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Inter–animal variation in glycemic and insulinemic response to different carbohydrate sources
Abstracts Vol 29, No 5 (2009)
transported to the laboratory, and dried to a constant weight in a forced-draught oven at 60 8C. Dried samples were milled to pass a 1 mm steel mesh. All samples were then analysed for their fructan content by the HPLC method3 or by the colorimetric technique.4 Inulin from Jerusalem artichoke was used as a control. The results were subjected to ANOVA.
RESULTS Values for fructan content of the samples ranged from 97-198 g fructan/kg DM as determined by HPLC and 11-110 g fructan/kg DM as determined by the colorimetric method (Table 1). Thus the HPLC technique consistently yielded higher fructan values than the colorimetric method for each sample. The magnitude of differences in values between the two methods varied with date of sampling. To determine if the discrepancy in fructan values between the two methods was due to ineffective activity of the fructanase, used in the colorimetric method, against grass fructan, the composition of extracts of two replicates of sample 2 both before and after treatment with the fructanase were analysed by HPLC. Inulin from Jerusalem artichoke served as a control. Degradation of fructan in the timothy sample averaged 53%; that of the inulin control was in excess of 90%.
DISCUSSION The colorimetric method yielded consistently lower values for fructan content of timothy samples than the HPLC technique (44-90 %), but reliably measured inulin from Jerusalem artichoke. The colorimetric method was originally developed and verified for use in the food industry where inulin-type fructans of moderate DP are frequently found. However, timothy, accumulates high DP levan-type fructan.5 Inulins are linear fructans linked by b-(2-1) glycosidic bonds, whereas although levans are also linear fructans they are linked by b-(2-6) glycosidic bonds. Thus, the type of fructan accumulated by timothy grass differs substantially from that accrued by Jerusalem artichoke, and it appears that the discrepancy between fructan values obtained by the two analytical methods was largely due to incomplete hydrolysis of timothy fructan. It may be advantageous to carefully monitor the diets of equines pre-disposed to the condition to prevent excess ingestion of fructans. It is therefore essential that the analyses used to measure the fructan content of their feeds accurately reflect the true fructan content of the feedstuff. The inconsistent efficacy of the colorimetric technique in quantifying the fructan content of timothy samples suggest that the colorimetric technique can result in variable and substantial underestimates of fructan content in such
377
Table 1. Comparison of mean (+SD) fructan values obtained by the HPLC or colorimetric methods for timothy grass samples harvested on seven occasions (g fructan/kg DM) Sample HPLC 1 2 3 4 5 6 7 Inulin control
SD
Colorimetric SD
99.9* 6.7 30.1 198.1* 4.6 110.5 99.2* 17.8 17.7 109.6* 5.9 10.9 97.1* 1.5 35.5 109.3* 9.5 44.4 102.4* 5.5 35.4 296.4 1.9 291.3
4.5 27.6 8.9 1.6 14.9 8.1 19.2 0.75
Ratio % 30 56 18 10 37 41 35
* differs significantly from the values obtained by colorimetry (p <0.05)
grasses. Further work is needed to determine the efficacy of the colorimetric fructan assay in other temperate grass species. Keywords: Fructan measurement; HPLC; Colorimetric techniques REFERENCES 1. Pollitt CC, Kyaw-Tanner M, French KR, Van Eps AW, Hendrikz JK, Daradka M. Equine Laminitis: 49th Annual Convention of the American Association of Equine Practitioners 2003. 2. Van Eps AW, Pollitt CC. Equine laminitis induced with oligofructose. Equine Vet J 2006;38:203-208. 3. Cairns AJ, Pollock CJ. Fructan synthesis in excised leaves of Lolium temulentum: 1) chromatographic characterisation of oligofructans and their labelling patterns following 14 CO2 feeding. New Phytologist 1988;109:399-405. 4. Mc Cleary BV, Murphy A, Mugford DC. Determination of oligofructans and fructan polysaccharides in foodstuffs by an enzymatic/spectrophotometric method. Collaborative study. J AOAC International 1997;83:356-36. 5. Spollen WG, Nelson CJ. Characterization of Fructan from Mature Leaf Blades and Elongation Zones of Developing Leaf Blades of Wheat, Tall Fescue, and Timothy. Plant Physiology1988;88(4):1349-1353.
31844 Inter–animal variation in glycemic and insulinemic response to different carbohydrate sources W.B. Staniar,* H.S. Grube, and E.A. Jedrzejewski, The Pennsylvania State University, University Park, PA, USA
INTRODUCTION Glycemic and insulinemic response to a meal is variable in the horse.1,2 Factors such as rate of intake and physical characteristics of feed have been investigated, but there
Abstracts Vol 29, No 5 (2009)
378
is little information on additive effects of digestion and absorption. These factors become important in regard to feed ingredients with different starch digestibilities or physical characteristics. Our hypothesis is that the amount of inter–animal variation will increase from low with an intravenous glucose dose to a high with a meal of crimped oats. The objective was to quantify and compare inter– animal variation in glycemic and insulinemic response to intravenous, oral, and dietary nonstructural carbohydrates.
MATERIALS AND METHODS Ten Quarter horse broodmares 11 5.6 y of age, weighing 597 43 kg, with body condition scores of 6.4 0.9 were given 5 treatments in a balanced latin square design. The study was conducted over a period 10 days in July and August. The treatments were; an intravenous administration of dextrose (300 mg/kg BW) (DIV), a nasogastric administration of dextrose (500 mg/kg BW) (DNT), a nasogastric administration of corn starch (1 g/kg BW) (SNT), a nasogastric administration of an oat extract (7.3 ml/kg BW) (ONT), or a meal of crimped oats (2.2 g/kg BW, 733 mg starch/kg BW) (OM). Treatments were formulated in an attempt to cause similar glycemic responses. Horses had ad libitum access to water but no hay during the feed tests. Catheters were placed prior to sampling. Treatments were given and samples taken on study days 2, 4, 6, 8 and 10. Mares had all feed removed by 2200 the evening prior to each test. Treatments were given at approximately 08:00 on each study day. Blood samples were taken at -15, 0, 5, 10, 15, 20, 25, 30, 35, 45, 60, 90, 120, 150, 180, 210, 270, 330 and 390 minutes. Following completion of sampling on study days, mares had access to pasture until they were brought in at 1600 the following afternoon. Plasma was frozen and stored at -20 8C for later measurement of plasma glucose by colorimetric assay and plasma insulin using a previously validated assay. To control for any influence of nasogastric tubing, all mares were nasogastric tubed; a volume of water equal to that used in other treatments was given to mares on the intravenous and meal treatments. Baseline concentrations for glucose and insulin are represented by a mean of the first two blood samples taken on the study day. Peak concentrations are the highest concentration measured during the day. The areas under the concentration-time curve (AUC) were calculated as summed trapezoids. Glucose and insulin data were analyzed with ANOVA for repeated measures with treatment and time and interactions as well as horse within treatment examined as main effects. Levene’s test was used to test for equality of variances between treatments. Differences were considered significant when P < 0.05, and data are presented as means SD.
RESULTS Mean Comparisons There were no detectable differences in basal glucose (92.2 9.7 mg/dl) and basal insulin (6.2 5.6 mIU/L) between the treatment groups. Peak glucose concentrations were significantly higher in the DIV (404 71 mg/ dl) and DNT (183 48 mg/dl) groups compared to a pooled mean for other treatments (162 40 mg/dl). Peak insulin concentrations were highest in response to OM (85 55 mIU/L) and ONT (78 39 mIU/L) and lowest for DIV (37 16 mIU/L) and SNT (41 23 mIU/L). Due to the different shape of the response curve the DIV glucose and insulin AUCs were not compared to the other treatments. There were no differences detected in the glucose AUC while the insulin AUC was higher for the OM (15585 11246 min*mIU*L^-1) and ONT (11324 5692 min*mIU*L^-1) when compared to the DNT (7112 4552 min*mIU*L^-1) or SNT (4155 2840 min*mIU*L^-1). There was significant influence of all main effects and interactions on both glucose and insulin in the analysis of variance. Variation Comparisons The variation in glucose values was lowest for SNT (overall SD, 18.3 mg/dl) and highest for DIV (overall SD, 103 mg/dl), while variation in insulin was also lowest for SNT (overall SD, 10.0 mIU/L), it was highest for the ONT and OM (overall SD, 29.5 and 38.4 mIU/L;respectively).
DISCUSSION Data from this study provides information on the expected inter–animal variation in glycemic and insulinemic to the 5 treatments. Insulinemic responses to the treatments, except for that to SNT, were inversely related to the glycemic response, i.e. the DIV had the highest glucose response and lowest insulin response, while the ONT and OM had relatively low glycemic responses, but both exhibited the highest insulin responses. This may be due to insulin’s response to absorbed glucose and amino acids. The relatively low responses to SNT may be due to the use of corn starch, while the ONT and OM were oat starch. Other work has shown a greater digestibility of oat starch.3 The differences in variation between the treatment groups seem due mainly to differences in the magnitude of response to each treatment as opposed to physiologic factors within the horse relating to digestion and absorption of glucose. Broadly, these factors will include peripheral absorption of glucose by tissues in the body, absorption of glucose across the gastrointestinal tract, enzymatic digestion of starch, and the digestion of starch within a common equine feed ingredient. It could be
Abstracts Vol 29, No 5 (2009)
expected that variation between treatments would increase sequentially DIV, DNT, SNT, ONT, to OM as each layer of digestion and absorption was added, but this was not shown in this study. This may lend strength to the argument that the majority of variation in glycemic and insulinemic response to a particular feed has less to do with between animal variation in digestion and absorption and more with systemic regulation of glucose and insulin. REFERENCES 1. Staniar WB, Cubitt TA, George LA, Harris PA, GeorR J. Glucose and insulin responses to different dietary energy sources in thoroughbred broodmares grazing cool season pasture. Livestock Sci 2007;111: 164-171. 2. Vervuert I, Coenen M, Bothe C. Effects of oat processing on the glycaemic and insulin responses in horses. J Anim Phys Anim Nutr 2003;87:96-104. 3. Julliand V, De Fombelle A, Varloud M. Starch digestion in horses: The impact of feed processing. Livestock Sci 2006;100:44-52.
31890 Nonstructural carbohydrate and glycemic response of feeds: how low is ‘‘low’’ starch? R.M. Hoffman,*1 J.C. Haffner,1 C.A. Crawford,1 H. Eiler,2 and K.A. Fecteau2, 1of Agribusiness and Agriscience, Middle Tennessee State University, Murfreesboro, TN, USA, 2Department of Comparative Medicine, College of Veterinary Medicine, The University of Tennessee, Knoxville, TN, USA
INTRODUCTION Horses are grazing, hind-gut fermenting herbivores, adapted to have forage as the main portion of their diet. Horse owners have provided grain supplements rich in starch in order to provide additional calories, protein and micronutrients necessary for optimal performance. In human nutrition, a shift to low fat, high starch diets has resulted in an increase in the prevalence of insulin resistance, which is fundamental in the pathology of type 2 diabetes. Several metabolic diseases have been categorized as equine grainassociated disorders1 and related to insulin resistance in horses.2 Managing horses prone to equine grain-associated disorders is difficult at best. In human nutrition, food glycemic indices, a measure of the glucose and insulin responses to specific foods, are available for developing nutritional strategies for insulin resistant people or those at risk for type 2 diabetes. In horses, glycemic indices for feeds are limited,3 although horses appear to be sensitive to small changes in dietary sugar and starch.4 There is currently a trend in the horse feed industry to manufacture low or controlled starch feeds, with claims of reducing the risk of grain-associated metabolic disorders; however, lack of
379
reports elucidating the effect of various starch intakes on blood glucose response leave questions regarding exact concentrations of dietary starch for horses that may be considered ‘‘low.’’ The objective of this study was to determine if a threshold value of dietary starch intake could be defined, by plotting incremental area under the curve of glucose response against varying non-structural carbohydrate intakes.
MATERIALS AND METHODS Eight horses with BW of 571 48 kg and BCS ranging from 5 to 7 were provided with eight meals in an 8 x 8 factorial design. Glycemic response of different horse feeds and forages were reported as low for unmolassed beet pulp and significantly higher for oats.3 Based on these data, 8 meals with different concentrations of oats and beet pulp were designed in order to provide different levels of starch to each horse in equal-weight meals. The ratios of oats:beet pulp in the meals were 100:0, 75:25, 67:33, 50:50, 33:67, 25:75, and 0:100. Non-structural carbohydrate concentrations of the diets were determined using ethanol extraction (Equi-Analytical, Ithaca, NY), and provided intakes of NSC ranging from 0.6 to 2.0 g/kg BW. To determine glucose response to meals, the horses were housed in stalls 18 h before the onset of the study and allowed ad libitum access to water but no hay. Body weights were determined using an electronic scale, and venous catheters were placed in order to facilitate the ease of repeated blood sampling. On each testing day, the horses were offered one of the eight meals, assigned in random order. Blood samples were collected into heparinized tubes, beginning at 0 (baseline), then 15, 30, 60, 90, 120, 150, 180, 240 and 300 min after the meal. Time to consume the meal was recorded, and orts were weighed after 1 hr of consumption time was allowed. It should be noted that not all meals were consumed fully, so NSC intake was calculated using total offered minus orts. Glucose concentrations in whole blood were analyzed immediately using a glucometer designed for use with fresh capillary blood (Precision Xtra, MediSense, Abbott Laboratories, Alameda, CA) and previously validated for horses.5 The magnitude of each glucose response was calculated as the incremental area under the curve (AUC) by graphical approximation. The threshold of glycemic sensitivity, i.e. the inflection point, or knot, after which higher NSC intakes produced less of a slope in AUC changes, was determined using NSC intake as the independent variable and blood glucose AUC as the dependent variable. The data were fitted using the NLIN procedure of SAS, a segmented regression model originally designed to obtain an objective estimate of a nutrient requirement.
transported to the laboratory, and dried to a constant weight in a forced-draught oven at 60 8C. Dried samples were milled to pass a 1 mm steel mesh. All samples were then analysed for their fructan content by the HPLC method3 or by the colorimetric technique.4 Inulin from Jerusalem artichoke was used as a control. The results were subjected to ANOVA.
RESULTS Values for fructan content of the samples ranged from 97-198 g fructan/kg DM as determined by HPLC and 11-110 g fructan/kg DM as determined by the colorimetric method (Table 1). Thus the HPLC technique consistently yielded higher fructan values than the colorimetric method for each sample. The magnitude of differences in values between the two methods varied with date of sampling. To determine if the discrepancy in fructan values between the two methods was due to ineffective activity of the fructanase, used in the colorimetric method, against grass fructan, the composition of extracts of two replicates of sample 2 both before and after treatment with the fructanase were analysed by HPLC. Inulin from Jerusalem artichoke served as a control. Degradation of fructan in the timothy sample averaged 53%; that of the inulin control was in excess of 90%.
DISCUSSION The colorimetric method yielded consistently lower values for fructan content of timothy samples than the HPLC technique (44-90 %), but reliably measured inulin from Jerusalem artichoke. The colorimetric method was originally developed and verified for use in the food industry where inulin-type fructans of moderate DP are frequently found. However, timothy, accumulates high DP levan-type fructan.5 Inulins are linear fructans linked by b-(2-1) glycosidic bonds, whereas although levans are also linear fructans they are linked by b-(2-6) glycosidic bonds. Thus, the type of fructan accumulated by timothy grass differs substantially from that accrued by Jerusalem artichoke, and it appears that the discrepancy between fructan values obtained by the two analytical methods was largely due to incomplete hydrolysis of timothy fructan. It may be advantageous to carefully monitor the diets of equines pre-disposed to the condition to prevent excess ingestion of fructans. It is therefore essential that the analyses used to measure the fructan content of their feeds accurately reflect the true fructan content of the feedstuff. The inconsistent efficacy of the colorimetric technique in quantifying the fructan content of timothy samples suggest that the colorimetric technique can result in variable and substantial underestimates of fructan content in such
377
Table 1. Comparison of mean (+SD) fructan values obtained by the HPLC or colorimetric methods for timothy grass samples harvested on seven occasions (g fructan/kg DM) Sample HPLC 1 2 3 4 5 6 7 Inulin control
SD
Colorimetric SD
99.9* 6.7 30.1 198.1* 4.6 110.5 99.2* 17.8 17.7 109.6* 5.9 10.9 97.1* 1.5 35.5 109.3* 9.5 44.4 102.4* 5.5 35.4 296.4 1.9 291.3
4.5 27.6 8.9 1.6 14.9 8.1 19.2 0.75
Ratio % 30 56 18 10 37 41 35
* differs significantly from the values obtained by colorimetry (p <0.05)
grasses. Further work is needed to determine the efficacy of the colorimetric fructan assay in other temperate grass species. Keywords: Fructan measurement; HPLC; Colorimetric techniques REFERENCES 1. Pollitt CC, Kyaw-Tanner M, French KR, Van Eps AW, Hendrikz JK, Daradka M. Equine Laminitis: 49th Annual Convention of the American Association of Equine Practitioners 2003. 2. Van Eps AW, Pollitt CC. Equine laminitis induced with oligofructose. Equine Vet J 2006;38:203-208. 3. Cairns AJ, Pollock CJ. Fructan synthesis in excised leaves of Lolium temulentum: 1) chromatographic characterisation of oligofructans and their labelling patterns following 14 CO2 feeding. New Phytologist 1988;109:399-405. 4. Mc Cleary BV, Murphy A, Mugford DC. Determination of oligofructans and fructan polysaccharides in foodstuffs by an enzymatic/spectrophotometric method. Collaborative study. J AOAC International 1997;83:356-36. 5. Spollen WG, Nelson CJ. Characterization of Fructan from Mature Leaf Blades and Elongation Zones of Developing Leaf Blades of Wheat, Tall Fescue, and Timothy. Plant Physiology1988;88(4):1349-1353.
31844 Inter–animal variation in glycemic and insulinemic response to different carbohydrate sources W.B. Staniar,* H.S. Grube, and E.A. Jedrzejewski, The Pennsylvania State University, University Park, PA, USA
INTRODUCTION Glycemic and insulinemic response to a meal is variable in the horse.1,2 Factors such as rate of intake and physical characteristics of feed have been investigated, but there
Abstracts Vol 29, No 5 (2009)
378
is little information on additive effects of digestion and absorption. These factors become important in regard to feed ingredients with different starch digestibilities or physical characteristics. Our hypothesis is that the amount of inter–animal variation will increase from low with an intravenous glucose dose to a high with a meal of crimped oats. The objective was to quantify and compare inter– animal variation in glycemic and insulinemic response to intravenous, oral, and dietary nonstructural carbohydrates.
MATERIALS AND METHODS Ten Quarter horse broodmares 11 5.6 y of age, weighing 597 43 kg, with body condition scores of 6.4 0.9 were given 5 treatments in a balanced latin square design. The study was conducted over a period 10 days in July and August. The treatments were; an intravenous administration of dextrose (300 mg/kg BW) (DIV), a nasogastric administration of dextrose (500 mg/kg BW) (DNT), a nasogastric administration of corn starch (1 g/kg BW) (SNT), a nasogastric administration of an oat extract (7.3 ml/kg BW) (ONT), or a meal of crimped oats (2.2 g/kg BW, 733 mg starch/kg BW) (OM). Treatments were formulated in an attempt to cause similar glycemic responses. Horses had ad libitum access to water but no hay during the feed tests. Catheters were placed prior to sampling. Treatments were given and samples taken on study days 2, 4, 6, 8 and 10. Mares had all feed removed by 2200 the evening prior to each test. Treatments were given at approximately 08:00 on each study day. Blood samples were taken at -15, 0, 5, 10, 15, 20, 25, 30, 35, 45, 60, 90, 120, 150, 180, 210, 270, 330 and 390 minutes. Following completion of sampling on study days, mares had access to pasture until they were brought in at 1600 the following afternoon. Plasma was frozen and stored at -20 8C for later measurement of plasma glucose by colorimetric assay and plasma insulin using a previously validated assay. To control for any influence of nasogastric tubing, all mares were nasogastric tubed; a volume of water equal to that used in other treatments was given to mares on the intravenous and meal treatments. Baseline concentrations for glucose and insulin are represented by a mean of the first two blood samples taken on the study day. Peak concentrations are the highest concentration measured during the day. The areas under the concentration-time curve (AUC) were calculated as summed trapezoids. Glucose and insulin data were analyzed with ANOVA for repeated measures with treatment and time and interactions as well as horse within treatment examined as main effects. Levene’s test was used to test for equality of variances between treatments. Differences were considered significant when P < 0.05, and data are presented as means SD.
RESULTS Mean Comparisons There were no detectable differences in basal glucose (92.2 9.7 mg/dl) and basal insulin (6.2 5.6 mIU/L) between the treatment groups. Peak glucose concentrations were significantly higher in the DIV (404 71 mg/ dl) and DNT (183 48 mg/dl) groups compared to a pooled mean for other treatments (162 40 mg/dl). Peak insulin concentrations were highest in response to OM (85 55 mIU/L) and ONT (78 39 mIU/L) and lowest for DIV (37 16 mIU/L) and SNT (41 23 mIU/L). Due to the different shape of the response curve the DIV glucose and insulin AUCs were not compared to the other treatments. There were no differences detected in the glucose AUC while the insulin AUC was higher for the OM (15585 11246 min*mIU*L^-1) and ONT (11324 5692 min*mIU*L^-1) when compared to the DNT (7112 4552 min*mIU*L^-1) or SNT (4155 2840 min*mIU*L^-1). There was significant influence of all main effects and interactions on both glucose and insulin in the analysis of variance. Variation Comparisons The variation in glucose values was lowest for SNT (overall SD, 18.3 mg/dl) and highest for DIV (overall SD, 103 mg/dl), while variation in insulin was also lowest for SNT (overall SD, 10.0 mIU/L), it was highest for the ONT and OM (overall SD, 29.5 and 38.4 mIU/L;respectively).
DISCUSSION Data from this study provides information on the expected inter–animal variation in glycemic and insulinemic to the 5 treatments. Insulinemic responses to the treatments, except for that to SNT, were inversely related to the glycemic response, i.e. the DIV had the highest glucose response and lowest insulin response, while the ONT and OM had relatively low glycemic responses, but both exhibited the highest insulin responses. This may be due to insulin’s response to absorbed glucose and amino acids. The relatively low responses to SNT may be due to the use of corn starch, while the ONT and OM were oat starch. Other work has shown a greater digestibility of oat starch.3 The differences in variation between the treatment groups seem due mainly to differences in the magnitude of response to each treatment as opposed to physiologic factors within the horse relating to digestion and absorption of glucose. Broadly, these factors will include peripheral absorption of glucose by tissues in the body, absorption of glucose across the gastrointestinal tract, enzymatic digestion of starch, and the digestion of starch within a common equine feed ingredient. It could be
Abstracts Vol 29, No 5 (2009)
expected that variation between treatments would increase sequentially DIV, DNT, SNT, ONT, to OM as each layer of digestion and absorption was added, but this was not shown in this study. This may lend strength to the argument that the majority of variation in glycemic and insulinemic response to a particular feed has less to do with between animal variation in digestion and absorption and more with systemic regulation of glucose and insulin. REFERENCES 1. Staniar WB, Cubitt TA, George LA, Harris PA, GeorR J. Glucose and insulin responses to different dietary energy sources in thoroughbred broodmares grazing cool season pasture. Livestock Sci 2007;111: 164-171. 2. Vervuert I, Coenen M, Bothe C. Effects of oat processing on the glycaemic and insulin responses in horses. J Anim Phys Anim Nutr 2003;87:96-104. 3. Julliand V, De Fombelle A, Varloud M. Starch digestion in horses: The impact of feed processing. Livestock Sci 2006;100:44-52.
31890 Nonstructural carbohydrate and glycemic response of feeds: how low is ‘‘low’’ starch? R.M. Hoffman,*1 J.C. Haffner,1 C.A. Crawford,1 H. Eiler,2 and K.A. Fecteau2, 1of Agribusiness and Agriscience, Middle Tennessee State University, Murfreesboro, TN, USA, 2Department of Comparative Medicine, College of Veterinary Medicine, The University of Tennessee, Knoxville, TN, USA
INTRODUCTION Horses are grazing, hind-gut fermenting herbivores, adapted to have forage as the main portion of their diet. Horse owners have provided grain supplements rich in starch in order to provide additional calories, protein and micronutrients necessary for optimal performance. In human nutrition, a shift to low fat, high starch diets has resulted in an increase in the prevalence of insulin resistance, which is fundamental in the pathology of type 2 diabetes. Several metabolic diseases have been categorized as equine grainassociated disorders1 and related to insulin resistance in horses.2 Managing horses prone to equine grain-associated disorders is difficult at best. In human nutrition, food glycemic indices, a measure of the glucose and insulin responses to specific foods, are available for developing nutritional strategies for insulin resistant people or those at risk for type 2 diabetes. In horses, glycemic indices for feeds are limited,3 although horses appear to be sensitive to small changes in dietary sugar and starch.4 There is currently a trend in the horse feed industry to manufacture low or controlled starch feeds, with claims of reducing the risk of grain-associated metabolic disorders; however, lack of
379
reports elucidating the effect of various starch intakes on blood glucose response leave questions regarding exact concentrations of dietary starch for horses that may be considered ‘‘low.’’ The objective of this study was to determine if a threshold value of dietary starch intake could be defined, by plotting incremental area under the curve of glucose response against varying non-structural carbohydrate intakes.
MATERIALS AND METHODS Eight horses with BW of 571 48 kg and BCS ranging from 5 to 7 were provided with eight meals in an 8 x 8 factorial design. Glycemic response of different horse feeds and forages were reported as low for unmolassed beet pulp and significantly higher for oats.3 Based on these data, 8 meals with different concentrations of oats and beet pulp were designed in order to provide different levels of starch to each horse in equal-weight meals. The ratios of oats:beet pulp in the meals were 100:0, 75:25, 67:33, 50:50, 33:67, 25:75, and 0:100. Non-structural carbohydrate concentrations of the diets were determined using ethanol extraction (Equi-Analytical, Ithaca, NY), and provided intakes of NSC ranging from 0.6 to 2.0 g/kg BW. To determine glucose response to meals, the horses were housed in stalls 18 h before the onset of the study and allowed ad libitum access to water but no hay. Body weights were determined using an electronic scale, and venous catheters were placed in order to facilitate the ease of repeated blood sampling. On each testing day, the horses were offered one of the eight meals, assigned in random order. Blood samples were collected into heparinized tubes, beginning at 0 (baseline), then 15, 30, 60, 90, 120, 150, 180, 240 and 300 min after the meal. Time to consume the meal was recorded, and orts were weighed after 1 hr of consumption time was allowed. It should be noted that not all meals were consumed fully, so NSC intake was calculated using total offered minus orts. Glucose concentrations in whole blood were analyzed immediately using a glucometer designed for use with fresh capillary blood (Precision Xtra, MediSense, Abbott Laboratories, Alameda, CA) and previously validated for horses.5 The magnitude of each glucose response was calculated as the incremental area under the curve (AUC) by graphical approximation. The threshold of glycemic sensitivity, i.e. the inflection point, or knot, after which higher NSC intakes produced less of a slope in AUC changes, was determined using NSC intake as the independent variable and blood glucose AUC as the dependent variable. The data were fitted using the NLIN procedure of SAS, a segmented regression model originally designed to obtain an objective estimate of a nutrient requirement.