Short communication
Inhibitory effect of dietary carboxymethylcellulose
on fat digestibility in rats fed diets containing
either starch or glucose
G.H.P. Vissia, A.C. Beynen
*Department of Nutrition and Department of Laboratory Animal Science, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80152, 3508 TD Utrecht, The Netherlands
Received 26 April 1999; received in revised form 28 September 1999; accepted 28 February 2000
Abstract
Dietary carboxymethylcellulose (CMC) inhibits lipid digestibility in broiler chickens by increasing the viscosity of small intestinal digesta. The aim of this study was to investigate the effect of dietary CMC on lipid digestibility in rats fed diets containing either starch or glucose. It was hypothesized that CMC would have a greater inhibitory effect on lipid digestibility when the diet contained starch instead of glucose. Male rats were fed for 2 weeks puri®ed diets containing either 619.2 g starch or glucose per kg and CMC levels of either 0, 20 or 40 g/kg. The experimental diets had no differential effect on growth of the rats. CMC signi®cantly lowered the apparent lipid digestibility coef®cient by more than 0.1, this effect being independent of the type of carbohydrate in the diet. Lipid digestibility was signi®cantly reduced by glucose when compared with starch, but
the mechanism is unknown.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Lipid digestion; Carboxymethylcellulose; Rats
1. Introduction
The ingestion of soluble, viscous and readily fermentable non-starch polysaccharides (NSPs), such as those present in rye, wheat and barley, reduces lipid digestibility in broiler chickens as reviewed by Smits and Annison (1996). At least part of the action of these NSPs relates to their increasing effect on the viscosity of the digesta. Using the
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*Corresponding author. Tel.:31-30-2534002; fax:31-30-2531817.
E-mail address: a.c.beynen@vet.uu.nl (A.C. Beynen)
model compound carboxymethylcellulose (CMC), which is non-fermentable, but highly viscous, Smits et al. (1997) demonstrated that an increase in the viscosity of small intestinal contents was associated with a decrease in lipid digestibility in broiler chickens. It was proposed (Smits et al., 1997) that gelling NSPs raise the viscosity of the digesta (White et al., 1983), which inhibits carbohydrate digestion through a reduced diffusion of digestive enzymes and less mixing of chyme (Edwards et al., 1988). The resulting accumulation of carbohydrates in the digesta leads to bacterial overgrowth (Smits and Annison, 1996) and subsequent excessive deconjugation of bile acids (Feighner and Dashkevicz, 1988) so that micelle formation becomes depressed and fat digestibility be lowered (Campbell et al., 1983). This mechanism, ®rst put forward by Smits et al. (1997), implies that the inhibitory effect of soluble NSPs on fat digestion is greater with starch in the diet than with glucose because the latter is absorbed without prior enzymatic digestion. To test the hypothesis, rats were fed diets without or with CMC, and either starch or glucose as sole source of digestible carbohydrate. Faeces of the rats were collected and digestibility was determined. The effects of CMC on mineral absorption have been reported elsewhere (Vissia and Beynen, 1999).
2. Material and methods
The protocol of this experiment was approved by the animal experiments committee of the Utrecht Faculty of Veterinary Medicine.
2.1. Animals, housing and diets
Thirty-six male Wistar (U:Wu) rats, aged about 3 weeks, were used. On arrival, the rats were housed in groups of three animals in macrolon type III cages (UNO BV, Zevenaar, The Netherlands), containing a layer of sawdust. They received a commercial pelleted feed (RMH-B, Hope Farms, Woerden, The Netherlands) for 4 days followed by the puri®ed experimental diet for another 7 days. The composition of the pre-experimental diet is given in Table 1. It contained 50 g cellulose and 669.2 g starch plus glucose per kg in a 1:1 ratio. After the pre-experimental period (Day 0), the rats were divided into six groups of six rats each, so that group mean body weights and distributions were similar. The groups were fed one of the experimental diets that contained either starch or glucose and either no CMC or CMC at a level of 20 or 40 g/kg (Table 1). CMC was added to the diets at the expense of the cellulose component.
The rats were gradually transferred from the pre-experimental diet to one of the experimental diets over a period of 3 days and were housed individually in metabolism cages with wire-mesh bottoms (Tecniplast Gazzada, Buguggiate, Italy). The cages were placed in a room with a controlled 12 h light-dark cycle (lights on: 07:00±19:00 hours ), temperature (20±228C) and humidity (60±63%). The powdered puri®ed diets were stored at 48C until feeding. Feed and demineralized water were given ad libitum. Feed intakes were recorded. The experimental feeding period lasted 14 days.
2.2. Chemical analysis
Faeces were quantitatively collected during the last 5 days of the experiment and were stored at 48C until analysis. Faeces of each day were pooled per rat, freeze-dried and homogenized using a coffee-grinder. Total lipids in diet samples and freeze-dried faeces were extracted with HCl/ethanol/diethylether/petroleum ether (bp 40±608C) (12:10:55:55) and quanti®ed gravimetrically (Bligh and Dyer, 1959).
2.3. Statistical analysis
The lipid digestibility coef®cient was calculated as dietary fat intake minus faecal lipid excretion, and expressed relative to intake. Statistical analysis was performed using the computerized statistical SPSS software (SPSS for windows Release 7.0, SPSS Inc., Chicago, IL). Effects were considered signi®cant ifP<0.05. All data were found to be normally distributed according to the Kolmogorov±Smirnov one-sample test. The data were subjected to regression analysis to identify linear effects of CMC within each type of carbohydrate treatment. The slopes were subsequently compared (Student'st-test) to evaluate an effect of carbohydrate source. To evaluate interactions between the effects of CMC and those of type of carbohydrate, we applied multivariate analyses of variance (MANOVA). When variances were inhomogeneous, according to Levene's test, log-transformed data which had homogeneous variances were used. In case of inhomogeneity
Table 1
The composition of the experimental diets
Components Diets
Constant components (g/kg)c 280.8 280.8 280.8 280.8 280.8 280.8 280.8
aCarboxymethylcellulose (AF2805, AKZO Chemicals, Arnhem, The Netherlands). bArbocel type B800 (Rettenmaier und SoÈhne, Ellwangen-HolzmuÈhle, Germany). cConstants components consisted of (g): casein, 150; animal fat, 80; CaCO
of variances after logarithmic transformation, the data were subjected to non-parametrical testing by means of the Kruskal±Wallis test.
3. Results
Mean initial body weight (Day 0) of the rats was 134.3 g, and ®nal body weight (Day 14) was 216.0 g. According to MANOVA, body weights of the rats were not signi®cantly affected by the type of dietary carbohydrate and amount of CMC. There were signi®cant differences (P<0.05) in food consumption: when the starch diets were fed, feed intake was on average 2.9 g/day lower than when the glucose diets were fed. This carbohydrate effect on feed intake was re¯ected by lipid intake (Table 2). CMC feeding raised fecal output of lipids, the effect being linear. The slopes of the CMC dose-response relationships did not differ between carbohydrate sources.
The apparent digestibility of lipids was signi®cantly reduced by CMC (Table 2). Irrespective of the dietary carbohydrate source, there was a signi®cant linear effect of CMC. The slopes of the dose-response relationships did not differ between carbohydrate sources. When the amount of dietary CMC was raised from 20 to 40 g/kg no further reduction in fat digestibility was seen. The digestibility was signi®cantly lower in the rats fed glucose when compared to those fed starch (Table 2). There was no interaction between the effects of CMC and the type of carbohydrate.
4. Discussion
The current study shows that CMC depresses apparent fat digestibility in rats. Thus, the CMC effect reported for broiler chickens (Smits et al., 1997) also holds true for young, growing rats. The effect of CMC is speci®c and not secondary to a difference in cellulose intake (Smits et al., 1997). When 20 g CMC/kg was incorporated into the diet of the rats, fat digestibility coef®cient was decreased by 0.13 when starch was the carbohydrate source, and by 0.12 when the carbohydrate source was glucose. The inhibitory effect of CMC on fat digestion was not in¯uenced by the type of carbohydrate in the diet. It follows that the above-described mechanism explaining why CMC inhibits fat digestion is not supported by the present data.
The question arises whether an alternative mechanism for the CMC effect on fat digestibility can be proposed. In a recent study with broiler chickens, it was found that CMC feeding signi®cantly increased the water content of the small intestinal digesta, and as a consequence, reduced the concentration of bile acids in the digesta (Smits et al., 1998). CMC also increased the loss of bile acids with the excreta (Smits et al., 1998), which may have contributed to the lower bile acid concentration in the digesta. The reduction in bile acid concentration may impair micelle formation, leading to less effective fat digestion. Additional work is necessary to prove or disprove this explanation for the inhibitory effect of CMC on fat digestibility.
A surprising ®nding emerged from the current study. When compared with starch, glucose signi®cantly lowered apparent fat digestibility. At present no satisfactory
Table 2
Apparent lipid digestibility in the rats fed the experimental dietsa
Diets Linear CMC effectc
Starch 0 g CMC/kg
Starch 20 g CMC/kg
Starch 40 g CMC/kg
Glucose 0 g CMC/kg
Glucose 20 g CMC/kg
Glucose 40 g CMC/kg
MANOVAb Starch Glucose
Lipid
Intake (g/day) 1.630.13 1.370.17 1.470.19 1.870.13 1.590.13 1.750.26 CMC, C NS NS Faecal output (g/day)d 0.290.05 0.430.08 0.450.10 0.510.06 0.630.08 0.780.24 CMC, C S S
Digestibility coefficiente 0.820.02 0.690.06 0.690.03 0.730.02 0.610.02 0.560.08 CMC, C S S
aValues are meansS.D. for six rats per dietary group.
bSigni®cance was calculated by multivariate analysis of variance (P<0.05) or Kruskal±Wallis test (P<0.05): CMCCMC effect; Ccarbohydrate effect; there were no signi®cant interactions.
cLinear effects of CMC were identi®ed by regression analysis: NSnot signi®cant (P>0.05); Ssigni®cant (P<0.05). dData have been subjected to logarithmic transformation prior to the MANOVA test.
eData were tested for the effect of diet by Kruskal±Wallis test.
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explanation for this effect can be given and further studies on the underlying mechanism should provide more insight in the process of fat digestion.
Acknowledgements
The authors are grateful to A.G. Lemmens for expert analytical assistance.
References
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