SUBROTO GHOSH, Ph.D
Department of Zoology
Fisheries and Aquaculture Unit (Post Graduate section)
Rishi Bankim Chandra College
West Bengal, India
Submitted to and Published by Biology Online.
Fish is the more common source of nutrition in the diet of the common Indian. As Ackman (2000) puts it "it is indisputable that fish is in all respects a healthy food" and in the backdrop of a need to understand the nutritional values of the fish items in the diet of the common man in India, it is thought essential to investigate the composition and distribution of lipids and fatty acids in a very common Indian fish vis-à-vis its role as a nutritional component in regard to the atherogenic and thrombogenic indices.
Interestingly enough one finds that most of the lipid studies are based on fish oil. Dyerberg et al. (1975, 1978) had reported a low rate of coronary mortality amongst the Greenland Eskimos whose diet included a strikingly higher intake of n-3 PUFA from their marine food sources like seals and whales. The Zutphen Study of Keli et al. (1994) was based on fish consumption and the risk of stroke. The Lugalawa study in Tanzania carried out by Pauletto et al. (1996a, b) showed similar results with fish in the diet. Silva et al. (1996) stated that fish consumption is an important modulator of fish oil efficacy and concluded that the triacylglycerol (TAG) lowering effect of fish oil is affected by fish consumption. Yamada et al. (2000) declare that diets rich in fish are associated with a decrease in the incidence of atherosclerosis and that is related to the n-3 fatty acid content of the fish. Bays et al. (2003) state that marine fish oils rich in n-3 fatty acids lower TAG and may be effective in combination with statins to treat combined hyperlipidaemia. Upadhya et al. (2002) state that high cholesterol diet increases lipid peroxidation in erythrocytes, spleen and aorta. But according to them dietary fish oil is protective against peroxidative damage caused by the high cholesterol diet. They believe that this could be due to the fact that PUFA in fish oil increase the utilization and uptake of cholesterol.
Channa punctatus (Bloch 1793) is a freshwater common green snake-headed spotted murrel belonging to the family Channidae of the order Channiformes and has accessory respiratory organs that help the fish to survive in inhospitable situations. It has a natural distribution in India and the other neighbouring countries. C. punctatus in having a low content of fat and found abundantly enough have strangely not found the attention of nutritionists.
Channa punctatus can very well be termed as a ‘lean' fish because of its very low lipid contents through out the year. Moreover these fishes have very little or no adipose tissues. According to Ghosh (2006) the Total Lipid (TL) of body flesh of C. punctatus is very low and is highest during the pre-breeding season, which is ca. 0.5%. The TL content in the pre-breeding and in the post-breeding seasons are pretty less in the body flesh of C. punctatus. The average TL content through out the year comes to about 0.37%. This may be an important reason for the easy digestibility of this species, according to Ghosh (2006).
Ghosh (2006) reports that in C. punctatus, TL decreases as the fish moves from pre-breeding to breeding and further on to the post-breeding season. The TL of liver is high. The breeding season profile is an indication that the fish utilizes lipid for oogenesis and this is reflected in the total lipid profiles of the roes. The TL of the roe of C. punctatus is exceptionally high.
According to Ghosh (2006) of the three fractions i.e. Phospholipids (PL), Neutral Lipids (NL) and Glycolipids (GL) PL are the major one in the flesh and are found maximally in the post-breeding season. The NL content remained more or less around 40 % w/w of TL through out the year. The GL remain the minor fraction. The Hydrocarbons-Wax esters-Steryl esters (HC-WE-SE) combine in the flesh is notably low in the pre-breeding season and noticeably high in the breeding. HC are the major component while WE and SE are nominally present. Triacylglycerols (TAG) in the flesh of C. punctatus are markedly high in the pre-breeding and very low in the breeding season. Free Sterols (ST) as % of TL are below 1 in the breeding season and as % of NL they are a little above 1. Amongst the PL components Phosphatidylethanolamine (PE) is the major fraction in the pre-breeding, Phosphatidylinositol (PI) in the breeding and Phosphatidylcholine (PC) in the post-breeding season. Sphingomyelin (SM) is found in a moderate amount in the breeding and pre-breeding but low in the post-breeding season.
It has been seen that those fishes which have very low lipid content in their flesh always have high lipid contents of all classes in their livers. According to Ackman et al. (2002) this suggests that the liver is the chief site of lipid synthesis and storage. The lipid content in the liver of C. punctatus is high (Ghosh, 2006).
The ST % falls appreciably in the breeding season and again rises in the post-breeding season but through out the year it is important to note that C. punctatus has a low % of ST. The ST as % w/w of TL in the body flesh and liver in the breeding season show a low profile, according to Ghosh (2006). He had also seen that ST in the flesh increase many folds in the post-breeding season.
The C. punctatus liver has the TAG as the major component while in all other tissues of it is the HC-WE-SE combine (Ghosh, 2006). HC are the main part of this combine. SE are low in the flesh through out the year but in the liver they are quite high unlike that in the breeding season. This high level of SE in the post-breeding liver is a vital indication of the accentuated activity of the liver in the fatty acid synthesis for the spawning phase of the life cycles. WE are an important constituent of simple lipids and are a major component of depot fats (Christie, 1982) and their low % in the flesh of C. punctatus irrespective of its reproductive cycle phase indicates the lean quality of the fish meat.
The TAG of the body flesh and liver of C. punctatus are much higher in the pre-breeding season. It is lowest in the breeding season. As the fish prepares for breeding the TAG contents of the liver increase.according to Ghosh (2006). The TAG are more often storage lipids and reflect the fatty acid composition of the diet to a greater extent than do the phospholipids. According to Ackman et al. (2002) the occurrence of a higher amount of TAG in the flesh suggests a need for a reserve to meet a higher physical and metabolic activity in the animal. The TAG of the flesh of C. punctatus is 10.23% (Ghosh, 2006).
In C. punctatus the liver of the breeding season has no detectable amount of cholesterol. This is very significant. In humans the liver together with the intestine account for about 10 % each of total synthesis (Mayes and Botham, 2003). In the fish, too, liver is a site of synthesis. C. punctatus adult being fundamentally a carnivore (Chondar, 1999) feeds on aquatic insects, shrimps, gastropods and fishes; and their feeding activity is highest in the pre-breeding season. Naturally, in the breeding season there is no detectable amount of cholesterol in the liver as dietary cholesterol inhibits hepatic synthesis (Mayes and Botham, 2003). The post-breeding season flesh of C. punctatus has high cholesterol. 22-Dehydro-cholesterol has been detected by Ghosh (2006) in the breeding and the post-breeding seasons. 22-Dehydro-cholesterol is an intermediate step in the synthesis of cholesterol and its presence in the liver once again proves that the liver is a site for its synthesis.
The principal fatty acids out of the 35 detected in the fish of C. punctatus does correspond with Ackman's list of 14 fatty acids that are "really needed to describe the fatty acids of fish" (Ackman, 2000). Major fatty acids found in C. punctatus are myristic (14:0), palmitic (16:0), stearic (18:0), oleic (18:1n-9), linoleic (18:2n-6), a-linolenic (18:3n-3), arachidonic (20:4n-6), eicosapentanoic acid (EPA) or timnodonic (20:5n-3) and decosahexanoic acid (DHA) or cervonic (22:6n-3). g-linolenic (18:3n-6) is present as a minor component. It is interesting to note that an important fatty acid like palmitoleic (16:1n-7) is absent in the fish as are lauric acid (12:0), myristoleic acid (14:1n-9) and hexatrienoic acid (16:3n-4) whereas pentacosanoic acid (25:0) is present only in breeding and post-breeding liver. DHA (22:6n-3) is not detected in the breeding C. punctatus flesh. Another important feature in this fish is the absence of an eicosatrienoic acid (20:3n-9). This indicates that there is no fatty acid deficiency in the fish (Ackman, 1994a).
Stearic acid, a saturated C18 fatty acid increases in both the tissues of C. punctatus as the fish prepares for spawning. Moreover, they have the essential fatty acids (EFAs) like linoleic (18:2n-6), arachidonic (20:4n-6) and linolenic (18:3n-3) which they get from plant sources via their food chain as fishes like all other animals lack the D12- and D15-desaturases and so cannot synthesise the EFAs. Linoleic can convert into linolenic and then further into arachidonic by desaturation and by chain elongation. Both these fatty acids are present in the fish in the right balance as worked out by Okuyama (2000). Arachidonic acid, which is the main product of the elongation and desaturation of linoleic acid, also has EFA activity. It is one of the principal precursors of prostaglandins (Christie, 1982). The presence of the minor component g-linolenic (18:3n-6) is important, as it is a precursor of a family of PUFA. EPA and DHA are both present in considerable amounts making the fish a good source of n-3 PUFAs.
C16 and C18 are more than C20 and C22 acids. Freshwater fish can convert C20 PUFA to a number of eicosanoids with the help of cycloxygenase and lipoxygenase enzymes (Henderson, 1996). Octadecatetraenoic acid in TL is of non-detectable amount (i.e.<0.001) except in the pre-breeding season when it is in a low amount. But DHA occurs in good amount. This indicates that C. punctatus efficiently converts the C20 PUFAinto EPA and to DHA.
The fat quality was evaluated by the atherogenic index (AI) and the thrombogenic index (TI) according to Ulbricht and Southgate (1991). It is evident that both the AI and the TI indices for C. punctatus are present in fair amount through out the year. AI varies from 0.95 to 0.56 in the flesh TL and TI varies from 0.37 to 0.41. The TI for the fish flesh suggests a high anti-thrombogenic quality of this fish meat. This is in stark contrast to the TI values of beef (1), lamb meat (1.4) and milk-based products (2.1) (Amerio et al., 1996).
C. punctatus being part of the diet have a definite benefit on human health. The long chain n-3, popularly called omega-3 (w-3), fatty acids like EPA (20:5n-3) and DHA (22:6n-3) have been proved beyond doubt of their beneficial roles (Burr et al., 1989; von Schacky, 1992; Bigger and El-Sherif, 2001; Christensen et al., 2001). EPA has a protective effect against thrombosis, atherosclerosis and other inflammatory diseases while DHA is effective in skin disorders, aids in the development of brain and a part of the retina. (Lee et.al., 1985). EPA also reduces the concentration of cholesterol and triacylglycerol in the plasma by lowering the rate of synthesis of LDL and VLDL by the liver and vascular tissues (Illingworth et.al., 1984). The n-3 fatty acids have been shown to modify several key risk factors for cardiovascular disease. The benefits include increased HDL2-cholesterol concentrations, reduced TAG-rich lipoprotein concentrations and others (Nestel, 2000). Pownall et al. (1994) had found that an addition of fatty acids to the diet lowers TAG levels particularly in patients with hypertriglyceridemia. This effect is not seen with plant sources of n-3 PUFA (Kestin et al., 1990).
Connor (1994) had listed five putative mechanisms of diatary n-3 PUFA on lipoprotein metabolism in humans The reduced platelet aggregation and prolonged bleeding times of the Greenland Eskimos suggested an important mechanism by which n-3 PUFA could affect CHD (Dyerberg et al., 1978). Early studies of the nuits highlighted their lower coronary mortality compared to their Danish counterparts. Their diet included a strikingly high intake of n-3 PUFAs. This resulted in lower blood cholesterol, lower TAG, lower LDL- and VLDL-cholesterol, increased HDL-cholesterol, increased bleeding times, and lower rates of CHD (Kromhout et al., 1985; Shekelle et al., 1985; Dolecekand Grandits, 1991; Kromhout et al., 1995). These effects have been seen better in women (Iso et al., 2001).
In the fish under investigation cholesterol is quite high in the flesh except in the breeding season when it was not detected. But the average consumable cholesterol of the fish which is around 155mg is far less than the recommended dietary intake of cholesterol of <200mg (Kris-Etherton et al., 2003). The contribution of dietary SAFA to serum Total Cholesterol (TC) is far greater and is almost ten times that of dietary cholesterol (Enas et al.,2003). Although many affluent Indians consume 50% of energy from fat mainly from the cooking medium of vegetable ghee, which has a load of transunsaturated fatty acids (TRAFA), and fatty fish and meat, most of the average Indians consume 20%-25% of energy from fat.
This fish can be safely recommended as it has low SAFA and high PUFA and MUFA. The SAFA/UFA ratios are below 1 in the pre-breeding and in the post-breeding seasons while in the breeding season it is marginally high. Enas (1996) showed that SAFA raises the serum TC level thrice as much as PUFA does, and MUFA lowers it. Most of this increase is due to an increase in LDL. Some increase in HDL also is registered but is not sufficient to offset the atherogenicity and thrombogenicity resulting from marked elevation of LDL. The AI and TI of the fish are low indicating low LDL. Stearic acid (18:0) is desaturated to oleic acid (18:1n-9) soon after its absorption and so dose not raise the TC level (Bonanome and Grundy, 1988; Denke and Grundy, 1992). In the fish oleic acid (18:1n-9) is in good quantity while stearic acid (18:0) is present substantially, therefore, the fish can be recommended.
It is to be mentioned that fishes have the unique capability of metabolising fats readily and, as a result can stay for long periods of time under conditions of food deprivation. This is reflected very well in this fish. The n-3/n-6 ratios once again prove that this lean fish may be classified as Type II (Takeuchi, 1996).
The high carbohydrate diet very common in the Indian families has its positive roles but it is also associated with highly atherogenic VLDLs. So a complementary of a fish like C. punctatus in the diet would be more effective in preventing CAD amongst Indians who have a high prevalence of this metabolic syndrome and diabetes.
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