Basic and functional nutrients in the muscles of fish: a review

ABSTRACT

Fish can be a valuable element of the human diet. Consequently, it is recommended in preventive and therapeutic diets. The subject of this review is a presentation of the nutritional value of fish species commercialized globally on a large scale. The content of nutrients important in terms of nutritional value was compared, and significant individual interspecies and intraspecies differences were highlighted. In addition, potential variability factors affecting the chemical composition of fish muscle tissue were shown. The presented data clearly indicated that in the case of food fish, available food nutrition tables cannot be used as universal reference data for planning a balanced diet and the current recommendations for frequency and amount of fish consumption are too general. Variability factors may substantively change the nutrient content in fish muscle tissue, even causing differences between specimens of the same species.

KEYWORDS:

• Food fish
• amino acid
• fatty acid
• nutritional value

Introduction

Fish can be a valuable component of the diet, mainly due to the content of complete protein and polyunsaturated fatty acids, in particular eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (AA).^[1,2] These nutrients are necessary for proper growth and physical development, as well as maintenance of good health and prevention of certain diseases.^[3–9] The results of some clinical trials assessing the effect of an increased fish proportion in the diet were promising. In a two-year study, a reduction in mortality by 29% and deaths from cardiovascular causes by 33% were observed in the group of people to whom increased fish consumption was recommended.^[10] Consequently, fish is recommended in preventive and therapeutic diets.^[10,11]

According to the literature, the content of nutrients in the muscles of food fish was variable. The protein content in the muscles of different fish species ranged from 12.2 to 21.79%,^[12–17] and the levels of essential amino acids (g/100 g protein) ranged from 0.2 to 6.2 for isoleucine, 0.4 to 10.4 for leucine, 0.3 to 16.1 for lysine, 0.1 to 4.0 for methionine, 0.04 to 0.6 for cystine, 0.3 to 6.3 for phenylalanine, 0.2 to 1.5 for tyrosine, 0.3 to 7.9 for threonine, 0.2 to 8.6 for valine, 0.1 to 2.3 for tryptophan, and 0.5 to 7.9 for histidine.^[18] The fat content in the muscles of different fish species was between 0.08 and 28.90%, ^{[2,12,17,19,20]} and the fatty acid profiles differed in the proportions of fatty acids types they contained over wide ranges. Saturated fatty acids were found to range from 10.72 to 48.94%, monounsaturated fatty acids from 14.84 to 55.65%, polyunsaturated fatty acids from 9.75 to 72.65%, eicosapentaenoic acid from 0.65 to 20.15%, docosahexaenoic acid from 0.72 to 27.08%, and arachidonic acids from 0.16 to 12.27%.^[15,21,22]

In view of this variability, knowledge of the content of nutrients in fish muscles and, thus, their nutritional value is crucial for their health advantages for the consumer to be gained.^[18,23,24]

The subject of the review is the nutritional value of food fish species commercialized globally on a large scale, including marine and freshwater species. The literature was revised and the nutrient contents, i.e. the protein, fat, amino acid, and fatty acid in fish muscle tissue were compared. Potential variability factors affecting the chemical composition of fish muscle tissue were also presented.

Protein and fat content

The protein content in the muscles of the fish species listed in Table 1 ranged from 11.8% for carp to 22.65% for trout. Differences in protein content between species reached as much as 10 g per 100 g of muscle (between carp and trout), while differences within the same fish species ranged in magnitude from 0.2 to 0.3 g per 100 g of muscle (in salmon, bream, cod, and perch) to 7.48 g per 100 g (in carp). Fat content in the muscles of the fish species (Table 1) ranged from 0.08% (in cod) to 35.06% (in eels). Differences in fat content between species reached 35 g per 100 g of muscle (between cod and eels), while differences within the same species ranged from 0.5 to 0.6 g per 100 g (in roach, grass carp, and cod) to 11.79 g per 100 g (in carp). Quantitative variations in fat in fish muscle are greater than variations in protein^[34,45] as confirmed by the data presented in Table 1. Fishes are classified according to their fat content as lean (up to 2% fat), medium-fat (2–7% fat), fatty (7–15% fat) or very fatty (over 15% fat).^[46] According to this classification, pike, roach, perch, pikeperch and cod were lean fishes; trout, grass carp, European catfish and herring were medium-fat fishes; bighead carp, Siberian sturgeon and salmon were fatty fishes; and the eel was a very fatty fish. The fat content in the muscle of bream was in the ranges for both lean and medium-fat fishes, and that of common carp was in the ranges for lean, medium-fat and fatty fishes.

Table 1. The content of protein and fat in the muscles of selected fish species (%)

Fish species	Protein	Fat	Reference
Bighead carp	15.98	8.93	^[Citation25]
	17.24	9.60	^[Citation26]
Carp	-	4.9	^[Citation27]
	11.8–17.74	8.29–13.32	^[Citation28]
	15.7–18.0	6.8–12.4	^[Citation29]
	16.7	5.1	^[Citation12]
	16.83	3.70	^[Citation26]
	17.21	4.44	^[Citation30]
	17.50–18.28	1.95–4.92	^[Citation31]
	17.75	4.84	^[Citation21]
	17.82	2.41	^[Citation32]
	19.28	1.53	^[Citation33]
Trout	-	5.1	^[Citation27]
	17.46	7.45	^[Citation34]
	18.72–19.74	3.63–7.40	^[Citation35]
	18.9	7.4	^[Citation12]
	19.23–20.34	5.39–3.13	^[Citation36]
	17.46–19.54	5.53–7.45	^[Citation34]
	19.85–22.65	3.15–4.47	^[Citation37]
	21.85	3.57	^[Citation30]
Perch	18.10	0.12	^[Citation1]
	18.06–18.38	0.11–0.29	^[Citation38]
	-	0.39–0.41	^[Citation39]
	-	1.97–2.17	^[Citation40]
	-	0.89	^[Citation41]
	-	0.40–0.51	^[Citation2]
Grass carp	17.07	2.85	^[Citation26]
	18.25	2.28	^[Citation25]
European catfish	-	2.33–3.42	^[Citation42]
	16.24	3.78	^[Citation25]
Siberian sturgeon	15.69	12.57	^[Citation25]
Bream	-	2.4	^[Citation27]
	18.10	2.59	^[Citation1]
	18.37	1.26	^[Citation32]
	-	0.96–1.04	^[Citation39]
		1.03	^[Citation41]
	-	2.11–4.05	^[Citation2]
Pike	21.41	0.58	^[Citation43]
	18.6	0.8	^[Citation29]
	18.66	0.64	^[Citation21]
	19.12	0.22	^[Citation32]
	-	0.56	^[Citation41]
	-	0.19–2.40	^[Citation44]
Roach	18.30	0.56	^[Citation1]
	20.5	0.9	^[Citation29]
	-	0.64	^[Citation41]
	-	0.51–1.01	^[Citation2]
Pikeperch	21.26	0.41	^[Citation43]
	18.61	0.87	^[Citation21]
	18.79	0.34	^[Citation32]
	19.50	0.13	^[Citation1]
	-	0.34–0.42	^[Citation2]
Eel	14.50	28.90	^[Citation1]
	-	24.70–35.06	^[Citation2]
Herring	16.60	3.09	^[Citation1]
	18.1	3.7	^[Citation12]
	-	1.98–3.38	^[Citation2]
Salmon	18.16	10.61	^[Citation21]
	18.4	13.1	^[Citation12]
Cod	17.73	0.68	^[Citation21]
	17.4	0.08	^[Citation12]

“-” means no data

Available data indicate that the differences in protein and fat content in fish muscle (Table 1), depend not only on the species but also on factors such as diet, body weight, environment, catch season, and farming system. The intensity of feeding, the type and quality of feed, and the amount of natural food consumed by fish affect the protein and fat content in the muscles.^[28] Higher fat content was found in the muscles of carp fed carbohydrate feeds, i.e., maize and wheat (13.32% and 10.58%, respectively), than in carp fed lupines (8.29%). The protein content, on the other hand, was highest in carp fed on lupines (17.74%), and in declining order of protein content, next were wheat-fed carp (15.7%) and maize-fed carp (11.8–14.6%). The same study showed a relationship between the quantity of these basic nutrients and the anatomical location of the carp muscle; irrespective of the type of diet, higher fat levels and lower protein levels were found in the ventral part of carp fillets.^[28] The type of feed that fish consumed had a significant impact on fat content in fish muscles, and this has been proven by comparison of predatory and omnivorous fish species. Pike and zander, as predatory fish, contained significantly less fat (0.22 and 0.34%, respectively) than carp and bream which are omnivorous fish (2.41 and 1.26%, respectively).^[32] In predatory fish of similar age, the protein and fat contents in the muscle were very similar: 21.26 and 0.41% respectively in zander, and 21.41 and 0.58% in pike.^[43] Differences in protein and fat levels were also associated with body weight. Rainbow trout with greater body weight (over 350 g) contained more protein and fat in the muscle tissue (22.65 and 4.47%, respectively) than trout with lesser body weight (up to 350 g) (19.85 and 3.15%, respectively).^[37] Somewhat different dependencies between body weight and the protein and fat levels in trout were reported by Litwińczuk et al.^[34] Smaller fish (up to 0.3 kg) contained significantly more protein and less fat (19.54 and 5.53%) than heavier fish (over 0.3 kg) (17.46 and 7.45%). The effect of the season on fat content in muscle tissue has been demonstrated in perch. Specimens of this species obtained in autumn contained significantly more fat (0.29%) than those caught in the spring (0.11%).^[38] Differences in fat content in the muscle of fish caught in different seasons (summer and winter) can reach about 11%.^[47] The lower fat content in fish muscle in the summer is associated with the use of perivisceral fat for gonadal development and the use of muscle fat as an energy source.^[48] The farming system is also a factor in muscle protein and fat content. Its impact on these contents in the muscle of rainbow trout was reported by Skałecki et al.^[36] Intensively farmed fish contained significantly more fat (5.39%) and less protein (19.23%) than fish obtained from extensive farming (3.13 and 20.34%, respectively).

The energy value of fish muscles ranges from 70.3 kcal/100 g in cod to 318.0 kcal/100 g in eels.

Protein digestibility and essential amino acids content

The digestibility of fish muscle protein ranged from 98.3 to 98.8% and was comparable for fish species from the Vistula Lagoon (herring, perch, roach, pikeperch, bream, and eel), farmed fish species (carp and trout), and Baltic fish caught in Polish waters (cod, herring, and Baltic salmon).^[1,12] The high digestibility of fish muscle protein results from the low content of connective tissue proteins, i.e., collagen and elastin.^[24] In evaluating protein, apart from its digestibility, its quality is also significant; protein quality is indicated by the presence of essential amino acids in amounts meeting the body’s requirements. The essential amino acids are isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), cysteine (Cys), phenylalanine (Phe), tyrosine (Tyr), threonine (Thr), tryptophan (Try), valine (Val) and histidine (His).^[49] The body’s requirements for essential amino acids have been established for different age groups (infants, children, adolescents and adults) and presented as amino acid scoring patterns for dietary protein.^[49] The sum of essential amino acids (excluding histidine) in the protein of fish species from the Vistula Lagoon, i.e. eel, perch, bream, herring, roach and pikeperch, ranged from 39.93 to 45.36 g per 100 g protein.^[1] The corresponding values (excluding histidine) for farmed fish, i.e. carp and trout, were 43.8 and 46.1 g per 100 g protein; for Baltic fish species caught in Polish waters, i.e. salmon, herring and cod, amino acids totaled 43.7–44.2 g per 100 g protein^[50]; and in another study, for farmed grass carp, bighead carp, Siberian sturgeon and wels catfish the range was 45.86–47.64 g per 100 g protein^[25] (Table 2). The muscle tissue protein of the fish species contained all essential amino acids in amounts exceeding the amino acid scoring pattern for adults (except for methionine + cysteine in wels catfish). The content of individual essential amino acids (g per 100 g protein) in the muscles of these fish species (Table 2) exceeded the standard by 2.3–4.3 g for phenylalanine + tyrosine, 1.1–1.8 g for isoleucine, 1.0–2.6 g for leucine, 3.8–5.6 g for lysine, 0.2–2.1 g for methionine + cysteine, 1.5–2.7 g for threonine, 0.2–2.4 g for tryptophan, 0.3–1.0 g for valine, and 0.7–1.7 g for histidine. Thus, all fish species presented in Table 2 were sources of high-quality protein.

Table 2. Comparison of essential amino acids in the protein of the muscle tissue of selected fish species (g/100 g protein)

Amino acid	Phe+Tyr	Isoleucine	Leucine	Lysine	Met+Cys	Threonine	Tryptophan	Valine	Histidine	∑ Essential amino acid	Reference
Standard protein*	4.1	3.0	6.1	4.8	2.3	2.5	0.66	4.0	1.6	29.06
Fish species
Carp	7.2	4.6	8.0	10.4	4.1	4.2	0.9	4.5	-	43.8	[50]
Trout	8.1	4.8	8.2	10.2	4.1	4.5	1.1	5.1	-	46.1
Cod	7.4	4.4	7.8	10.7	4.3	4.0	1.1	4.5	-	44.2
Salmon	8.4	4.6	7.4	8.8	4.4	4.1	1.1	4.8	-	43.7
Herring	7.2	4.6	8.1	9.5	4.1	4.3	1.2	5.0	-	44.0
Herring	6.69	4.16	7.83	9.04	4.09	4.33	1.38	4.82	-	42.34	[1]
Perch	6.46	4.14	7.62	8.95	4.03	4.25	1.49	4.31	-	41.25
Roach	7.10	4.32	8.03	9.51	3.83	3.99	1.48	4.70	-	42.96
Pikeperch	8.10	4.77	8.67	10.41	4.20	4.97	1.38	5.02	-	45.36
Bream	6.85	4.14	7.57	9.12	3.81	4.09	1.16	4.53	-	41.27
Eel	6.42	4.07	7.10	8.55	3.93	4.14	1.24	4.48	-	39.93
Grass carp	7.23	4.28	7.81	9.83	3.14	4.75	3.02	4.91	2.69	47.64	[25]
Bighead carp	7.37	4.24	7.89	9.69	2.54	4.71	3.04	4.93	2.67	47.08
S. sturgeon	7.82	4.33	7.97	10.02	2.67	4.86	1.80	4.85	3.29	47.62
Wels catfish	7.36	4.35	7.98	9.71	2.25	5.15	1.98	4.74	2.34	45.86

*Recommended amino acid scoring pattern for adult, FAO 2011.

“-” means no data

Fatty acid profile

The percentage content of saturated fatty acids (SFA; 16.32–64.71%), monounsaturated fatty acids (MUFA; 8.5–55.34%), and polyunsaturated fatty acids (PUFA; 10.67–67.4%) in the fat of the fishes (Table 3) varied over a wide range and did so within and between species. The differences in the proportions of main groups of fatty acids within a single species ranged from 3% in herring to 60% in pike for SFA, from 1.6% in herring to 63.5% in cod for MUFA, and from 3% in herring to 82% in pike for PUFA. The recommended ratio of PUFA to SFA should be at least 0.45:1.^[54] The PUFA/SFA ratios in the presented fish species (Table 3) ranged from 0.44:1 (bighead carp) to 2.79:1 (cod), and therefore met or very nearly met the recommendation. Exceptions were pike caught in spring after spawning, in which this ratio was much lower (0.16:1) than the recommended minimum. The prophylactic properties of fatty acids are determined by the n-3:n-6 fatty acid ratio. The optimal ratio is 1:5 (0.2:1).^[12] In the lipids of fish and fish products this ratio is usually much higher, which is nutritionally highly beneficial in the human diet.^[12] The n-3:n-6 ratio in the presented fishes (Table 3) ranged from about 0.4:1 in carp to 13.0:1 in cod with only one exception, and therefore these fishes had high nutritional value. The exception with a lower ratio than the recommended value, 0.1:1, was shown in only one study in farmed carp.^[53] The significant differences in the content of the major fatty acid groups and in the PUFA to SFA and n-3 to n-6 PUFA ratios can be explained by the effect of the same factors that influenced the total fat content in the fish muscles. According to Steffens,^[55] the fatty acid profile of the diet of fish has a qualitative and quantitative effect on the fatty acid composition of fish lipids. This effect has been demonstrated in studies on the chemical composition of the muscles of carp and European catfish, as well as on those of predatory and non-predatory fish. The fatty acid profile of carp muscles depended on that of zooplankton, a natural food of these species. The content of n-3 PUFA and n-6 PUFA increased with the level of these fractions in the fatty acid profile of zooplankton.^[56] The muscle tissue of European catfish fed on artificial feed contained significantly more n-3 highly unsaturated fatty acid (HUFA) content and less n-6 HUFA content (24.46% and 0.99%, respectively) than that of fish of the same species fed natural feed (18.22% and 3.96%, respectively).^[42] The muscles of non-predatory fish, i.e. roach and bream, had smaller amounts of SFA (35.38 and 29.91%, respectively) than those of predatory fish, i.e. perch and pike (37.44 and 36.46%, respectively).^[41] Usydus et al.^[12] noted the location where fish are caught to be significant in the case of wild fish because it determines the composition of the available food and thus can influence fatty acid composition. The fatty acid profile of pike muscle has also been shown to be largely influenced by the season when the fish were caught. Fillets of pike caught in autumn (before spawning) contained predominantly unsaturated fatty acids (74.30% UFA), while fillets of fish caught in spring, i.e. after spawning, were dominated by saturated fatty acids (64.71% SFA). There is also an interrelation between season and fatty acid composition. The content of ∑ PUFA was six times lower in the fillets of fish after spawning, and the content of n-3 PUFA was nearly nine times lower.^[52] As stated by Jankowska et al.,^[44] the fatty acid profile of fish can also be significantly affected by water temperature, and a decrease in water temperature may increase the degree of unsaturation of fish lipids. Ligaszewski et al.^[56] reported that an increase in water temperature led to an increase in MUFA content and a decrease in DHA content in carp muscles. Summer to winter changes in water temperature were shown to result in differences in the PUFA to SFA and PUFA to MUFA ratios in farmed trout, with their metabolic adaptation to the cold resulting in a significant increase in these ratios.^[57] Sushchik et al.,^[58] ascribed more importance to phylogenetic factors than water temperature and diet composition in determining the fatty acid composition and HUFA content in fish. That study also indicated that the effect of changes in temperature on EPA and DHA in fish tissues may depend on the species, as only one of the two fish species inhabiting both cold and warm rivers had higher EPA and DHA content at lower temperatures.

Table 3. Comparison of fatty acids in the fat of selected fish species (%)

Gatunek	∑ SFA	∑ MUFA	∑ PUFA	∑ n-3	∑ n-6	n-3/n-6	C20:5 n-3	C22:6 n-3	C20:4 n-6	Reference
Bighead carp	30.04	55.31	13.4	9.29	4.11	2.25	2.92	2.29	1.34	^[Citation51]
Bream	24.6	35.8	39.6	26.6	10.5	2.6	8.8	9.1	4.8	^[Citation27]
	38.40	38.76	22.84	12.23	10.61	1.18	5.85	3.24	3.42	^[Citation32]
	26.44	46.50	27.01	17.39	9.56	1.81	5.00	5.96	2.90	^[Citation2]
	-	-	-	-	-	2.20	8.91	9.83	5.83	^[Citation41]
Grass carp	32.96	41.84	24.62	10.94	13.64	0.8	1.10	1.08	2.72	^[Citation51]
Perch	26.81	23.59	49.60	41.74	7.83	5.33	6.34	31.02	4.27	^[Citation2]
	25.54– 34.60	38.87– 39.62	18.15– 29.61	-	-	1.5– 1.9	3.25– 5.26	4.21– 8.51	2.14– 3.86	[38]
	47.53	23.83	28.65	13.72	12.71	-	8.28	4.15	7.95	[40]
	46.02	32.24	21.73	10.39	9.91	-	6.17	2.65	4.26
	-	-	-	-	-	2.75	6.06	17.77	6.55	[41]
Pike	25.70	15.97	58.33	41.26	15.79	2.61	6.67	27.96	8.60	[52]
	64.71	24.61	10.67	4.84	5.84	0.82	0.73	3.13	1.78
	36.28	27.46	36.26	24.85	11.41	2.18	5.71	15.73	4.38	^[Citation32]
	-	-	-	-	-	3.82	6.43	24.67	5.46	^[Citation41]
	34.15	21.96	42.19	28.64	13.55	2.11	4.29	22.84	5.76	^[Citation21]
Carp	25.43	49.75	24.80	6.28	17.50	0.37	1.48	1.34	1.18	^[Citation33]
	28.7	55.0	16.2	4.7	10.8	0.4	1.2	1.7	2.0	^[Citation27]
	27.0	51.1	21.9	11.6	10.3	1.1	2.3	2.1	1.4	^[Citation12]
	29.96	48.57	21.46	6.66	14.81	0.42	2.61	1.73	2.52	^[Citation32]
	26.33	46.32	26.38	9.29	17.09	0.54	2.40	3.14	3.76	^[Citation21]
	24.73	55.34	19.93	1.96	-	0.1	ND	ND	-	^[Citation53]
Trout	24.3	35.9	39.8	27.7	8.4	3.3	6.4	15.9	0.8	^[Citation27]
	22.1	31.6	46.3	37.5	8.8	4.3	8.0	17.5	0.5	^[Citation12]
	31.06	39.98	28.96	24.02	-	4.9	4.40	15.54	-	^[Citation53]
Pikeperch	37.26	28.11	34.63	23.46	11.17	2.11	5.47	13.64	4.57	^[Citation32]
	25.51	22.30	52.19	43.93	8.15	5.39	6.21	33.25	4.66	^[Citation2]
	31.28	33.13	35.60	32.07	-	9.1	8.78	13.46	-	^[Citation53]
European catfish	25.41– 26.36	39.86– 37.82	34.73– 35.82	22.88– 25.75	9.91– 6.56	2.31– 3.93	4.46– 7.67	9.94– 13.53	3.14– 0.67	^[Citation42]
	16.32	54.08	26.31	9.68	16.63	0.58	1.63	3.01	0.4	^[Citation51]
Siberian sturgeon	18.37	54.72	24.24	10.23	14.00	0.73	2.38	3.7	0.48	^[Citation51]
Roach	25.10	32.49	42.41	33.15	9.20	3.60	5.85	19.82	4.55	^[Citation2]
	-	-	-	-	-	2.94	6.71	14.16	5.49	^[Citation41]
	34.98	46.83	18.19	16.78	-	11.9	4.03	8.76	-	^[Citation53]
Eel	29.34	52.17	18.49	12.84	5.59	2.29	2.46	4.97	1.39	^[Citation2]
Cod	24.1	8.5	67.4	62.6	4.8	13.0	7.6	50.8	2.8	^[Citation12]
	33.12	23.28	41.60	36.47	5.13	7.11	10.95	23.85	0.30	^[Citation21]
Herring	28.6	30.7	40.7	34.4	6.3	5.5	6.2	20.4	0.7	^[Citation12]
	29.44	31.20	39.35	31.77	7.44	4.27	7.08	17.80	0.69	^[Citation2]
Salmon	24.3	26.1	49.6	43.7	5.9	7.4	3.8	26.6	0.8	^[Citation12]
	31.47	39.10	27.11	22.08	5.03	4.39	6.81	13.86	0.16	^[Citation21]

“-” or ND means no data

Eicosapentaenoic acid, docosahexaenoic acid, and arachidonic acid are also important in terms of health benefits. These acids are essential for growth and development and for prevention of disease.^[3,5,59] Their levels in the muscles of the fishes presented in Table 3 varied widely. The content of eicosapentaenoic acid (C20:5 n-3) in the fish muscles ranged from 0.73% in pike to 10.95% in cod. The largest percentage differences in the content of this acid (89%) were observed in pike and were associated with the season when the fish were caught. The EPA content in specimens of this species caught in spring (0.73%) was nine times lower than in individuals obtained in autumn (6.67%).^[52] In other species, differences in EPA content within the same species were 79% in wels catfish, 61% in perch, 54% in carp, 45% in trout, 44% in bream, 44% in salmon, 40% in roach, 31% in cod, 38% in pikeperch, and 13% in herring. The content of docosahexaenoic acid (C22:6 n-3) in the fish muscles (Table 3) ranged from 1.08% in grass carp to 50.08% in cod. Within the same fish species, differences in DHA content were 92% in perch, 89% in pike, 78% in wels catfish, 67% in bream, 60% in pikeperch, 57% in carp, 56% in roach, 53% in cod, 48% in salmon, 13% in herring, and 11% in trout. The content of arachidonic acid (C20:4 n-6) in the fish muscles (Table 3) ranged from 0.16% in salmon to 8.60% in pike. Within single species, differences in AA content were 89% in cod, 87% in wels catfish, 80% in pike and salmon, 73% in perch, 69% in carp, 50% in bream, 38% in trout, 17% in roach, and 2% in pikeperch. The muscles of herring did not differ in the content of this acid. In most of the fish presented in Table 3, the percentage of AA was lower than that of EPA and DHA (except for grass carp and some specimens of perch, pike and common carp, in which EPA or DHA was present in greater amounts than AA). The data indicate that all the fish species were sources of EPA, DHA and AA, but varied significantly in the percentage content of these acids, which was associated with factors such as species, environment, diet, season, and water temperature.^{[2,12,38,40,41,52,56]}

The European Food Safety Agency recommends a combined EPA+DHA daily intake of 250 mg.^[60] Large amounts of EPA+DHA (1–1.5 g/day) are recommended for people with diseases of the cardiovascular system.^[10,11] This considered, the recommended fish consumption frequency of once or twice a week^[61,62] may not be sufficient to produce therapeutic effects if the fish species is one with low levels of these acids. Research showed that fishes in the diet such as sutchi catfish or tilapia imported from China and Vietnam or carp farmed in Poland were not significantly preventive of coronary heart disease due to their low levels of eicosapentaenoic and docosahexaenoic acid (2.0%, 3.7% and 4.4%, respectively).^[12] Alburnus mossulensis and Liza abu were not found to be good sources of n-3 fatty acids (8.93% and 3.90%, respectively) in comparison to seven other species of freshwater fish from the River Tigris in Turkey (24.91–54.45%).^[22] Carp from a fish farm in Gryfino, Poland were not a good source of n-3 fatty acids (1.96%), in contrast with rainbow trout from a fish farm in Łożnica (24.02%).^[53] Recommendation for fish consumption may also not be meaningful even if it specifies species, because pike caught in spring were found to contain significantly smaller quantities of valuable n-3 and n-6 fatty acids (4.84% and 5.84%, respectively) than pike caught in autumn (41.26 and 15.79%, respectively).^[52]

The presented data indicate that the analysis of the chemical composition of food fish muscles is necessary, especially if they are part of a balanced diet.

Conclusion

The content of the nutrients in fish muscles, and thus their nutritional value, varied greatly. Both inter- and intraspecies differences were evident. Various factors such as species, diet, body weight, environment, catch season, and farming system affect the chemical composition of fish muscle tissue; however, despite the causes being known, no discernible pattern of these chemical composition variations was found. Recommendations for the frequency and amount of fish to include in preventive or therapeutic diets fail to take account of possible variations in its nutritive value. Therefore, the available food nutrition tables in relation to food fish should not be treated as universal reference data. In the light of the general recommendation to consume fish, the possibility of its use in preventive and therapeutic diets, and the large inter and intraspecies differences in chemical composition, continuing research on the nutritive value of food fish is highly justified. This knowledge is necessary to gain the optimal nutritional and health benefits from the consumption of fish.

Conflict of interest

The authors declare that there is no conflict of interest.