any ingredient in a diet can potentially alter experimental results, depending upon the nature of the experiment and the ingredient; soybean meal and casein have been the most often cited protein sources shown to influence study outcomes.
Sourcing of Fish Meal
The primary source of fish oil and fish meal in laboratory animal diets is derived from the menhaden fish. Menhaden are short-lived omnivores that feed primarily on phytoplankton, thus, they accumulate very little methylmercury or other heavy metals, unlike carnivorous predatory fish. Currently, two companies in the U.S. supply fish meal and fish oil from menhaden. Fish meal is produced from freshly harvested menhaden that are promptly delivered chilled and whole, in refrigerated vessels, to the processing facilities. Processing at the five plants in the U.S. is immediate to ensure product quality and minimize the presence of non-nutrient factors like nitrosamines.
Fish Meal in Animal Diets
Animals do not require protein per se, but instead utilize essential and non-essential amino acids. Thus, the quality of a protein source is positively correlated with the digestibility, bioavailability, and proportions of the amino acids in the protein source.6 Animal proteins are the gold standard for protein quality because they provide the closest amino acid composition relative to an animal’s requirements. Furthermore, the nutritional quality of diets combining grains and animal protein sources is greater than diets containing grains alone.^4,9
To date, very little information is available on the effects of incorporating fish meal into grain-based diets for rodents; however, the advantages have been published in other laboratory species such as dogs, poultry, and swine. Whole menhaden fish meal has been shown to increase the number of live pigs per litter at birth as well as the overall body weight of pigs at birth and two weeks post parturition.¹⁴ Similar results have been observed in laying hens fed a diet containing 3% dietary fish meal for a 12 week period. Hens receiving the fish meal diet had higher egg production rates, egg weights and egg volume (p<0.05) compared with hens fed a corn-wheat-soybean meal (SBM) diet.¹⁸ Or-Rashid et al¹³ evaluated the effects of providing fish meal to ewes during late gestation and early lactation on the proportion of DHA and EPA in colostrum and milk as well as the subsequent effect on the plasma fatty acid profile of nursing lambs. Ewes fed a fish meal supplemented diet had greater (p<0.013) percentages of EPA, DHA, total n-3-PUFA, total CLA and total very long chain n-3-PUFA in colostrum and milk compared to control ewes receiving a SBM based diet. At birth, lambs born to fish meal supplemented ewes had greater concentrations of plasma EPA, DHA, and total very long chain n-3-PUFA compared to lambs born to control ewes. Fatty acid concentrations also increased over time for lambs nursing ewes supplemented with fish meal.
Dietary fish meal may also be advantageous for immunocompromised animals. In poultry, both chronic and acute infections induced by coccidiosis (Eimeria acervulina) decrease growth performance and MEn and AA digestibility in chicks fed a corn-SBM diet. However, this same effect was not observed when chicks were fed a diet containing 15% dietary fish meal.¹⁵ The authors concluded the observed positive effects on growth and nutrient digestibility may be attributed to decreased intestinal inflammation caused by increased levels of dietary omega 3 fatty acids provided by the fish meal. Although concentrations of EPA and DHA are greater in fish oil, fish meal remains a good supplier of these omega 3 fatty acids as well as an excellent concentrated source of high quality protein.
Heavy Metal and Non-nutritive Contaminants
Mercury
Mercury occurs naturally in the environment and is an industrial pollutant that accumulates in bodies of water in the form of methylmercury (MeHg).²¹ It can be found in most species of fish and shellfish, but bioaccumulates in larger, longer living deep water species like shark, tuna, swordfish, and king mackerel. Methylmercury toxicity in humans has been associated with neural dysfunctions and cognitive deficits.
MeHg-induced dysfunctions and deficits observed in humans have been reproduced in laboratory animals and were observed as decreased motor abilities, coordination, and overall activity in C57/B6 mice¹¹ as well as impaired temporal and spatial visual function in primates.¹⁷ The degree and severity of neurotoxicity is dependent upon the dose and duration of exposure as well as the stage of development at which the animal is exposed.⁷ Roegge et al¹⁹ provided Long Evans rats 0.5 ppm MeHg via drinking water, continuously for 4 weeks prior to conception, during pregnancy and through postnatal day 16. MeHg did not significantly affect motor tasks or morphological parameters in the cerebellum of the weanling or adult rats. Colomina et al³ gavaged mice with 2 mg/kg BW/d MeHg chloride from days 15-18 of gestation and did not observe any significant effects on postnatal development in the offspring, even when the dams were exposed to additional stress during pregnancy. Laboratory diets routinely tested for mercury are typically below the limit of detection (0.025 to 0.1 ppm). Thus, an average size rat consuming 15 grams of diet per day could consume up to 0.00125 to 0.005 mg/kg MeHg BW/d, which is less than the known no-effect levels (NOELs) for neurotoxicity. However, published research findings indicate that both low- and high-dose MeHg can affect the behavior of many species. Thus, researchers investigating oncology or behavioral teratology, for example, may choose to use specially designed diets such as purified diets or grain based diets containing no fish meal. Additional information regarding the effects of MeHg on the neurodevelopment of laboratory animals can be found in the review paper authored by Castoldi et al.²
Nitrosamines
Nitrosamines were identified as carcinogens in a rodent model in 1956,¹⁰ and in the early 1960s sheep in Norway died of liver toxicity after eating herring treated with sodium nitrite. Nitrosamines are produced from nitrites and secondary amino compounds, and can be formed endogenously or prior to ingestion. Oxides of nitrogen formed during food processing, preservation and preparation can react with amino compounds and other nucleophiles to produce N-, C-, O-, and S-nitroso compounds.
A number of studies have demonstrated carcinogenic effects of N-nitroso compounds in a variety of animal species, suggesting that specific N-nitroso compounds are carcinogenic in all species. However, the affected organs and cellular targets within those tissues vary with species, dose, frequency of dose, and route of administration.¹⁰ The minimum dietary dose of N-nitrosodimethylamine (NDMA) that induces a detectable increase in cancer incidence in rats (which are more sensitive to NDMA than other rodent or lab species) is 2 mg/kg, and oral NDMA doses as low as 1 ug/kg BW are absorbed intact and reach the liver and kidney of the rat.⁸ At levels of <109 ug NDMA/kg BW/day (via drinking water) most rats survived to old age (28-31 months), with a tumor incidence of 3-13%. At levels of >109 ug NDMA/kg BW/day, most animals died of tumors.16 Concentrations (ug/kg) of NDMA in fish meal average 254 ± 16.4, whereas the average levels in most fish meal-containing diets are only 8.17 ± 0.40, suggesting some loss or degradation during the manufacturing process. This is also well below the dietary limit for carcinogenicity (2 mg/kg limit vs 8 ug/kg average concentration).
The two major sources of nitrogen oxides are from the addition of nitrate and/or nitrite to foods (e.g., curing of meat products and cheeses, pickling of vegetables) and from the heating and/or drying of foods in combustion gases (e.g., smoked foods, dried malt, dried milk products, dried spices). Prior to the use of refrigerated vessels, preservatives such as sodium nitrite were used to prevent spoilage of harvested fish for use in the animal feed industry. Of course, these additives are no longer used in menhaden fish ingredients. Laboratory animal diets are an essential and important component of animal research. If non-nutritive compounds in diets are of concern to you, consult with a nutritionist as other diet options may exist.
Conclusions
The advantages of supplementing diets with fish meal and/or fish oil have been demonstrated in a number of species due to the positive effects of omega 3 fatty acids on performance, immunity and overall health. Fish meal is a highly digestible protein source with a high content of amino acids, minerals, and vitamins. Under certain circumstances, researchers may prefer using diets without fish meal, but for most types of research the benefits of fish meal (amino acid composition, protein digestibility and contribution of omega 3 fatty acids) warrant its dietary use.
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