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Protein and lipid sources affect cholesterol concentrations of juvenile Pacific white shrimp, Litopenaeus vannamei (Boone)¹

Hagerman Fish Culture Experiment Station, University of Idaho, Hagerman 83332

Abstract

Two experiments were conducted to evaluate the effects of protein and lipid sources on cholesterol, AA, and fatty acid content, and on biological performance of juvenile Pacific white shrimp, Litopenaeus vannamei (Boone). In Exp. 1, seven isonitrogenous and isocaloric diets were prepared using fish meal; soybean meal; casein; fish meal + soybean meal; fish meal + casein; soybean meal + casein; and fish meal + soybean meal + casein. In Exp. 2, seven isonitrogenous and isocaloric diets were prepared using fish oil; soy oil; poultry fat; fish oil + soy oil; fish oil + poultry fat; soy oil + poultry fat; and fish oil + soy oil + poultry fat. Nine shrimp (average BW 570 mg) were stocked per 60-L tank, with three tanks per diet in each experiment. Shrimp were fed to apparent satiation twice daily for 28 d. Protein sources affected shrimp cholesterol, feed consumption, feed efficiency, protein consumption, protein efficiency ratio, and crude body fat (P 0.05), but not weight gain, survival, hepatosomatic index, body protein, ash, and AA composition. Body (without hepatopancreas) cholesterol concentrations were the highest in shrimp fed the diet containing fish meal (0.81%), lowest for those fed the casein diet (0.64%), and intermediate in the other dietary treatment groups (range 0.71 to 0.74%). Lipid source also affected shrimp body cholesterol, body fatty acid profiles, and fatty acid profiles in the hepatopancreas (P 0.05), but not growth performance, body protein, fat, ash, and cholesterol concentrations in the hepatopancreas. Shrimp fed the fish oil diet had the highest body cholesterol (0.75%), whereas those fed the soy oil or poultry fat diets were lowest (0.66 and 0.65%, respectively). Results indicate that by replacing fish meal and fish oil with soybean meal and soy oil, shrimp growth performance is not affected, but body cholesterol concentration is reduced.

Key Words: Amino Acids • Cholesterol • Fatty Acids • Lipid • Protein • Shrimp

Introduction

The effects of protein and lipid sources on cholesterol concentrations in plasma, tissue, and the body as a whole have been studied in swine and other animals (Hutagalung et al., 1969; Pond et al., 1992; Potter et al., 1996). However, the effects of protein source on cholesterol and AA concentrations and the effects of lipid source on cholesterol and fatty acid profiles of the Pacific white shrimp, Litopenaeus vannamei, the most widely cultured shrimp species in North, Central and South America, have not been investigated. In the United States, per capita consumption of shrimp is the highest of any seafood, despite the fact that the cholesterol concentration in shrimp is high and second only to squid among seafood (King et al., 1990). High cholesterol consumption by humans may be related to high incidence of cardiovascular diseases, stroke, and certain types of cancer (NRC, 1989). Therefore, given that over 35% of shrimp consumed in the United States are produced by aquaculture and that this percentage is expected to increase, lowering cholesterol levels in farmed shrimp could be advantageous to consumers wishing to lower their dietary cholesterol intake while continuing to consume shrimp.

Generally, animal proteins have been associated with a hypercholesterolemic effect, plant proteins have a hypocholesterolemic effect, and unsaturated fats have a hypocholesterolemic effect compared with saturated fats. In the past, the animal protein used in most studies was casein and the plant protein was soy protein, whereas the animal lipid was tallow and the plant lipid was soy oil (SO) (Barrows et al., 1980; Potter et al., 1996). However, the primary animal protein and lipid sources used in practical shrimp feeds are fish meal (FM) and fish oil (FO) (Cheng et al., 2002a,b); therefore, they were used in this study. The objectives of the study were: 1) to evaluate the effects of FM, soybean meal (SBM), casein, and their combination on cholesterol and AA concentrations and on biological performance of the juvenile Pacific white shrimp; and 2) to evaluate the effect of FO, SO, poultry fat (PF), and their combination on cholesterol concentration and fatty acid profiles and on biological performance of the shrimp.

Materials and Methods

Experiment 1
Experimental Design and Diet Preparation.
Seven experimental diets (Table 1) were formulated with 250 g of FM/kg of diet, 250 g of SBM/kg of diet, 250 g of casein/kg of diet, combinations of FM + SBM (125 g of each/kg of diet), FM + casein (125 g of each/kg of diet), SBM + casein (125 g of each/kg of diet), and FM + SBM + casein (83 g of each/kg of diet). Diets were formulated to be isonitrogenous and isocaloric, which was done by adjusting the amounts of wheat gluten, wheat, corn gluten meal, and SO. All diets were formulated to contain 42% CP and a calculated DE level of 3,600 kcal/kg of diet. The CP level of each ingredient was analyzed (data not shown), and the value of DE was calculated from the nutrient composition of those ingredients according to NRC (1993). All diets were pelleted without steam at the Hagerman Fish Culture Experiment Station (Hagerman, Idaho) according to the methods of Cheng et al. (2002c) using a laboratory pellet mill (California Pellet Mill Co., San Francisco, CA) equipped with a 2.4-mm die. Diets were air-dried at approximately 20°C for 48 h, and samples were removed for chemical analysis. The diets were then shipped to the Universidad of Nuevo León (Nuevo León, Mexico). Analyzed chemical values of the diets were close to expected values (Table 1).

Shrimp were fed to apparent satiation twice daily. Daily feeding was fixed at 10% of the biomass present in the tanks for the first 2 d and then adjusted for each tank such that a small excess of feed remained at the next feeding. The pellets were broken into small pieces to allow each shrimp to eat at least one piece at each feeding time. The feeding trial lasted for 28 d. The experiment followed the guidelines of the Animal Care and Use Committee of the University of Idaho.

Shrimp Tissue Sampling Procedure.
On d 28, after being individually weighed, shrimp were dissected and the hepatopancreas placed into in a previously weighed Eppendorf tube, which was weighed again and identified by tank number plus shrimp number from the shrimp weight record for each tank. Eppendorf tubes and dissected bodies were immediately packed together in aluminum foil marked with the tank number and frozen by immersion in liquid nitrogen. Lyophilization was conducted on the opened packs and Eppendorf tubes with the following steps: 8 h at -30°C, 6 h at -25°C, 10 h at -15°C, 16 h at -10°C, and 6 h at 10°C. After lyophilization, all shrimp samples were shipped back to the Hagerman Fish Culture Experiment Station for chemical analyses.

Chemical Analysis.
Feed samples were dried in a convection oven at 105°C for 2 h to determine moisture level according to AOAC (1995). The dried samples were finely ground by mortar and pestle and were analyzed for N using a LECO FP-428 nitrogen analyzer (LECO Instruments, St. Joseph, Michigan) to determine CP content (total N x 6.25 = CP). Crude fat was measured using a soxhlet extraction apparatus (Soxtec System HT, Foss Tecator AB, Hoganas, Sweden) with methylene chloride as the extracting solvent and ashed by incineration at 550°C in a muffle furnace. Cholesterol and AA were determined according to AOAC (1995). For AA analysis, samples were hydrolyzed in 6 N HCl for 24 h at 110°C, and AA contents in hydrolysates were determined with by ion-exchange chromatography. Amino acid concentrations were not corrected for incomplete recovery resulting from hydrolysis.

Calculations and Statistical Methods.
Individual weights were measured at the beginning and the end of the experiment. Shrimp were blotted on a wet cloth and individually weighed on a digital scale to the nearest milligram. Protein efficiency ratio (PER) was determined as: PER = (final weight - initial weight) x 100/protein consumption, and hepatosomatic index (HSI, %) was calculated as: HSI = liver weight x 100/somatic weight. Prism (version 3.0, GraphPad, Inc., San Diego, CA) was used to perform statistical calculations; data were analyzed by ANOVA as a completely randomized design with shrimp tanks as experimental units. The Student-Neuman-Keuls procedure was used to detect differences among treatments; an alpha level of P 0.05 was used to determine statistical significance.

Experiment 2
Seven experimental diets (Table 2) were formulated with 30 g of FO/kg of diet, 30 g of SO/kg of diet, 30 g of PF/kg of diet; combinations of FO + SO (15 g of each/kg of diet), FO + PF (15 g of each/kg of diet); SO + PF (15 g of each/kg of diet); and FO + SO + PF (10 g of each/kg of diet). Diet preparation, shrimp culture conditions, shrimp tissue sampling, chemical analyses, calculations and statistical methodology were the same as those described in Exp. 1. In addition, fatty acids were analyzed using AOAC (1995) methods.

During the time span of the two experiments, water temperature, salinity, and pH were 25.5 to 30.5°C, 29 to 37 g/L, and 7.9 to 8.1, respectively. Unionized ammonia, nitrate, and nitrite were 0, 120 to 140, and 0.5 to 0.8 ppm, respectively. All of these measured water quality variables were in the normal range for shrimp growth (Cheng et al., 2002a,b).

Experiment 1
Biological performance and body composition of shrimp fed experimental diets for 28 d are presented in Table 3. There were no differences in shrimp final BW, weight gain, survival, and HSI among treatments. However, differences existed in feed consumption, feed efficiency, protein consumption, and PER (P 0.05). Shrimp fed the diets containing FM, SBM, or casein had lower feed consumption than those fed the diets containing FM + casein, SBM + casein, and FM + SBM + casein, but were not different from those fed the FM + SBM diet. Shrimp fed the casein diet had higher feed efficiency than those fed the diets containing SBM, FM + SBM, FM + casein, SBM + casein, and FM + SBM + casein, but were not different from those fed the FM diet. Shrimp fed the diets containing FM and casein had lower protein consumption than those fed diets containing FM + casein, SBM + casein, and FM + SBM + casein, and shrimp fed the SBM diet had lower protein consumption than those fed diets containing SBM + casein, and FM + SBM + casein. Shrimp fed diets containing FM and casein had higher PER than those fed diets containing SBM, FM + SBM, FM + casein, SBM + casein, and FM + SBM + casein.

Feeding trials with most farmed species of fish are generally conducted for 12 wk or more. Shrimp are often harvested at approximately 20 to 25 g of total weight after a growth cycle of 16 wk, and most of this weight gain occurs in the final 4 to 6 wk of production. Thus, although shrimp feeding trials are commonly conducted for relatively short periods, weight gains during shrimp trials are at least five times initial BW, making them equivalent to longer trials with finfish, such as rainbow trout. Results in Exp. 1 show that shrimp fed the FM diet had the highest cholesterol concentration in the body (without hepatopancreas), followed by those fed the SBM diet, and shrimp fed the casein diet had the lowest body cholesterol. When fed combinations of FM + SBM, FM + casein, or FM + SBM + casein, shrimp body cholesterol concentrations were reduced compared with those fed the FM diet. This effect has an important implication in practical shrimp production. As FM production is not growing worldwide and aquaculture production is increasing, higher amounts of plant protein ingredients, such as SBM, will have to be formulated into shrimp feeds. By replacing FM with SBM, shrimp production costs may be reduced and shrimp body cholesterol will be reduced. Also, shrimp fed the SBM diet had the lowest hepatopancreas cholesterol content, followed by those fed the FM diet, and shrimp fed the casein diet had the highest hepatopancreas cholesterol content. Results suggest that the lowest body cholesterol concentrations in shrimp fed the casein diet may be due to the fact that dietary cholesterol is digested, absorbed, and stored in the hepatopancreas. Hypercholesterolemic effects of casein and hypocholesterolemic effects of soy protein in blood have been demonstrated in other animal species and humans (Carroll and Hamilton, 1975; Hagemeister et al., 1990). However, in contrast to blood cholesterol in other animals and human, shrimp fed the casein diet had the lowest body cholesterol and the highest hepatopancreas cholesterol concentrations. The hepatopanceas is not eaten by most consumers in developed countries; only shrimp tail (muscle) is eaten. Thus, shrimp having the lowest body concentration of cholesterol would potentially be of interest in such markets.

Both FM and casein are of animal origin and their different actions on cholesterol may be due to their AA compositions and/or cholesterol concentrations. The effects of dietary lysine, methionine, cystine, tryptophan, arginine, taurine, glycine, lysine:arginine ratio, and diets containing unbalanced AA on cholesterol have been investigated (Seidel and Harper, 1963; Kritchesky et al., 1982; Kohls et al., 1987). The AA compositions in diets containing FM and casein, especially arginine, glycine, lysine, and methionine, were different, as were lysine:arginine ratios, which were 1.02 and 0.88 for diets containing FM and casein, respectively. Dietary cholesterol concentrations were 0.25 and 0.17% for diets containing FM and casein, respectively. These differences may have affected shrimp cholesterol concentrations.

In Exp. 1, no differences were found in shrimp weight gain, survival, and HSI among treatments. However, feed efficiency and PER were lower for shrimp fed the SBM diet than for those fed the diets containing FM and casein. Plant protein meals, such as SBM, have lower protein quality than high-quality animal protein meals, such as FM and casein (NRC, 1993). No differences were found in body CP, ash, and AA concentrations except body crude fat and cholesterol concentrations. Shrimp body protein and AA concentrations were high and crude fat levels were low, in agreement with Cheng et al. (2002a,b). Shrimp body AA concentrations reflected their dietary AA concentrations with glutamic acid being the highest and hydroxyproline being the lowest.

Cholesterol is a vital component of cell membranes and is the precursor of bile acids, steroids, and molting hormones. It is reported to be an essential nutrient for growth and survival of all crustacean species (Kanazawa et al., 1971). In both Exp. 1 and 2, all diets were supplemented with cholesterol, because shrimp, like other crustaceans, cannot synthesize cholesterol de novo (Teshima and Kanazawa, 1971). Previous researchers have demonstrated that cholesterol supplementation in diets improves biological performance of prawns (P. japonicus; Teshima and Kanazawa, 1986), lobsters (Homarus americanus; D’Abramo et al., 1984), mud crabs (Scylla serrata; Sheen, 2000), crayfish (Pacifastacus leniusculus; D’Abramo et al., 1985), tiger shrimp (P. monodon; Sheen et al., 1994), and Pacific white shrimp (L. vannamei; Duerr and Walsh, 1996).

In Exp. 2, shrimp fed the FO diet had the highest whole-body cholesterol concentration (without hepatopancreas), followed by shrimp fed the diets containing SO or PF, and no differences in body cholesterol existed between shrimp fed the diets containing SO or PF. When fed combinations of FO + SO, FO + PF, or FO + SO + PF, the body cholesterol concentrations were reduced compared with shrimp fed the FO diet. Results suggest that shrimp body cholesterol concentrations can be altered by replacing FO with SO or PF. Surprisingly, there were no differences in body cholesterol concentrations between shrimp fed diets containing SO or PF. Shrimp fed the FO diet had higher body cholesterol levels than those fed the SO diet. Both FO and SO are unsaturated fats, the differences being that FO is of animal origin and contains high level of cholesterol, whereas SO is of plant origin and does not contain cholesterol. Shrimp fed the FO diet also had higher body cholesterol concentration than those fed the PF diet. Unsaturated fats, such as FO, are reported to have hypocholesterolemic effects and saturated fats have hypercholesterolemic effects in blood (Feldman et al., 1979; Richard et al., 1982; Jackson et al., 1984), but this was not the case for shrimp body cholesterol concentration. Concentrations of cholesterol in shrimp bodies or the hepatopancreas were not correlated with dietary cholesterol either; diets containing FO and PF had the same dietary cholesterol concentrations (0.28%). This suggests that mechanisms other than simply intake may be responsible for the cholesterol lowering effects of PF observed in this study.

No differences are found in shrimp growth performance and body composition except body cholesterol concentration among treatments in Exp. 2. This may due to the fact that almost all ingredients in diets were the same, the only difference among the experimental diets was lipid source, and the difference in the amount of lipids was only 30 g/kg of diet or 3%. The dietary lipid concentration in the experimental diets was 7 to 8%, which is the optimal level for crustaceans (Kanazawa et al., 1977; Davis and Robinson, 1986; Sheen and D’Abramo, 1991). A dietary lipid level of 8% supports the best growth for P. japonicus (Castell and Covey, 1976). A dietary lipid level of 8.1% gives the highest gonadosomatic index, a lipid level higher than 9% negatively affects maturation of L. vannamei (Wouters et al., 2001), and a dietary lipid level greater than 13.9% reduces the percentage of matings per night of female P. stylirostris (Bray et al., 1990).

Results in Exp. 2 indicate that by replacing FO with SO, only the levels of 18:0 and 22:5 are reduced in shrimp body; it does not affect other fatty acid compositions, or the other three n-3 fatty acids in shrimp body. The n-3 polyunsaturated fatty acids play important roles in human and animal nutrition (Twibell et al., 2000; Satoh et al., 1989; Ip et al., 1991). Normolipidemic men who consumed oyster, clam and crab diets (high in n-3 fatty acids and low in cholesterol) exhibited reduced VLDL cholesterol and triglycerides, and LDL and total cholesterol concentrations (Childs et al., 1990). Adults who consumed FO diets (rich in n-3 fatty acids) exhibited reduced platelet aggregation and plasma triglyceride levels (Sanders and Roshanai, 1983), reduced plasma cholesterol and triglyceride levels, and LDL and VLDL cholesterol concentrations (Harris et al., 1983).

Polyunsaturated fatty acids are also essential for the growth of crustaceans. They have been reported to improve the growth performance of freshwater prawns (Macrobrachium resenbergii; D’Abramo and Sheen, 1993), and various other prawns, such as P. chinensis (Xu et al., 1993), P. japonicus (Kanazawa et al., 1979), P. inducus (Read, 1981), and P. duodarum (Sick and Andrews, 1973). In the present study, dietary n-3 fatty acid concentrations were the highest in the FO diet, followed by the diets containing SO or PF. These fatty acid profiles are reflected in shrimp body fatty acid profiles, with shrimp fed the FO diet having the highest concentration of n-3 fatty acids, followed by those fed the diets containing SO or PF. Shrimp weight gain and survival are not affected by dietary fatty acid profiles, indicating that the dietary levels of these polyunsaturated fatty acids met the n-3 fatty acid requirements of the shrimp.

There are essentially no 14:0 and 22:1 fatty acids in shrimp body. It appears that dietary fatty acids, 14:0 and 22:1, are not stored in shrimp body but in the hepatopancreas, indicating that these fatty acids are metabolized in the hepatopancreas as energy sources, or elongated and desaturated to form other fatty acids. None of the diets used in Exp. 2 contained 14:1, 18:4, 20:0, or 22:0. These fatty acids were found in the hepatopancreas but not in shrimp body, indicating that this species of shrimp can synthesize 14:1, 18:4, 20:0, and 22:0, and use them as energy sources. In contrast to M. rosenbergii (de Man) (Tidwell et al., 1998), L. vannamei do not contain 14:0, but do contain 22:5 in their bodies, and contain 14:1, 18:4, 20:0, 22:0, 22:1, 22:5, and 24:0 in their hepatopancreas.

Implications

This is the first demonstration of the body cholesterol-lowering effects of diets containing soybean meal and soy oil in shrimp. Fish meal contains 5 to 10% oil, and thus it contains polyunsaturated fatty acids that are desirable in human nutrition and health. But the cholesterol concentration of fish meal also is high, resulting in higher shrimp cholesterol levels when used in practical shrimp diets. Replacing fish meal and fish oil with soybean meal and soy oil can decrease shrimp body cholesterol, and yet not affect growth performance of shrimp. Because soybean meal and soy oil are widely available and produced through sustainable practices, their use in shrimp feed should be encouraged. As human living standards improve, the demand for seafood, such as shrimp, will increase. Further investigations to determine the mechanisms by which soybean meal or soy oil reduce shrimp body cholesterol are needed.

Footnotes

¹ We thank M. G. López, C. G. Barbosa, D. R. Marie, M. T. Salazar, E. C. Suárez from Univ. of Nuevo León, Monterrey, Mexico, for taking care of shrimp feeding; W. G. Dominy, from The Oceanic Institute, Honolulu, Hawaii, for supplying mineral and vitamin premixes used in this study; and C. Hoffman and J. Cole from Hagerman Fish Culture Exp. Stn., Univ. of Idaho, for their assistance.

² Correspondence: 3059F National Fish Hatchery Rd. (phone: 208-837-9096; fax: 208-837-6047; e-mail: feedtecheng{at}yahoo.com).

Received for publication February 17, 2003. Accepted for publication November 14, 2003.

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