A total of 1050 Jian carp, Cyprinus carpio var. Jian (23.39 ± 0.06 g) were randomly divided into seven groups of each three replicates, which were fed respectively with seven semi-purified diets contained 1.25, 2.71, 4.22, 5.78, 7.23, 8.83 and 11.44 mg riboflavin kg⁻¹ diet for 6 weeks. The results showed that riboflavin significantly improved percent weight gain, specific growth rate, feed efficiency and protein efficiency ratio at the level of 4.22 mg kg⁻¹ diet. Gross protein retention efficiency and lipid production value improved with increasing dietary riboflavin levels from 1.25 to 5.78 mg kg⁻¹. Activities of trypsin, lipase, α-amylase, Na⁺,K⁺-ATPase and alkaline phosphatase in the intestinal tract were significantly improved with increasing riboflavin levels. Weight and protein content of hepatopancreas, intestine protein and intestine length index were also significantly improved.

Introduction

Jian carp (Cyprinus carpio var. Jian) is an artificially bred new variety of C. carpio, which exhibits 30% faster growth rate compared with common carp (Sun et al. 1995; Dong & Yuan 2002). Its production accounts for 35% of the total annual production of C. carpio in the world [Zhu & Wang 2004; FAO (Food Agriculture Organization) 2005].

Riboflavin, which participates in oxidation–reduction reactions, is required by all animals (Deng & Wilson 2003). Previous studies focused mainly on the effect of dietary riboflavin on growth performance of fish. Experiments conducted in common carp (Aoe et al. 1967; Takeuchi et al. 1980), channel catfish (Serrini et al. 1996) and sunshine bass (Deng & Wilson 2003) have demonstrated that dietary riboflavin can improve fish growth. Feed efficiency (FE) and protein efficiency ratio (PER) were enhanced with increasing dietary riboflavin (Serrini et al. 1996; Deng & Wilson 2003).

The intestinal tract, where digestion and absorption of nutrients take place, is very important for fish, especially for stomachless fish such as carp. Intestinal enzymes involve in luminal digestion and brush border membrane digestion of nutrients, and their activities play an important role in the growth rate of fish (Hakim et al. 2006). Yates et al. (2003) have demonstrated that riboflavin can improve the development of the gastrointestinal tract of rat. However, to our knowledge, reports about the relationship between riboflavin and the structure and function of intestine in fish are lacking. Shiau & Su (2004) reported that mid-gut gland index was decreased by myo-inositol. Results of earlier studies indicated that intestine protein content (IPC), intestine weight index and activities of protease, lipase, alkaline phosphatase (AKP) and Na⁺,K⁺-ATPase in the intestine can be improved by the addition of lysine (Zhou et al. 2007a) and glutamine (Lin & Zhou 2006). Consequently, the relationship between dietary riboflavin and intestine function is worthy of being investigated.

The purpose of this study was to investigate the effect of dietary riboflavin levels on growth, feed utilization, body composition and intestinal enzyme activity of juvenile Jian carp.

Materials and methods

Experimental diets

Fish meal (Pesquera Lota Protein Ltda., Chile), soy protein concentrate (Archer Daniels Midland company, USA) and rice gluten meal (Jindege Sugar Co. Ltd, Wuhan, China), α-starch (Sigma, St Louis, USA) and corn starch (Sigma, St Louis, USA), fish oil (CIA. Pesquera Camanchaca S.A., Chile) and soybean oil (Sigma, St Louis, USA), were used as protein, carbohydrate and lipid sources respectively. The experimental diets were formulated to contain 320 g crude protein kg⁻¹ diet and 45.6 g crude fat kg⁻¹ diet (Table 1). Test diets except riboflavin were formulated to meet the nutrient requirements for Jian carp according to Zhou et al. (2007b). The content of riboflavin in each ingredient was determined by spectrophotofluorimetry as described by Woodward (1982). A model SP890 fluorescence spectrophotometer (Turner Designs, CA, USA) was used for the riboflavin analysis. Riboflavin (Sigma) was added the test diets to provide graded concentrations (calculated levels) of 1.3 (unsupplemented control), 2.8, 4.3, 5.8, 7.3, 8.8 and 11.5 mg riboflavin kg⁻¹ diet. The experimental diets were dried with forced air at room temperature for 24 h in a dark room, and then stored at −20 °C until used. At the end of the experiment, the analysed riboflavin contents of the seven experimental diets were: 1.25, 2.71, 4.22, 5.78, 7.23, 8.83 and 11.44 mg kg⁻¹ diet.

Table 1. Formulation and proximate composition of experimental diet fed to juvenile Jian carp (Cyprinus carpio var. Jian)

Ingredients	g kg⁻¹ dry diet
Fish meal	40.0
Soy protein concentrate	241.8
Rice gluten meal	208.0
α-Starch	401.5
Fish oil	26.9
Soybean oil	2.5
Lys–HCl	3.5
dl-Met	4.5
CaH₂PO₄	24.5
Trace mineral premix¹	5.0
Vitamin premix²	10.0
Corn starch	30.5
Choline chloride	1.3
Riboflavin premix³	–
Proximate composition (g kg⁻¹)
CP	320
EE	45.6

¹ Per kilogram of mineral mix: FeSO₄·7H₂O (21.7% Fe) 152.28 g, CuSO₄·5H₂O (25.00% Cu) 2.40 g, ZnSO₄·7H₂O (22.5% Zn) 26.65 g, MnSO₄·H₂O (31.8% Mn) 8.18 g, KI (3.8% I) 5.79 g, NaSeO₃ (1.0% Se) 5.00 g. All ingredients were diluted with CaCO₃.
² Per kilogram of vitamin mix: retinyl acetate (500 000 IU g⁻¹) 0.800 g, cholecalciferol (500 000 IU g⁻¹), 0.480 g, dl-α-tocopherol acetate (50%) 20.000 g, menadione (50%) 0.200 g, thiamine nitrate (98%) 0.063 g, cyanocobalamin (10%) 0.010 g, ascorhyl acetate (92%) 7.247, d-biotin (20%) 0.500 g, folic acid (96%) 0.521 g, meso-inositol (98%) 44.898 g, pyridoxine hydrochloride (98%) 0.744 g, d-calcium pantothenate (98%) 3.337 g, niacin (98%) 2.857 g. All ingredients were diluted with corn starch.
³ Riboflavin premix: 10 g riboflavin kg⁻¹ corn starch. Premix was added to obtain graded levels of riboflavin. The analysed riboflavin contents of the diets were: 1.25, 2.71, 4.22, 5.78, 7.23, 8.83 and 11.44 mg riboflavin kg⁻¹ diet.

Fish maintenance and feeding

Juvenile Jian carp (C. carpio var. Jian) were obtained from Ya’an fisheries and acclimated to experimental condition for 2 weeks. A total of 1050 Jian carp [initial weight (IW) 23.39 ± 0.06 g, mean ± SD, n = 3] were randomly distributed into 21 glass aquaria (90 × 40 × 30 cm, water volume: 90 L). The fish were hand-fed eight times daily at 07:00, 09:00, 11:00, 13:00, 15:00, 17:00, 19:00 and 21:00 h to satiation but without overfeeding for 6 weeks. A closed water and oxygen auto-supplemented system was used to maintain the optimal water quality for Jian carp. Water exchange rates in each aquarium were maintained at 1.2 l min⁻¹and the water was circulated through biofilters to remove solid substances and reduce ammonia concentration. Water quality parameters were determined daily and there were no significant differences between dietary treatments. Ranges of temperature, pH, total ammonia–nitrogen [(NH₄⁺ + NH₃)–N] and dissolved oxygen were from 22–24 °C, 7.0–7.4, 0.30–0.4 mg L⁻¹ and 5.0–5.5 mg L⁻¹ respectively, during the experimental period. During the experiment, the diurnal cycle was 12 light : 12 dark.

Sample collection and analysis

At the beginning of the trial, 30 fish were selected randomly from the same population used in the experiment and sacrificed with an overdose of anaesthetic to determine the initial carcass proximate composition. When the feeding trial was completed, five fish from each aquarium were collected and frozen, then freeze-dried, ground and analysed for protein, lipid and ash composition.

Total bulk weights were recorded for each aquarium at the beginning and the end of the experiment. At the end of growth trial, 12 h after the last feeding, 45 fish were collected from each group. The fish were killed with an overdose of anaesthetic, and then hepatopancreas and intestine were quickly removed, weighed and frozen in liquid nitrogen, then stored at −70 °C until analysed.

Tissue samples were homogenized in 10 volumes (w/v) of ice-cold physiological saline and centrifuged for 20 min at 6000 × g and 4 °C, the supernatant was conserved. The protein content of tissue was assayed according to Bradford (1976), using bovine serum albumin as the standard. The activity of trypsin was determined using N_α-p-toluenesulphonyl-l-arginine methyl ester (TAME) (Sigma) as the substrate (Hummel 1959). The α-amylase and lipase activity was determined by the starch-hydrolysis method and by the evaluation of the degradation of triacylglycerols, dacylglycerols and monoacylglycerols to free fatty acids respectively, according to Furne et al. (2005). AKP was measured according to Bessey et al. (1946). In this reaction, p-nitrophenoxide obtained is proportional to the enzymatic activity and has a distinct yellow colour that can be quantified. Na⁺, K⁺-ATPase activity was determined by the method described by Ilenchuk & Davey (1982). Crude Protein (CP), lipid, ash and moisture of the experimental diets and fish samples were analysed according to the methods of AOAC (Association of Official Analytical Chemists) (2000).

Calculations and statistical analyses

Data on IW, final weight (FW), feed intake (FI) and proximate composition carcass, hepatopancrease and intestine weight and body or intestine length were used to calculate:

All data were subjected to one-way anova followed by the Duncan’s method to determine differences among treatment groups. Results were considered significantly different at the level of P < 0.05. PWG data was evaluated by the broken-line model (Robbins et al. 1979) to estimate the dietary riboflavin requirement of Jian carp.

Results

Table 2 showed the growth performance of fish fed diets containing graded levels of dietary riboflavin. PWG, FI, FE, SGR, PER, GPR and LPV of fish fed the diet containing dietary riboflavin at 1.25 mg kg⁻¹ are significantly lower than those of fish fed diets containing higher dietary riboflavin concentrations (P < 0.05). PWG, SGR, FI and FE are significantly improved with increasing dietary riboflavin from 1.25 to 4.22 mg kg⁻¹ diet (P < 0.05), but there were no significant differences among groups containing riboflavin from 5.78 to 11.44 mg kg⁻¹ diet (P > 0.05). PER was significantly improved up to 4.22 mg riboflavin kg⁻¹ diet, and then reached a plateau. GPR and LPV are significantly enhanced by dietary riboflavin at the level of 5.78 mg kg⁻¹ diet (P < 0.05), and no further improvement was seen among higher levels.

Table 2. Initial weight (IW, g fish⁻¹), final weight (FW, g fish⁻¹), per cent weight gain (PWG, %), feed intake (FI, g fish⁻¹), feed efficiency (FE), specific growth rate (SGR), protein efficiency ratio (PER), gross protein retention efficiency (GPR) and lipid production value (LPV) of juvenile Jian carp fed diets containing graded levels of dietary riboflavin

	Dietary riboflavin levels (mg kg⁻¹ diet)
	1.25	2.71	4.22	5.78	7.23	8.83	11.44
IW	23.4 ± 0.0^a	23.4 ± 0.1^a	23.4 ± 0.1^a	23.4 ± 0.1^a	23.4 ± 0.1^a	23.4 ± 0.0^a	23.4 ± 0.0^a
FW	34.3 ± 2.3^a	49.0 ± 2.8^b	58.9 ± 0.7^c	62.2 ± 2.4^c	58.9 ± 0.8^c	60.7 ± 2.2^c	59.5 ± 0.7^c
PWG	46.4 ± 9.8^a	109.6 ± 11.9^b	151.7 ± 3.9^c	166.0 ± 10.1^c	152.9 ± 1.2^c	160.1 ± 9.3^c	154.4 ± 3.0^c
FI	24.8 ± 3.2^a	39.1 ± 1.7^b	44.7 ± 1.3^c	44.8 ± 2.8^c	43.9 ± 1.5^c	43.2 ± 2.1^c	44.0 ± 1.4^c
FE	43.5 ± 4.4^a	65.4 ± 4.4^b	79.5 ± 0.5^c	86.8 ± 4.0^c	81.7 ± 2.3^c	86.7 ± 6.1^c	82.2 ± 3.9^c
SGR	0.9 ± 0.2^a	1.8 ± 0.1^b	2.2 ± 0.0^c	2.3 ± 0.1^c	2.2 ± 0.0^c	2.3 ± 0.1^c	2.2 ± 0.0^c
PER	1.4 ± 0.1^a	2.1 ± 0.1^b	2.6 ± 0.1^c	2.7 ± 0.1^c	2.7 ± 0.0^c	2.7 ± 0.1^c	2.7 ± 0.1^c
GPR	12.7 ± 2.8^a	30.4 ± 4.9^b	35.8 ± 2.8^bc	43.7 ± 4.7^d	39.9 ± 3.5^cd	41.8 ± 6.8^cd	43.2 ± 6.8^d
LPV	251.6 ± 31.3^a	264.8 ± 43.3^ab	282.6 ± 27.8^b	322.0 ± 23.1^c	334.8 ± 20.3^c	331.0 ± 19.4^c	333.1 ± 23.1^c

Mean ± SD (n = 3). Mean values within the same row sharing the same superscript are not different (P > 0.05).

When broken-line analysis was used, based on PWG for estimating the requirement of dietary riboflavin (Fig. 1), the regression equations were as follows: Y = 26.437X + 26.161, R² = 0.923; Y = 158.350. The dietary riboflavin requirement of Jian carp is estimated to be 5.0 mg kg⁻¹ diet.

Details are in the caption following the image — **Figure 1**
Open in figure viewer PowerPoint

Broken-line analysis of weight gain in Jian carp (*Cyprinus carpio* var. Jian) fed the diets containing graded levels of dietary riboflavin for 6 weeks.

Body composition of fish are presented in Table 3. Whole-body moisture significantly decreased with increasing dietary riboflavin levels, in contrast, body lipid content of fish showed an opposite tendency (P < 0.05). Body protein content was significantly enhanced by riboflavin at 2.71 mg kg⁻¹ or higher levels (P < 0.05). However, ash content was not affected by dietary riboflavin.

Table 3. Whole-body composition (g kg⁻¹) of Jian carp fed diets containing graded levels of dietary riboflavin

	Dietary riboflavin levels (mg kg⁻¹ diet)
	1.25	2.71	4.22	5.78	7.23	8.83	11.44
Moisture	736.8 ± 8.7^a	708.9 ± 12.2^b	710.9 ± 7.0^b	710.1 ± 17.3^b	699.8 ± 16.1^b	702.7 ± 24.9^b	700.7 ± 26.8^b
Protein	118.0 ± 6.6^a	144.6 ± 6.7^b	141.8 ± 2.7^b	142.3 ± 9.8^b	143.9 ± 5.3^b	144.8 ± 12.6^b	148.2 ± 10.3^b
Lipid	83.5 ± 7.6^a	101.8 ± 8.2^b	102.8 ± 8.4^bc	103.2 ± 4.9^bc	114.6 ± 5.2^d	110.2 ± 6.7^bcd	111.2 ± 4.5^cd
Ash	28.9 ± 1.0^a	27.5 ± 1.6^a	27.1 ± 1.6^a	26.6 ± 1.0^a	27.5 ± 1.4^a	26.7 ± 0.9^a	27.7 ± 2.2^a

Mean value of triplicate groups, with five fish each group. Mean values within the same row sharing the same superscript are not different (P > 0.05).

Fish fed higher levels of dietary riboflavin had greater ILI and IPC, HWI and hepatopancreas protein content (HPC) as shown in Table 4. And these parameters were significantly different at the level of 5.78 mg riboflavin kg⁻¹ diet or higher levels (P < 0.05). HWI and ILI were significantly improved in response to the increasing dietary riboflavin levels (P < 0.05), but there were no differences between 5.78 and 11.44 mg riboflavin kg⁻¹ diets (P > 0.05). However, there were no significant differences observed in IWI among all groups (P > 0.05).

Table 4. Hepatopancreas weight index (HWI), hepatopancreas protein content (HPC, g kg⁻¹), intestine length index (ILI), intestine weight index (IWI), and intestine protein content (IPC, g kg⁻¹) of Jian carp fed diets containing graded levels of dietary riboflavin

	Dietary riboflavin levels (mg kg⁻¹ diet)
	1.25	2.71	4.22	5.78	7.23	8.83	11.44
Hepatopancreas
HWI	2.8 ± 0.3^a	3.0 ± 0.2^b	3.4 ± 0.2^c	3.8 ± 0.3^d	3.7 ± 0.2^d	3.9 ± 0.2^d	3.9 ± 0.2^d
HPC	43.9 ± 2.3^a	50.9 ± 2.9^b	66.6 ± 5.1^c	76.0 ± 2.4^d	74.0 ± 4.4^d	73.2 ± 4.7^d	75.1 ± 8.8^d
Intestine
ILI	145.5 ± 6.9^a	156.4 ± 7.6^b	168.7 ± 8.3^c	172.5 ± 5.0^d	173.3 ± 9.4^d	177.5 ± 4.8^d	175.8 ± 11.6^d
IWI	2.8 ± 0.2^a	2.6 ± 0.1^a	2.7 ± 0.2^a	2.7 ± 0.2^a	2.8 ± 0.1^a	2.7 ± 0.2^a	2.7 ± 0.1^a
IPC	34.8 ± 2.0^a	51.1 ± 1.5^b	56.1 ± 3.7^c	58.0 ± 4.3^c	55.3 ± 2.4^c	56.7 ± 1.7^c	57.6 ± 3.6^c

Mean value of triplicate groups, with 15 fish each group. Mean values within the same row sharing the same superscript are not different (P > 0.05).

Intestinal enzyme activities are presented in Table 5. Intestinal enzyme activity of fish fed the diet containing riboflavin 1.25 mg kg⁻¹ diet is significantly lower than that of fish fed higher dietary riboflavin levels (P < 0.05). Trypsin, lipase, α-amylase, AKP, and Na⁺,K⁺-ATPase activities were significantly enhanced with increasing dietary riboflavin levels (P < 0.05). Around 4.22 mg kg⁻¹ diet or higher dietary riboflavin levels can significantly improve the trypsin activity (P < 0.05). The activity of lipase was significantly enhanced by dietary riboflavin at 5.78 mg kg⁻¹ or higher levels. Amylase activity was significantly improved up to 2.71 mg riboflavin kg⁻¹ diet and remained similar thereafter. Na⁺, K⁺-ATPase activity was significantly enhanced by the dietary riboflavin levels ranging from 5.78 to 11.44 mg kg⁻¹ diet (P < 0.05). Activity of AKP in the proximal intestine was significantly improved in the groups of fish fed 5.78 mg riboflavin kg⁻¹ diet or higher dietary riboflavin levels (P < 0.05). However, lower AKP activity was observed in the mid intestine, which was significantly improved up to 4.22 mg riboflavin kg⁻¹ diet (P < 0.05), then reached a plateau. Lowest AKP activity was observed in the distal intestine and exhibited a similar tendency to that in the mid intestine.

Table 5. The effect of riboflavin on trypsin (U g⁻¹ tissue), lipase (U g⁻¹ tissue), α-amylase (U g⁻¹ tissue), Na⁺,K⁺-ATPase (P μmol g⁻¹ tissue h⁻¹), and AKP (nitrophenol mmol g⁻¹ tissue h⁻¹) in the intestine of Jian carp fed diets containing graded levels of dietary riboflavin

	Dietary riboflavin levels (mg kg⁻¹ diet)
	1.25	2.71	4.22	5.78	7.23	8.83	11.44
Trypsin	2.1 ± 0.1^a	2.5 ± 0.2^b	2.8 ± 0.2^c	2.9 ± 0.2^c	3.0 ± 0.2^c	2.9 ± 0.2^c	3.0 ± 0.2^c
Lipase	88.0 ± 0.0^a	202.5 ± 24.1^b	280.9 ± 14.4^c	435.0 ± 9.5^d	486.9 ± 20.8^e	487.8 ± 24.4^e	487.8 ± 24.4^e
α-Amylase	1273.1 ± 30.0^a	1379.0 ± 47.7^b	1452.7 ± 44.9^b	1466.5 ± 55.0^b	1466.5 ± 89.8^b	1443.5 ± 71.7^b	1448.1 ± 72.1^b
Na⁺,K⁺-ATPase	107.8 ± 2.9^a	137.5 ± 3.8^b	178.1 ± 5.2^c	198.8 ± 6.2^d	197.8 ± 6.2^d	201.6 ± 6.2^d	204.7 ± 7.3^d
AKP
Proximal intestine	12.5 ± 0.7^a	13.1 ± 0.8^a	14.5 ± 0.7^b	16.4 ± 0.7^c	16.9 ± 1.0^c	16.7 ± 1.0^c	16.0 ± 1.0^c
Mid intestine	7.9 ± 0.4^a	8.2 ± 0.5^a	8.5 ± 0.4^bc	8.7 ± 0.4^bc	8.8 ± 0.5^bc	8.9 ± 0.4^c	8.8 ± 0.5^bc
Distal intestine	1.0 ± 0.1^a	1.4 ± 0.1^b	1.6 ± 0.1^c	1.5 ± 0.1^bc	1.5 ± 0.1^bc	1.6 ± 0.1^c	1.5 ± 0.1^bc

Mean value of triplicate groups, with 15 fish each group. Mean values within the same row sharing the same superscript are not different (P > 0.05).

Discussion

The results of this study clearly showed that the growth performance was affected by dietary riboflavin levels. With increasing dietary riboflavin levels up to 4.22 mg kg⁻¹ diet, PWG and SGR were significantly improved, which is consistent with the results obtained in common carp (Aoe et al. 1967; Takeuchi et al. 1980) and other fish (Serrini et al. 1996; Deng & Wilson 2003). There was a similar trend in FI to that of PWG and SGR (Table 2). These data indicate that improvement in fish growth is attributed to increased FI (Table 2). Experiments conducted with common carp (Aoe et al. 1967; Takeuchi et al. 1980), channel catfish (Serrini et al. 1996) and sunshine bass (Deng & Wilson 2003) have shown that fish exhibited anorexia and retardation of growth when there was inadequate riboflavin supplemented in the diets. Fish fed higher levels of dietary riboflavin has higher FE, which was confirmed in our results by higher PER, GPR and LPV. PER, GPR and LPV significantly increased at an optimal riboflavin concentration and thereafter reached a plateau with higher dietary riboflavin levels. Similar tendency was observed in other fishes fed with riboflavin diets (Serrini et al. 1996; Deng & Wilson 2003) and in previous studies with glutamine (Lin & Zhou 2006) and lysine (Zhou et al. 2007b) for Jian carp.

Broken-line model was used to estimate the riboflavin requirement of Jian carp. This study showed that dietary riboflavin requirement of Jian carp was 5.0 mg kg⁻¹ diet based on PWG, which was higher than 4 mg kg⁻¹ diet for common carp reported previously by Aoe et al. (1967) and similar to the result (5 mg kg⁻¹) obtained in common carp by Takeuchi et al. (1980). It seems that this result is not consistent with the research that Jian carp had higher nutrient requirements than those of common carp (Zhou et al. 2007b), which suggested that it may have higher dietary riboflavin requirement. However, fish used in the present study (23.4 g) was much bigger than common carp used in the experiment by Aoe et al. (1967) (12.8 g) and Takeuchi et al. (1980) (3.4 g). This explains why the dietary riboflavin requirement of juvenile Jian carp is not higher than that of common carp.

This study indicated that body moisture content decreased with increasing dietary riboflavin levels, which was easily explained by the fact that whole-body lipid content enhanced with increasing dietary riboflavin levels (Table 3). This is in agreement with the findings in common carp (Takeuchi et al. 1980). Whole-body protein content increased by increasing dietary riboflavin levels, which was consistent with the findings in rainbow trout (Takeuchi et al. 1980).

In stomachless fish such as carp, the intestine plays a crucial role in digestion and absorption of nutrients. Digestion ability is correlated with digestive enzyme activity, as nutrients are digested by these enzymes (Lin & Zhou 2006). The present study showed that activities of trypsin, lipase and α-amylase in the intestinal tract improved with the increment of dietary riboflavin levels. To our knowledge, no studies have reported about the effect of dietary riboflavin on digestive enzymes. However, previous studies on dietary lysine (Zhou et al. 2007a) and glutamine (Lin & Zhou 2006) have demonstrated that both can improve activities of protease and lipase in intestine. The enhancement of activities of trypsin, lipase and α-amylase in the intestinal tract may be the result of the growth and development of hepatopancrease improved by dietary riboflavin. Most fishes except for sturgeon have no crypt, and the intestinal enzymes mostly come from the hepatopancreas (Lin 1998). So the enzyme activity closely relates to the growth and development of the hepatopancreas. The present study showed that hepatopancreas weight, HWI and HPC were significantly improved with increasing dietary riboflavin levels from 1.25 to 5.78 mg kg⁻¹ diet, and then reached a plateau.

Intestinal absorption ability is the guarantee of utilizing nutrients adequately in vivo as well. Pedersen & Sissons (1984) demonstrated that absorption function is correlated with intestinal growth and development. In the present study, intestine weight, intestine length, ILI and IPC were improved by increasing dietary riboflavin levels. Although there is no report about the effect of dietary riboflavin on intestine weight, intestine length, ILI and IPC, similar observations were obtained in previously conducted experiment of glutamine for Jian carp (Lin & Zhou 2006). Na⁺,K⁺-ATPase is considered to be involved in the absorption of nutrients such as amino acids and glucose. Its activity indirectly reflects intestinal absorption ability (Rhoads et al. 1994). Activity of Na⁺,K⁺-ATPase was enhanced with increasing dietary riboflavin. This tendency was similar to that obtained in previous studies with glutamine (Lin & Zhou 2006) and lysine (Zhou et al. 2007a) for Jian carp. Intestine AKP localized in the brush border reflects the epithelium development and absorption ability (Cuvier-Peres & Kestemont 2001). Activity of AKP was improved with increasing dietary riboflavin levels. This result is consistent with the fact that riboflavin can improve the development of gastrointestinal tract (Yates et al. 2003). AKP activity tested in different intestinal sections exhibited that enzyme activity decreased gradually from proximal intestine to distal intestine. This is in agreement with the findings made by Villanueva et al. (1997) of an obvious proximal–distal gradient in the activity of AKP in the intestine of C. carpio.

In conclusion, dietary riboflavin can improve the growth performance and the intestinal enzyme activities of Jian carp, and the dietary riboflavin requirement of Jian carp was 5.0 mg kg⁻¹ diet based on PWG.

Acknowledgements

This study was financed by Program for Changkiang Scholars and Innovative Research Team in University (IRTO555) and Sichuan Province Outstanding Youth Fund. The authors sincerely thank Miss Xiu-Qun Wu for assisting with the analysis of fish-body compositions, and Dr Jing-Yi Cai for his aid in laboratory analysis.

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Effect of dietary riboflavin on growth, feed utilization, body composition and intestinal enzyme activities of juvenile Jian carp (Cyprinus carpio var. Jian)

Abstract

Introduction