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Articles

In Vitro Starch Digestion Correlates Well with Rate and Extent of Starch Digestion in Broiler Chickens¹

Roelof E. Weurding^*,,**2, Albertus Veldman^*, Willem A. G. Veen^*, Petrus J. van der Aar^* and Martin W. A. Verstegen^**

^* Institute for Animal Nutrition, De Schothorst, P.O. Box 533, 8200 AM Lelystad, The Netherlands; Brameco · ZON, P.O. Box 8510, 5605 KM Eindhoven, The Netherlands; and ^** Animal Nutrition Group, Wageningen University & Research Center, P.O. Box 338, 6700 AH Wageningen, The Netherlands

²To whom correspondence should be addressed. E-mail: eweurding{at}schothorst.nl.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Current feed evaluation systems for poultry are based on digested components (fat, protein and nitrogen-free extracts). Digestible starch is the most important energy source in broiler chicken feeds and is part of the nitrogen-free extract fraction. Digestible starch may be predicted using an in vitro method that mimics digestive processes in the gastrointestinal tract of broiler chickens. An experiment was designed to use this method for predicting site, rate and extent of starch digestion in broiler chickens. In vitro starch digestion was studied in 12 experimental diets differing in starch sources. These diets were also used in a digestibility trial with broiler chickens. Correlations between in vitro and in vivo starch digestion were calculated. Starch digestion after 2 h incubation correlated well with in vivo starch digestion in the first half of the small intestine (r = 0.94). A 4-h incubation period resulted in a good correlation between in vitro starch digestion and ileal starch digestion (r = 0.96). In vitro starch digestion rate (h^-1) correlated well with in vivo starch digestion rate (r = 0.87). In vitro starch digestion of individual starch sources was additive. It appeared that legume seeds and waxy corn contained two starch fractions, which were digested at different rates. We conclude that starch digestion rate in broiler chickens is well predicted by the in vitro method.

KEY WORDS: • starch • in vitro • digestion rate • broiler chickens

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Starch provides >50% of the apparent metabolizable energy (AME)³ in broiler feeds. The Dutch feed industry (1) evaluates feedstuffs on the basis of digested components (fat, protein and nitrogen-free extract). Starch is part of the nitrogen free-extract (NFE) fraction. The AME content of wheat is affected by starch digestibility (2 ,3) . Undoubtedly, this also applies to other feedstuffs containing starch. Starch digestibility is affected by intrinsic factors such as starch structure and composition (4) and associations between starch granules and protein and cell wall structures within the feedstuff (5 ,6) . Furthermore, extrinsic factors such as processing of starch sources and conditions in the gastrointestinal tract also affect starch digestibility. Soluble nonstarch polysaccharides in the diet increase gut viscosity, which impairs starch digestion (5 ,7) . Finally, passage rate of digesta in the small intestine determines the time available for starch hydrolysis.

Englyst et al. (8) introduced the term Resistant Starch (RS) in 1982; the term was later defined as "the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals" (9) . This RS fraction was further subdivided into physically inaccessible starch (RS₁), resistant starch granules (RS₂) and retrograded amylose (RS₃). Results from Weurding et al. (10) showed that starch digestion rate in the small intestine of broiler chickens varies considerably among feedstuffs. Digestion rate of the digestible starch fraction and the enzymatically resistant starch fraction will affect the extent of starch digestion. In human nutrition, the kinetics of starch digestion are already considered to be an important food characteristic. In 1981, Jenkins et al. (11) introduced the glycemic index, which reflects the effect of glucose absorption rate on plasma glucose levels. Englyst et al. (12) showed that starch digestion rate correlates well with glycemic index. These two traits can be used for managing diabetes, sports performance and appetite research (13) . Starch digestion rate may also be important in broiler nutrition because it may affect plasma insulin levels and the availability of nutrients at a specific time (synchronization of energy and amino acid digestion). This may have an effect on the efficiency of protein deposition in the broiler chicken, which is of economic interest to the poultry farmer.

If starch digestion rate turns out to be an important trait for broiler chickens, then a reliable, rapid and inexpensive laboratory analysis to predict this trait is warranted. Englyst et al. (14) proposed an in vitro method that simulates starch digestion in the small intestine of humans. On the basis of this in vitro method, he fractionated starch in rapidly digestible starch (RDS), slowly digestible starch (SDS) and RS. The objective of this experiment was to investigate whether a modified version of the in vitro starch digestion method proposed by Englyst et al. (14) can be used to predict site, rate and extent of starch digestion in broiler chickens.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Experimental diets and feedstuffs.

In vitro starch digestion of 12 experimental diets was measured in a laboratory experiment. Diet composition and starch digestion in three different segments of the small intestine is given by Weurding et al. (10) . Eight diets contained only one starch source and four diets contained two starch sources. In vitro starch digestion was also determined in each of the 12 starch sources. The 12 starch sources are given in Table 1 .

In vitro starch digestion was determined using a modified version of the method described by Englyst et al. (14) . This modified in vitro method simulates digestive behavior in the alimentary tract of broilers. In contrast to Englyst et al. (14) , diets and feedstuffs were milled in a Retsch mill to pass a 1-mm screen, thus simulating grinding action in the gizzard. Test tubes containing feed sample, glass balls and a pepsin-HCl solution were incubated in a water bath (37°C) for 30 min to simulate passage through the proventriculus. After this preincubation, the procedure described by Englyst et al. (14) was carried out. Buffer solution and an enzyme cocktail were added and tubes were placed horizontally in a shaking water bath (37°C) for 0.25, 0.50, 0.75, 1, 2, 3, 4, 5 and 6 h. After each incubation time, aliquots were taken from the tubes and the amount of released glucose was measured colorimetrically according to a glucose oxidase method (Glucose oxidase diagnostic kit 166391, Boehringer Mannheim, Mannheim, Germany). We used nine incubation times instead of the two (20 and 120 min) used by Englyst et al. (14) . This was done to improve the estimation of starch digestion rate. The four incubation times during h 1 represent the steep part of the digestion curve. A good estimation of starch digestion rate requires sufficient measurements in this part of the curve. For most feedstuffs, the digestion curve will have reached the asymptotic level after 6 h. In vitro starch digestion was determined separately for the experimental diets and the starch sources. Four analyses were performed. Each analysis contained either 12 different experimental diets or 12 different starch sources. Total starch (TS) and free glucose (FG) content was determined as described for a finely divided sample by Englyst et al. (14) . Digestion coefficient of starch at time t (DC_t) was calculated as follows:

(1)

where G_t represents the amount of glucose present at time t. Starch digestion coefficients of common beans, peas, horsebeans and potato starch in both in vivo and in vitro experiments were calculated from digestion coefficients of the total diet and corn (10) . It was assumed that starch digestion coefficients in compound feeds are additive. Preliminary data suggested that in vitro starch digestion follows first-order kinetics and in vitro starch digestion rate was estimated using the following equation:

(2)

where DC_t is the starch fraction digested at time t, fraction D is the potentially digestible starch fraction that will digest at a fractional rate of k_d (h^-1). This terminology is similar to that used for protein degradation in the rumen of dairy cows (15) . The Marquardt method of the PROC NLIN procedure (an iterative curve fitting procedure) (16) was used to reduce the residual sums of squares associated with the regression model. In vitro starch digestion at each incubation time was correlated with in vivo starch digestion at the different sites of the small intestine [posterior jejunum (PJ), anterior ileum (AI) and posterior ileum (PI)] and to total starch digestion as determined by Weurding et al. (11) . The incubation times for which in vitro starch digestion showed the best correlation with in vivo starch digestion in the PJ and PI were determined. In vivo and in vitro estimates for potential starch digestibility (D) and fractional digestion rate (k_d) were compared using regression analysis. Relations between in vitro and in vivo starch digestion were investigated by regressing in vivo on in vitro. The influence of a specific treatment on the regression equation was investigated by calculating the leverage of each treatment (a measure for the relative position of an observation in relation to the other observations). Treatments that have a high leverage (>5/n, in which n is the number of observations) are outside the range of the x-axis (17) . When leverage was high, the influence of that specific treatment on the regression equation was studied.

Additivity of in vitro starch digestion.

Additivity of in vitro starch digestion was tested by comparing in vitro starch digestion of diets containing two different starch sources with that of the separate measurements of the two starch sources (P < 0.05). Additivity was tested using a t test, which tested a · DC_corn + b · DC_x - DC_y = 0, where DC is the starch digestion coefficient, x is the experimental starch source and y is the experimental diet containing corn and x in proportions a and b, respectively. The effect of other feed ingredients on starch digestion was also studied by comparing starch digestion in individual starch sources and in the corresponding experimental diets with only one starch source (8 diets). A t test was used, which tested DC_feedstuff - DC_diet = 0. Before both t tests, homogeneity of variance was tested.

Goodness of fit of starch digestion curves.

Predicted starch digestion values (based on ^{equation 2} ) were compared with observed starch digestion values. If alternating periods of underestimation and overestimation of starch digestion were observed (resulting in a systematic pattern in the plot of residuals), the curve fitting procedure was repeated using a two-phase model:

(3)

An F-test as described by Motulsky and Ramsnas (18) was used to determine whether the two-phase model gave a significant improvement of the fit (P < 0.05).

where F is the F-value for the comparison of both curves; SS₁ is the sum of squares of fit for the one-phase model; SS₂ is the sum of squares of fit for the two-phase model; df₁ represents the degrees of freedom for the one-phase model; and df₂ represents the degrees of freedom for the two-phase model.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
In vitro starch digestion differed among the diets (Table 1) . Potato starch and legumes displayed lower starch digestion values than cereal grains and tapioca. Sorghum had lower starch digestion values within the first 30 min compared with the other cereal grains. In vitro and in vivo starch digestion (PJ) differed in a few aspects. In vitro starch digestion (2–6 h) of common beans was similar to that of peas and higher than horsebeans. In vivo common bean starch digestion, however, was much lower than pea and horsebean starch digestion. Ileal starch digestion of wheat and sorghum was low compared with that of other cereal grains. In vitro starch digestion for these cereals was similar to that of the other cereals. Table 2 presents correlation coefficients for in vitro starch digestion at different incubation times and in vivo starch digestion at different segments of the gastrointestinal tract of broilers. After 2 h of incubation, in vitro starch digestion showed the best correlation with in vivo starch digestion in the PJ, (r = 0.94). Starch digestion at the AI was best predicted after 3 h of incubation (r = 0.96). Starch digestion at the PI was best predicted by a 4-h incubation period (r = 0.96). Figure 1A and Bshows the relation between in vitro starch digestion and starch digestion in the PJ and PI. In vivo starch digestion coefficients at the PJ and PI can be predicted from these regression equations:

(4)

(5)

Potato starch was at the outer reach of the x-axis in both figures and had a leverage of 7/n and 9/n in Figures 1 A and B, respectively. Removing potato starch data from the dataset scarcely affected the prediction of starch digestion in the PJ. The correlation coefficient, however, was lowered to 0.88 after removing the potato starch data. Prediction of starch digestion at the PI, however, was affected when potato starch data were removed from the dataset. Scatter plots, in which mean values for the clusters of slowly (potato starch), gradually (legume grains) and rapidly digestible (cereal grains and tapioca) starch sources were used to show the correlation between in vitro and in vivo starch digestion, are presented in Figure 1 C and D for PJ and PI, respectively. Starch from tapioca was digested most rapidly (highest k_d), followed by cereal grains, legumes and finally potato starch (Table 3). Figure 2A and Bshows correlations between in vitro and in vivo D and k_d. The potato starch data in Figure 2 A were outside the range of the x-values (leverage = 9/n). After removal of the potato starch data, no correlation existed between in vivo and in vitro D values. In Figure 2 B, tapioca data were outside the range of the x-values (leverage = 11/n). After removal of the tapioca data, a good correlation was found between in vivo and in vitro starch digestion rate (k_d).

View larger version (28K): [in this window] [in a new window]   Figure 1. Relation between in vivo starch digestion coefficients (DC) at specific sites of the small intestine of broiler chickens (PJ, posterior jejunum; PI, posterior ileum) and in vitro starch digestion after 2 and 4 h of incubation. Panels A and B show mean values ± SD per feedstuff (hm, hammer-milled; rm, roller-milled; wx, waxy); panels C and D show mean values per group (see text). nvivo = 6; nvitro = 2.

View larger version (12K): [in this window] [in a new window]   Figure 2. Relation between in vivo and in vitro starch digestion characteristics of 12 different starch sources for broiler chickens. nvivo = 6; nvitro = 2.

View larger version (32K): [in this window] [in a new window]   Figure 3. Predicted and observed in vitro starch digestion curves for experimental diets containing common beans (A), peas (B), horsebeans (C) and potato starch (D). The predicted curves are based on observed in vitro starch digestion curves for the individual feedstuffs; n = 2 observations per feedstuff and diet.

View larger version (25K): [in this window] [in a new window]   Figure 4. In vitro starch digestion of horsebeans (n = 1) fitted using a one- or two-phase model. In the two-phase model, two fractions can be distinguished, i.e., a slowly and a rapidly digestible starch fraction.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

The in vitro results indicated that starch digestion up to the PJ can be predicted well after a 2-h incubation period. This fraction can be defined as rapidly digestible starch for poultry (RDS_p). It appears that for a prediction of ileal starch digestion, a 4-h incubation period is required. Slowly digestible starch for poultry (SDS_p) can be calculated from the difference between the starch fraction digested after 4 and 2 h of in vitro incubation. From the starch fraction that was not digested after 4 h, in vitro incubation resistant starch for poultry (RS_p) can be derived. Englyst et al. (14) defined RDS as starch digested after 20 min of in vitro incubation and SDS as starch that was digested between 20 and 120 min of in vitro incubation. RS is the starch fraction that was not digested after 120 min of in vitro incubation. Our definitions are not identical because we used different starch sources and processing methods compared with Englyst et al. (14) . Furthermore, there are differences in the gastrointestinal tract of broilers and humans.

Figure 1 A and B shows that more rapidly digestible starch sources than slowly digestible sources were used in the experiment. Starch in legume grains and potato starch was digested more slowly than starch in cereals and tapioca. From scatter plots in Figure 1 A and B, it is clear that potato starch is located at the lower end of the x-axis. Omitting potato starch from the dataset scarcely affects the prediction of preileal starch digestion. Omitting potato starch from the regression analysis altered the regression line. In that case, predicted in vivo values for slowly digestible starch sources were higher. To make the vitro method applicable for a wide range of products, potato starch was included. Figure 1 A and B clearly indicates three product groups, i.e., tapioca and cereal grains, legumes grains and potato starch. Figure 1 C and D suggests that it is justifiable to keep potato starch in the dataset.

A distinction can be made between slowly (potato starch: 0.3 h^-1), gradually (legume grains: 0.6–0.9 h^-1), rapidly (cereal grains: 1.1–1.6 h^-1) and extremely rapidly (tapioca: 5.3 h^-1) digestible starch sources (Table 3) . Starch digestion rate was higher in vivo than in vitro (Fig. 2 B). Tapioca was digested extremely quickly (both in vivo and in vitro). For tapioca, the starch digestion rate was lower in vivo than in vitro. This may be due to the absence of sufficient sampling sites in the anterior part of the small intestine (i.e., the steep part of the digestion curve). This may lead to an underestimation of in vivo starch digestion rate of rapidly digestible starch sources. We did not observe an interaction effect of starch sources on in vitro starch digestion (P > 0.05). Also, other feed components in the compound feeds did not affect in vitro starch digestion (P > 0.05). The potato starch diet showed the most pronounced difference between predicted and observed in vitro starch digestion. This is probably due to the fact that when potato starch is mixed with the buffer solution, it coagulates easily, which results in impaired starch digestion.

The two-phase model gave a much better fit for in vitro starch digestion of legumes and waxy corn (P < 0.05). Apparently, two distinct different starch fractions with different digestion rates are present in these feedstuffs. It is not known whether this two-phase starch digestion is caused by different starch structures or by differences in accessibility to starch granules.

On the basis of the results of this experiment we conclude that an in vitro procedure can be used to predict both jejunal and ileal starch digestion as well as starch digestion rate. In turn, predicted ileal starch digestion can be used to improve the prediction of AME content (2 ,3) . Furthermore, Wiseman et al. (3) found that wheat with rapidly digestible starch had a higher AME content. This suggests that a faster starch digestion rate results in a more efficient energy utilization by broilers. This may be related to the observation that a faster starch digestion rate results in a more complete starch digestion. This was seen in vivo (10) as well as in vitro. In vitro D-values for rapidly digestible starch sources were between 96 and 100%. Slowly and gradually digestible starch sources had lower in vitro D-values. These observations seem to be inconsistent with observations reported by Truswell (20) . He reported no general correlation between glycemic index and the percentage of resistant starch in foods. Some foodstuffs he discussed, however, were heat-treated. These treatments result in starch gelatinization (increased starch digestion rate) and retrogradation (increased RS₃ fraction). Furthermore, the glycemic index is also affected by absorption of sugars other than glucose, whereas starch digestion rate affects only glucose absorption. Some of the feedstuffs discussed by Truswell contained high amounts of sugars compared with starch. It is of interest to know whether growth performance of broiler chickens is affected by isoenergetic diets with different starch digestion rates. A gradual starch digestion results in a more or less continuous availability of glucose. It is conceivable that ingested dietary protein will be utilized more efficiently with a continuous glucose supply. The response to insulin is also affected by glucose absorption rate (20) . Insulin is the major hormone that promotes anabolism in the body. It promotes the cellular uptake of amino acids and their incorporation into proteins (21) .

In vitro digestion of sorghum starch started slowly compared with starch from other cereals, but after 2 h, it was higher than that of corn (both hammer-milled and waxy), rice and barley. After a 15-min incubation, wheat starch digestion was similar to that of other cereals (except sorghum). It was, however, much higher after longer incubation times. Common bean starch digestion was similar to that of most cereals after 15 min of incubation. After longer incubation times, common bean starch digestion became more like that of pea starch. The observed differences between in vivo and in vitro digestion of wheat, sorghum and legumes (Table 1) may be due to the presence of antinutritional factors such as lectins, tannins and arabinoxylans. Their effect on digestive processes in the gastrointestinal tract is not simulated in the in vitro method. Other factors to which in vitro techniques do not respond, but which may affect starch digestion in birds, should also be noted. Effects of diets on passage rate and viscosity are not simulated at all. The in vivo trial (10) revealed that diets containing substantial amounts of slowly digestible and resistant starch resulted in longer retention times in the small intestine. This may affect digestion coefficients. Furthermore, microbial fermentation of nutrients is not likely to occur in vitro. In addition, the in vitro technique implies that digestion products are not removed. Finally, there are no feedback mechanisms in the in vitro assay. There is an excessive amount of enzymes at the onset of the incubation period. On the other hand, a number of advantages of the in vitro method can be mentioned. The in vitro method is standardized, reliable, rapid and less expensive than in vivo measurements. There is no animal variation involved. Furthermore, the in vitro method enables simulation of many sites of the digestive tract of broiler chickens. Finally, in vitro methods are preferred in view of the welfare concerns related to animal experiments.

	ACKNOWLEDGMENTS

 
The authors thank Arnold Dijkstra for the effort he put into the development of the in vitro method.

	FOOTNOTES

 
¹ Supported by the Dutch feed co-operatives and the Dutch Technology Foundation STW (project no. WPR66.4050).

³ Abbreviations used: AME, apparent metabolizable energy; AI, anterior ileum; D, potentially digestible starch fraction; DC_t, digestion coefficient at time t; FG, free glucose; k_d, fractional starch digestion rate; NFE, nitrogen-free extract; PI, posterior ileum; PJ, posterior jejunum; RDS, rapidly digestible starch; SDS, slowly digestible starch; RS, resistant starch; TS, total starch.

Manuscript received March 13, 2001. Initial review completed April 9, 2001. Revision accepted June 21, 2001.

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

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