Research Article | Open Access
Amino Acid Compositions of 27 Food Fishes and Their Importance in Clinical Nutrition
Proteins and amino acids are important biomolecules which regulate key metabolic pathways and serve as precursors for synthesis of biologically important substances; moreover, amino acids are building blocks of proteins. Fish is an important dietary source of quality animal proteins and amino acids and play important role in human nutrition. In the present investigation, crude protein content and amino acid compositions of important food fishes from different habitats have been studied. Crude protein content was determined by Kjeldahl method and amino acid composition was analyzed by high performance liquid chromatography and information on 27 food fishes was generated. The analysis showed that the cold water species are rich in lysine and aspartic acid, marine fishes in leucine, small indigenous fishes in histidine, and the carps and catfishes in glutamic acid and glycine. The enriched nutrition knowledge base would enhance the utility of fish as a source of quality animal proteins and amino acids and aid in their inclusion in dietary counseling and patient guidance for specific nutritional needs.
Amino acids are important biomolecules that both serve as building blocks of proteins and are intermediates in various metabolic pathways. They serve as precursors for synthesis of a wide range of biologically important substances including nucleotides, peptide hormones, and neurotransmitters. Moreover, amino acids play important roles in cell signaling and act as regulators of gene expression and protein phosphorylation cascade , nutrient transport and metabolism in animal cells , and innate and cell-mediated immune responses.
Amino acids are mainly obtained from proteins in diet and the quality of dietary protein is assessed from essential to nonessential amino acid ratio. High quality proteins are readily digestible and contain the dietary essential amino acids (EAA) in quantities that correspond to human requirements . Proteins, the most abundant macromolecules found in biological systems, are present in diverse forms such as structural elements, enzymes, hormones, antibodies, receptors, signaling molecules, and so forth, having specific biological functions. Protein is necessary for key body functions including provision of essential amino acids and development and maintenance of muscles. Inadequate uptake of quality proteins and calories in diet leads to protein-energy malnutrition (PEM) (or protein-calorie malnutrition, PCM) which is the most lethal form of malnutrition/hunger. Kwashiorkor and marasmus, the extreme conditions of PCM mostly observed in children, are caused by chronic deficiency of protein and energy, respectively. PCM also occurs in adults who are under chronic nutritional deficiency. About 870 million people in the world are suffering from chronic protein malnutrition; 80% of children suffering from PCM are from developing countries [3, 4]. Fish, in this context, can play a vital role as it is an important and cheaper source of quality animal proteins. Therefore, there is a need to generate and document nutritional information on the numerous varieties and species of food fishes available. In comparison to the other sources of dietary animal proteins, consumers have wide choice for fish as far as affordability is concerned as there are many varieties and species of fishes available, especially in the tropical countries . The present study was undertaken to generate information on protein content and amino acid composition of important food fishes with the objective of enhancing the scope for their utility in clinical nutrition for dietary counseling.
2. Materials and Methods
2.1. Ethical Statement
The authors confirm that all the research done meets the ethical guidelines, including adherence to the legal requirements of the study country.
2.2. Sample Collection and Processing
Freshly caught fishes were collected from either the landing centers or the local fish markets and were brought to the laboratory in ice. A total of 27 species included for amino acid profiling were the carps Catla catla, Labeo rohita, and Cirrhinus mrigala, catfishes Sperata seenghala, Heteropneustes fossilis, and Clarias batrachus, the small indigenous fishes Amblypharyngodon mola, Puntius sophore, Anabas testudineus (all fresh water fishes), and Tenualosa ilisha (anadromous), the cold water fishes Oncorhynchus mykiss, Tor putitora, Schizothorax richardsonii, Neolissochilus hexagonolepis, and Cyprinus carpio; the marine fishes Thunnus albacares, Stolephorus waitei, Stolephorus commersonii, Rastrelliger kanagurta, Nemipterus japonicas, Sardinella longiceps, Katsuwonus pelamis, Epinephelus spp., Leiognathus splendens, and Trichiurus lepturus, and the shellfishes Crassostrea madrasensis, Perna viridis. Fishes were cleaned, descaled, degutted, minced, homogenized, and stored at −40°C until used.
2.3. Amino Acid Analysis
The crude protein content was determined by Kjeldahl method . Amino acid composition was determined following Ishida et al.  and has been described earlier . Briefly, muscle protein was hydrolyzed with 6N hydrochloric acid at 110°C under anaerobic condition for 24 h. The hydrolyzed samples were neutralized with 6N NaOH and were derivatized using a kit (AccQ-Fluor Reagent, WAT052880, Waters). The derivatized samples were injected in high performance liquid chromatography (HPLC) (1525, Waters) equipped with a C18 RP column and a fluorescence detector (2475, Waters). The amino acids were identified and quantified by comparing with the retention times and peak areas of standards (WAT088122, Waters). For the tryptophan analysis, minced meat was digested with 5% (w/v) NaOH for 24 h and neutralized to pH 7.0 with 6N HCl. Tryptophan content was measured spectrophotometrically at 530 nm . All data have been presented as mean ± standard deviation.
3. Results and Discussion
The physiological role of dietary proteins is to provide substrates required for the synthesis of body proteins and other metabolically important nitrogen-containing compounds. Therefore, the content of the nutritionally indispensable amino acids (AAs) in food proteins is usually the primary determinant of nutritional quality of protein . Moreover, amino acids are associated with health issues and amino acid deficiencies lead to a number of diseases. Hence, knowledge of the amino acid composition of foods serves as a basis for establishing their potential nutritive value. It may also allow evaluation of changes in nutritive value that may arise in the preparation, processing, and storage of foods .
AAs have been traditionally classified as nutritionally essential (EAA), “nonessential” (NEAA) or conditionally essential (CEAA) . Arginine, cystine, histidine, leucine, lysine, methionine, threonine, tryptophan, tyrosine, and valine are the EAAs, glutamine, glutamic acid, glycine, proline, and taurine are CEAA, and aspartic acid, serine, and alanine are the NEAA for human nutrition. However, recently the concept of functional amino acids (FAAs) has been proposed. FAAs are those which participate and regulate key metabolic pathways to improve health, survival, growth, development, lactation, and reproduction of the organisms [1, 12]. The FAAs also hold great promise in prevention and treatment of metabolic diseases (e.g., obesity, diabetes, and cardiovascular disorders), intrauterine growth restriction, infertility, intestinal and neurological dysfunction, and infectious disease. Arginine, cystine, leucine, methionine, tryptophan, tyrosine, aspartate, glutamic acid, glycine, proline, and taurine have been classified as FAA in human nutrition .
Fish is an important source of quality animal proteins and it has been reported that fish protein has greater satiety effect than other sources of animal proteins like beef and chicken . In comparison to the other sources of dietary animal proteins, consumers have wide choice for fish as far as affordability is concerned as there are many varieties and species of fishes available, especially in the tropical countries . Here, we report the crude protein content and amino acid composition of 27 food fishes from the Indian subcontinent (Tables 1 and 2) which could be useful in patient counseling and recommending species for patients with specific requirements and thus could be useful in clinical medicine. The distributions of amino acid in different species are discussed below. There was no appreciable variation in amino acid composition of fishes of the same species from different locations.