Calf milk replacers (MR) commonly contain 40% to 50% lactose. For economic reasons, starch is of interest as a lactose replacer. Compared with lactose, starch digestion is generally low in calves. It is, however, unknown which enzyme limits the rate of starch digestion. The objectives were to determine which enzyme limits starch digestion and to assess the maximum capacity for starch digestion in milk-fed calves. A within-animal titration study was performed, where lactose was exchanged stepwise for one of four starch products (SP). The four corn-based SP differed in size and branching, therefore requiring different ratios of starch-degrading enzymes for their complete hydrolysis to glucose: gelatinised starch (α-amylase and (iso)maltase); maltodextrin ((iso)maltase and α-amylase); maltodextrin with α-1,6-branching (isomaltase, maltase and α-amylase) and maltose (maltase). When exceeding the animal’s capacity to enzymatically hydrolyse starch, fermentation occurs, leading to a reduced faecal dry matter (DM) content and pH. Forty calves (13 weeks of age) were assigned to either a lactose control diet or one of four titration strategies (n=8 per treatment), each testing the stepwise exchange of lactose for one SP. Dietary inclusion of each SP was increased weekly by 3% at the expense of lactose and faecal samples were collected from the rectum weekly to determine DM content and pH. The increase in SP inclusion was stopped when faecal DM content dropped below 10.6% (i.e. 75% of the average initial faecal DM content) for 3 consecutive weeks. For control calves, faecal DM content and pH did not change over time. For 87% of the SP-fed calves, faecal DM and pH decreased already at low inclusion levels, and linear regression provided a better fit of the data (faecal DM content or pH v. time) than non-linear regression. For all SP treatments, faecal DM content and pH decreased in time (P<0.001) and slopes for faecal DM content and pH in time differed from CON; P<0.001 for all SP), but did not differ between SP treatments. Faecal DM content of SP-fed calves decreased by 0.57% and faecal pH by 0.32 per week. In conclusion, faecal DM content and pH sensitively respond to incremental inclusion of SP in calf MR, independently of SP characteristics. All SP require maltase to achieve complete hydrolysis to glucose. We therefore suggest that maltase activity limits starch digestion and that fermentation may contribute substantially to total tract starch disappearance in milk-fed calves.

The primary reason for processing maize is to enhance feeding value. Total tract starch digestion is similar for coarsely processed (dry rolled, cracked) dry maize. Enhancements in starch digestion due to dry rolling maize v. feeding maize whole may be greater in light-weight calves than in yearlings, and when DM intake is restricted ( < 1·5 % of body weight). The net energy (NE) maintain (NEm) and NE gain (NEg) values for whole maize are 8·83 and 6·02 MJ (2·11 and 1·44 Mcal)/kg, respectively. Compared with conventional dry processing (i.e. coarse rolled, cracked), finely processing maize may increase the initial rate of digestion, but does not improve total tract starch digestion. Tempering before rolling (without the addition of steam) may enhance the growth performance response and the NE value of maize. Average total tract starch digestion is similar for high-moisture and steam-flaked maize. However, the proportion of starch digested ruminally is greater (about 8 %) for high-moisture maize. The growth performance response of feedlot cattle to the feeding of high-moisture maize is highly variable. Although the NEm and NEg value of whole high-moisture maize was slightly less than that of dry processed maize (averaging 9·04 and 6·44 MJ (2·16 and 1·54 Mcal)/kg, respectively), grinding or rolling high-moisture maize before ensiling increased (6 %) its NE value. Substituting steam-flaked maize for dry processed maize increases average daily gain (6·3 %) and decreases DM intake (5 %). The comparative NEm and NEg values for steam-flaked maize at optimal processing (density = 0·34 kg/l) are 10·04 and 7·07 MJ (2·40 and 1·69 Mcal)/kg, respectively. These NE values are greater (3 %) than current tabular values (National Research Council, 2000), being more consistent with earlier standards (National Research Council, 1984). When maize is the primary or sole source of starch in the diet, concentration of starch in faeces (faecal starch, % of DM) of feedlot steers can serve as an indicator of total tract starch digestion, and, hence, the feeding value of maize.

Seven mature Welsh-cross pony geldings provided the faecal inocula in a cross-over design experiment, consisting of two 14-day periods. In period 1, four ponies (group 1) were offered a low-starch fibre mix (LS), and three (group 2) were offered a conventional high-starch coarse mix (HS). Both groups were offered these mixes in a 50:50 ratio with mature grass hay, to give a total daily dry-matter intake of 17·5 g/kg live weight per day. Diets were then switched in period 2. At the end of each experimental period freshly voided faeces were collected from each animal and assessed for their ability to ferment grass hay (H), fibre mix (FMix) or starch-based coarse mix (SMix) using the gas production (GP) technique of Theodorou et al. (1994). Donor animal diet and donor animal had no effect on any end-point measurements. Lag times recorded for the SMix were significantly (P<0·001) greater in LS-inoculated bottles compared with the HS inocula (1·74 v. 2·25 h, respectively). Lag times for FMix and SMix varied significantly (P<0·001) between ponies (0·82 to 1·78 h in the FMix and 1·64 to 2·51 h in the SMix). The degradation rate of H also differed significantly (P<0·001) between ponies with the time taken to reach 50% of GP (T50) ranging from 12·70 to 17·30 h. Consequently, it would appear that the effect of feeding LS or HS on the in vitro fermentative capacity of equine faecal inocula is minimal; moreover, the GP technique appears to be valuable tool for evaluating such effects.

The objective of the present study was to investigate the postprandial metabolism of two starches with contrasting rates of hydrolysis in vitro. Characterized using the Englyst method of in vitro starch classification, C*Set 06 598 contained predominantly rapidly digestible starch and C*Gel 04 201 contained predominantly slowly digestible starch. Each test starch, naturally enriched with 13C, was fed to ten healthy female volunteers as part of a moderate fat test meal (containing 75 g test starch and 21 g fat), in a double-blind randomized crossover design. The metabolic response to each starch was measured after an overnight fast, in an acute 6 h study, before and after 14 d of daily consumption of 75 g test starch. During each acute study, blood samples were taken at 15 min intervals for the first 2 h and at 30 min intervals for the remaining 4 h. Breath 13CO2 enrichment was measured at the same time points and indirect calorimetry was performed for 20 min every 40 min immediately before and throughout the study. Significantly more rapid, greater changes in postprandial plasma glucose, NEFA and serum insulin concentrations were observed after consumption of the rapidly digestible starch. Breath 13CO2 output over the first 3–4 h rose rapidly then began to decline following consumption of the rapidly digestible starch, but plateaued for the slowly digestible starch. The 14 d adaptation period did not affect any of the glycaemic or lipaemic variables but there was a reduction in postprandial plasminogen activator inhibitor-1 concentrations. These data confirm that starches characterized as predominantly rapidly digestible versus slowly digestible by the Englyst procedure provoke distinctly different patterns of metabolism postprandially.

Acarbose (Glucobay®; Bayer) is an α-glucosidase inhibitor used to treat diabetes and which may have a role in the prevention of type 2 diabetes. The present study investigated the effects of acarbose treatment on the site and extent of starch digestion, large-bowel fermentation and intestinal mucosal cell proliferation. Eighteen young male Wistar rats were fed ‘Westernised’ diets containing 0, 250 and 500mg acarbose/kg (six rats/diet) for 21d. For most variables measured, both acarbose doses had similar effects. Acarbose treatment suppressed starch digestion in the small bowel but there was compensatory salvage by bacterial fermentation in the large bowel. This was accompanied by a substantial hypertrophy of small- and large-bowel tissue and a consistent increase in crypt width along the intestine. Caecal total SCFA pool size was increased more than 4-fold, with even bigger increases for butyrate. These changes in butyrate were reflected in increased molar proportions of butyrate in blood from both the portal vein and heart. There was little effect of acarbose administration on crypt-cell proliferation (significant increase for mid-small intestine only). This is strong evidence against the hypothesis that increased fermentation and increased supply of butyrate enhances intestinal mucosal cell proliferation. In conclusion, apart from the increased faecal loss of starch, there was no evidence of adverse effects of acarbose on the aspects of large-bowel function investigated.

The objective of the present study was to investigate the effects of starches with differing rates of hydrolysis on exposure to pancreatin in vitro on postprandial carbohydrate metabolism in healthy subjects and in subjects with type 2 diabetes. Two test starches, prepared from uncooked native granular starch products, and naturally enriched with 13C, were consumed in a randomized crossover design by eight healthy and thirteen type 2 diabetic subjects. One starch was characterized in vitro as being rapidly hydrolysed (R, 94% after 180min), and the other was more slowly hydrolysed (S, 51% after 180min). Each subject consumed 50g of each test starch. In addition, the type 2 diabetic subjects consumed 89·7g of the S starch on a separate occasion. Blood samples were taken at 10min intervals for 3h, and at 20min intervals for a further 3h during a 6h postprandial period. Breath 13CO2 enrichment was measured at the same time points, and indirect calorimetry was performed for seven 20min sessions immediately before and during the 6h postprandial period. With the R starch, plasma glucose concentrations and serum insulin concentrations rose faster and the maximum glucose change was approximately 1·8 times that for the S starch, averaged across both subject groups. The areas under the curves for glucose and insulin were, respectively, 1·7 and 1·8 times higher for the R starch compared with the S starch, averaged across both subject groups. The rate of 13CO2 output and the proportion of 13C recovered in breath after consumption of the R starch was similar for both subject groups. The results provide evidence that starches which have different rates of hydrolysis in vitro result in different patterns of glycaemia and insulinaemia in both healthy adults and in diet-controlled type 2 diabetic subjects. Data from the hydrolysis of novel starch products in vitro, therefore, are useful in predicting glycaemic responses in vivo.

The purpose of the present study was to determine the effect of abomasal casein infusion on glucose uptake and abundance of the Na+/glucose co-transporter (SGLT1) 1 in the ovine small intestine. Lambs (body weight 35 (SEM 1·0) kg) were surgically fitted with abomasal infusion catheters and were fed diets containing equal portions of wheat hay and cracked maize. Lambs were infused with either 500 g water/d or with 500 g water containing 35 g casein/d. The infusion period lasted 10 d, after which lambs were killed, exsanguinated and eviscerated. Brush border membrane vesicles (BBMV) were prepared using mucosa from different small intestinal regions. Intake and total tract digestibility of nutrients were similar between treatments and averaged 1134, 1142 and 486 g/d and 67, 70, and 94 % for DM, organic matter and non-structural carbohydrates respectively. Crude protein (N×6·25) digestibility was 15 % greater in the casein-infused than control lambs. Glucose uptake to BBMV ranged from 101 to 337 pmol/mg protein per s along the small intestine and was greatest in the mid-section of the small intestine. In the mid-jejunum, glucose uptake was greater (P<0·07) in lambs infused with casein and averaged 120 pmol/mg protein per s compared with 68 pmol/mg protein per s in the control group. SGLT1 affinity was similar between treatments and averaged 104 μM in the different segments of the small intestine of lambs. However, lambs infused with casein exhibited similar values along the small intestine and affinity averaged 106 μm, while in the control group a greater affinity (85 μm) was measured in the mid-jejunum. SGLT1 protein abundance was correlated with glucose uptake in the BBMV in the casein-treated lambs, but not in the control group. These results suggest that glucose uptake along the small intestine of lambs is influenced by casein or its derivatives in the small intestine via SGLT1 affinity and activity at the brush border membrane, and that SGLT1 activity may be regulated by post-translational events affected by amino acids and peptides.

Three castrated male Holstein cattle (423 (SD 19) kg live weight) fitted with elevated carotid artery, portal, and mesenteric venous catheters, and abomasal and ileal cannulas were used to study small-intestinal starch digestion. The cattle were infused abomasally with water (275 ml/h), glucose (66 g/h), maize dextrin (66 g/h) or maize starch (66 g/h) in an incomplete Latin square design, with eight infusion periods. Infusion with carbohydrate resulted in higher arterial glucose concentrations and greater net portal glucose flux than when cattle were infused with water. Arterial glucose concentration and net portal glucose flux were highest when glucose was infused. In the small intestine, 85% of abomasally infused glucose, 78% of infused dextrin, and 66% of infused starch disappeared. Of the carbohydrate that disappeared in the small intestine, that which could be accounted for as net portal glucose flux was 73% for glucose, 60% for dextrin, and 57% for starch. Ileal digesta contained unpolymerized glucose, and short-chain soluble α-glucoside. Of the infused dextrin flowing past the ileum (14 g/h), 0·3 g/h was glucose, 6·2 g/h was soluble α-glucoside, and 7·5 g/h was insoluble α-glucoside. Of the infused starch flowing at the ileum (22·2 g/h), 0-9 g/h was glucose, 5·3 g/h was soluble α-glucoside, and 15·9 g/h was insoluble α-glucoside. The average chain lengths of the soluble α-glucosides in ileal digesta were 2·07 and 2·36 for dextrin and starch infusions respectively, indicating mostly di- and to a lesser extent trisaccharides. We conclude that (1) when 66 g raw starch is presented to the small intestine per h, about half of the intestinal disappearance appears as glucose in the portal vasculature, and (2) α-1,4 glucosidase (EC 3.2.1.20) activity at the brush border is the rate-limiting step to small-intestinal starch digestion in cattle.

Cotyledons and hulls were prepared from twelve varieties of field beans (Vicia faba L.). Adult cockerels were tube-fed either beans, cotyledons or hull diets containing high or low levels of protein. Metabolizable energy coefficients and starch digestibility coefficients were determined for beans, cotyledons and hull diets. Lipid digestibility coefficients from hull diets were also determined. When cotyledons were fed there were no significant differences in the way in which adult cockerels metabolized energy or digested starch from the proanthocyanidin-free and proanthocyanidin-rich varieties (0·780, 0·908, 0·775 and 0·918 respectively). When beans were fed, however, both energy metabolizability and starch digestibility decreased due to the presence of hulls, with proanthocyanidin-rich hulls decreasing values the most to 0·660 and 0·819 respectively, and proanthocyanidin-free hulls decreasing values to a lesser extent to 0·709 and 0·886 respectively. Diets composed of proanthocyanidin-rich hulls depressed metabolizable energy and maize starch digestibility. Their effect on maize starch digestibility, however, was considerably less than that on bean starch. Lipid digestibility was enhanced by proanthocyanidins but only when the protein content of the diet was high. There was a significant correlation (P < 0·05) between the vanillin and anthocyanidin formation methods for the estimation of proanthocyanidins (r 0·779). There was also a highly significant regression of bean starch digestibility v. proanthocyanidin content of coloured-flowered bean hulls (P < 0·001). The regression of maize starch digestibility v. hull proanthocyanidins was also significant at P < 0·005.

To study in vivo the contribution of the bacterial flora to amylomaize starch degradation in the rat, germ-free and conventional rats were fed on a diet containing either a normal maize starch or an amylomaize starch. In germ-free rats maize starch was almost totally digested in the small intestine, whereas 40% of the ingested amylomaize starch reached the caecum and 30% was excreted, despite the very high endogenous amylase activity. Study by transmission electron microscopy of germ-free caecal contents showed an endocorrosion of the starch granule. In conventional rats, as in germ-free rats, digestibility of maize starch reached 98% in the small intestine, whereas that of amylomaize starch was only 60%. In the caecum of these rats amylomaize starch was fermented, and this led to a decrease in caecal pH and to formation of short-chain fatty acids (SCFA), especially propionate. Comparison between conventional rats fed on maize starch or amylomaize starch showed that caecal SCFA concentrations during a circadian cycle varied in the same way whereas total SCFA and lactic acid concentrations were much higher in rats fed on amylomaize starch. Amylase (EC 3.2.1.1) activity was similar in the caecal contents of conventional rats whatever the ingested starch. It was lower in conventional than in germ-free rats, but no starch granule remained in the caecum of conventional rats. These results showed that bacterial amylase was more efficient at degrading resistant amylomaize starch than endogenous amylase.

Four diets were prepared in which cooked potatoes provided approximately 0.8 of the dry matter (DM) and all the polysaccharides. The potatoes were either boiled conventionally (unprocessed) or prepared by rehydrating a commercial ‘instant’ potato product with boiling water. The diets were fed to rats (six per diet) immediately after preparation or after storage at 4° for 48 h and observations made on site and extent of digestion and large bowel (LB) fermentation. All diets were equally well digested (overall means 0.95 and 0.96 for DM and organic matter (OM) respectively) with a significant proportion of this digestion occurring in the LB. More OM flowed to the LB with the ‘instant’ (0.18 of intake) than with the unprocessed potato diets (0.13 of intake) and was associated with markedly different fermentation patterns. When compared with rats fed on the unprocessed potato diets, those given ‘instant’ potatoes had smaller caecums with much shorter transit times, lower caecal pH, higher total volatile fatty acid (VFA) concentrations and a greater molar proportion of butyrate in these VFA. Storage of the diets for 48 h at 4° had relatively smaller effects on these variables. Possible mechanisms for the observed effects and implications for tissue metabolism and health are discussed. In addition, a simple and apparently novel method for quantifying coprophagy is proposed.