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J. Anim. Sci. 2003. 81:1764-1771
© 2003 American Society of Animal Science

Glucose clearance in grazing mares is affected by diet, pregnancy, and lactation1

R. M. Hoffman*,2, D. S. Kronfeld*, W. L. Cooper* and P. A. Harris{dagger}

* Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061-0306 and and {dagger} Equine Studies Group, WALTHAM Centre for Pet Nutrition, Melton Mowbray, U.K.

2 Correspondence:
Virginia Tech MARE Center, 5527 Sullivans Mill Rd., Middleburg, VA 20117 (phone: 540-687-3521; fax: 540-687-5362; E-mail:
Rhonda.Hoffman{at}vt.edu).


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
The glucose tolerance test in the horse may be used to determine metabolic responses to diet, disease, or physiologic state. The objective of this study was to determine the effect of reproductive stage (gestation and lactation) and supplemental dietary energy source (sugar and starch [SS] or fiber and fat [FF]) on glucose metabolism in grazing mares using an oral glucose tolerance test. Twelve mares, six on each supplement, were examined on three occasions: one in the third trimester of pregnancy, the second in early lactation, and the third in late lactation. During each test, venous samples were taken at 30 and 1 min before, and 30, 60, 90, 120, 150, 180, 240, and 300 min after a nasogastric dose of glucose at 0.2 g/kg of BW. Plasma was assayed for glucose, insulin, and cortisol. Statistical analysis was a mixed model with repeated measures with horse, diet, and reproductive stage as fixed effects. The incremental glucose area under the curve (AUC) in response to oral glucose was lower in SS than in FF mares (P = 0.022). Mares tended to have a lower incremental glucose AUC in early lactation than in late gestation (P = 0.057), and insulin AUC was lower in early lactation than in late gestation (P = 0.002) and late lactation (P = 0.013). Glucose clearance was more rapid (P = 0.007) in SS than in FF mares. The glycemic response to the oral glucose tolerance test was consistent with adaptation to dietary sugar and starch as well as metabolic changes associated with pregnancy and lactation. Feeding twice-daily grain meals rich in SS influenced glucose metabolism in horses to an extent that the natural adaptation of glucose metabolism to pregnancy was moderated. Feeding a diet rich in FF more closely mimics the natural grazing state of pasture and allows for adaptation of glucose metabolism to pregnancy and lactation.

Key Words: Fat • Glucose Tolerance Test • Lactation • Mares • Pregnancy


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
Plasma glucose and insulin responses to a glucose challenge is known as the glucose tolerance test (Roberts and Hill, 1973). It provides information about the animal’s glucose metabolism and homeostasis. Factors affecting glucose tolerance include diet adaptation (Jacobs and Bolton, 1982), fasting, pregnancy and lactation (Evans, 1971), obesity, and diseases such as laminitis, pituitary disturbances, and polysaccharide storage myopathy (De La Corte et al., 1999).

A similar procedure, the glycemic index, measures plasma glucose and insulin responses to a meal and provides information about the food but not necessarily the animal. The glycemic index has been applied primarily in human nutrition for diabetics in order to formulate diets with a low glycemic impact (Wolever et al., 1991). In horse nutrition, the glycemic index has been used to describe meal-related responses of blood glucose and insulin to different diets (Stull and Rodiek, 1988; Williams et al., 2001). Factors affecting glycemic index include meal size, concentrations of hydrolyzable carbohydrates, fiber and fat, processing, intake time, gastric emptying, digestibility, and rate of absorption.

Glucose metabolism adapts to pregnancy and lactation in many species (Bell, 1995; Boden, 1996), including horses (Evans, 1971). The objective of this study was to determine the effect of physiological state (gestation and lactation) and supplemental dietary energy source (sugar and starch or fiber and fat) on glucose metabolism in grazing mares, as assessed using an oral glucose tolerance test. The hypothesis was twofold: 1) the glycemic response to oral glucose will be greater during gestation than lactation, and 2) because the oral glucose load would be similar to a meal rich in sugar and starch, the largest glycemic response would be noted in mares not adapted to sugar and starch.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Implications
 Literature Cited
 
The research was conducted at the Middleburg Agricultural Research and Extension (MARE) Center. The protocol was approved by the Institution’s Animal Care and Use Committee. Twenty Thoroughbred mares were maintained in groups of 10 on adjacent, 30-acre, mixed bluegrass/white clover pastures. Prior to the study, representative samples of the pastures were collected by random clippings from all areas of each pasture during different seasons. Analysis of these samples (Dairy One, Ithaca, NY) indicated no difference (P > 0.10) in nutrient composition of DM, CP, ADF, NDF, fat, nonstructural carbohydrate, Ca, P, Mg, K, Na, S, Fe, Zn, Cu, and Mn between the pastures. Pasture samples were collected monthly for the duration of the study. The mares and their foals were supplemented with mixed grass/legume hay between the months of November and April. A core sampler was used to collect hay samples, each being a composite from 10 bales.

Supplements
In midgestation, the mares were paired by age, breeding date, and sire of their foal, and then randomly assigned into two groups. In addition to pasture, 10 mares and their foals were fed a corn grain and molasses supplement (SS) and 10 were fed a corn oil and fiber supplement (FF). The supplements (Table 1Go) were formulated to be isocaloric and isonitrogenous, with mineral contents balanced to complement the pasture and to meet or exceed current recommendations (NRC, 1989). The supplements were fed to the mares in varying amounts, with goals of approximately a 1:2 supplement:forage ratio (Kronfeld, 1998) and BCS of 5 and 6 for mares, using a scale of 1 to 9 (Henneke et al., 1983). The amounts fed were 2.9, 4.0, and 3.5 kg/d per mare in late gestation, early lactation, and late lactation, respectively, divided into two meals. The supplements were fed in pans placed in a circular pattern on the ground with a distance of approximately 6 m between pans to minimize dominant/submissive behavioral effects during feed ingestion. The mares were observed to ensure each consumed their allotted amount of supplement.


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  		Table 1. Ingredient composition (%), as-fed basis, of the sugar and starch (SS) and fat and fiber (FF) supplements fed to the mares
 
Oral Glucose Tolerance Test
Twelve of the twenty Thoroughbred mares, six on each supplement, were chosen for the oral glucose tolerance test based on their acceptance of being housed in a stall. Three oral glucose tolerance tests were completed, one during the third trimester of gestation (310 ± 3.9 d gestation, 4 mo after initiation of the dietary treatments), one in early lactation (55 ± 3.6 d after foaling), and the last in late lactation (182 ± 3.6 d after foaling).

For each oral glucose tolerance test, the mares were weighed using an electronic scale (Tyrel Platform, model TC-10S, Allweights Hamilton Scale Corp., Richmond, VA) and moved from pastures into stalls 15 to 18 h before the onset of the test. Mares from the same groups were housed so they could see each other in order to avoid social dislocative stress. Fasting has been shown to reduce tissue sensitivity to the glucoregulatory action of insulin in equids (Forhead and Dobson, 1997), and because the effect of fasting on insulin resistance may be exacerbated by pregnancy (Fowden et al., 1984), the mares were allowed ad libitum access to grass hay and water in order to mimic the nonfasted, grazing state on pasture.

Catheters were placed in jugular veins, and after an adjustment period of approximately 1 h, baseline blood samples were taken at 30 min and 1 min before the glucose challenge. Each mare was given a glucose dose of 0.2 g/kg of BW in a 50% solution via nasogastric tube. This dose is lower than that reported in other studies (Roberts and Hill, 1973; Jacobs and Bolton, 1982) but was chosen because it was calculated to provide a glucose load similar to that provided by the hydrolyzable carbohydrate in a 0.5-kg meal of heavy oat grain (IFN 4-18-520).

Venous samples were collected at 30, 60, 90, 120, 150, 180, 240, and 300 min after the glucose challenge. The blood samples were immediately placed in heparinized sample tubes (Vacutainer, Fisher Health Care, Chicago, IL), kept in ice water, centrifuged, and then plasma was removed within 10 to 20 min of collection. Plasma was frozen at -4°C pending analysis.

Analyses
Samples of supplements, pastures, and hay were submitted for proximate and mineral analysis (Dairy One DHIA Forage Testing Laboratory). Nonstructural carbohydrate of the supplements and forage samples was calculated by difference from the proximate analysis. Samples were also analyzed for hydrolyzable carbohydrate using direct methods (Smith, 1981; Hoffman et al., 2001).

Plasma glucose concentrations were determined by colorimetric assay (Glucose Procedure #16-UV, Sigma Diagnostics, St. Louis, MO), and insulin and cortisol were determined using radioimmunoassays (Coat-A-Count insulin, Coat-A-Count cortisol, Diagnostic Products, Los Angeles, CA) previously validated for equine insulin and cortisol (Freestone et al., 1991). Duplicate assays had an intraassay CV of <1% for glucose, 5% for insulin, and 4% for cortisol. The interassay CV was 2% for glucose, 5.5% for insulin, and 4% for cortisol.

Glucose, insulin, and cortisol data were summarized as least squares means and standard errors and plotted over time. The magnitude of each glucose or insulin response was calculated as the incremental area under the curve (AUC) by graphical approximation. The AUC was negligible for cortisol concentrations. Glucose clearance was calculated as dose (g/kg of BW) divided by AUC (g•min•L-1).

Statistical Analysis
The data were tested for normality by the Shapiro-Wilk statistic. Data did not fit a normal distribution for glucose clearance (P = 0.0001) or for insulin AUC (P = 0.0027), so log10 transformations were applied and used for statistical analysis. Log10 transformations were also used for statistical analysis of insulin AUC:glucose AUC ratios.

The pasture and hay samples were compared using an ANOVA mixed model with repeated measures, with individual pasture and collection months corresponding to reproductive stage as fixed effects (SAS Inst., Inc., Cary, NC). Plasma data were compared using an ANOVA mixed model with repeated measures, with diet and reproductive stage as fixed effects. Means from both forage and plasma mixed models were compared using the Tukey test. Results were considered statistically significant at P < 0.05 and as a trend toward statistical significance at 0.05 < P < 0.10 (Rosner, 1995).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Implications
 Literature Cited
 
Nutrient analysis of the supplements, pastures, and hay are shown in Tables 2Go and 3Go. There were no differences (P > 0.42) in DM, DE, CP, ADF, NDF, fat, nonstructural carbohydrate, hydrolyzable carbohydrate, ash, Ca, P, Mg, K, Na, and S between individual pastures, so data from all pastures were combined. Seasonal variation in pasture composition influenced some of the nutrients available for grazing, with higher DM (P = 0.009) in late gestation vs. early or late lactation, higher NDF (P = 0.005) in early vs. late lactation, higher nonstructural carbohydrate (P = 0.001) in late lactation vs. late gestation or early lactation, lower hydrolyzable carbohydrate (P = 0.001) in early lactation vs. late gestation or early lactation, lower ash (P = 0.017) in early lactation vs. late gestation or late lactation, and higher Ca (P = 0.012) in late lactation vs. late gestation or early lactation. Compared to pasture, the mixed grass hay supplemented during late gestation and during the oral glucose tolerance tests was higher in DM (P = 0.01), ADF (P = 0.001), and NDF (P = 0.014) and lower in DE (P = 0.001), CP (P = 0.001), fat (P = 0.001) and hydrolyzable carbohydrate (P = 0.004). The SS supplement had approximately 4.2 times as much hydrolyzable carbohydrate, and the FF supplement had 4.3 times as much fat, 3.1 times as much ADF, and 2.7 times as much NDF.


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  		Table 2. Nutrient profiles of the supplements fed to mares during gestation and lactation. Data are summarized on a dry matter basis as means ± standard errors for supplements rich in sugar and starch (SS) or fat and fiber (FF)a
 

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  		Table 3. Nutrient profiles of the pasture and hay fed to mares during gestation and lactation. Data are summarized on a dry matter basis as means ± standard errorsa
 
The mares weighed 634 ± 13, 599 ± 11, and 609 ± 13 kg and had BCS of 5.7 ± 0.1, 5.9 ± 0.2, and 6.1 ± 0.1, in late gestation, early lactation, and late lactation, respectively. There was no difference in mare BW (P = 0.30) or body condition (P = 0.26) attributable to diet. Gestation length was 344 ± 3 d, with no difference (P = 0.88) attributable to diet. The foals of SS and FF mares weighed 54.8 ± 2.4 and 50.4 ± 2.6 kg at birth, with no difference (P = 0.26) attributable to diet. There were no diet x reproductive stage interaction effects noted (P > 0.10) in any of the plasma variables measured.

Glucose
Basal glucose concentrations (Table 4Go) were higher in SS than FF mares during early lactation (P = 0.030), but no diet effects were noted (P > 0.18) in late gestation or late lactation. For both diets, basal glucose concentrations were lower in early lactation than in late gestation (P = 0.001) or late lactation (P = 0.030). In FF mares, basal glucose was lower in early lactation than late gestation (P = 0.001) and late lactation (P = 0.002). In SS mares, basal glucose was lower in early lactation than late gestation (P = 0.005) and tended to be lower in late lactation than late gestation (P = 0.095).


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  		Table 4. Basal concentrations (mean ± SE) of plasma glucose, insulin and cortisol in Thoroughbred mares, fed supplements rich in sugar and starch (SS) or fiber and fat (FF), during late gestation, early and late lactationa
 
After oral glucose administration, the highest plasma glucose concentrations were measured at 60 min in late gestation and late lactation, and at 30 min in early lactation (Figure 1Go). Plasma glucose concentrations were not different (P > 0.10) from baseline concentrations at 150 min in late gestation and late lactation, and 120 min in early lactation. For both SS and FF diets, the highest plasma glucose concentrations were measured at 60 min and were not different from baseline concentrations (P > 0.10) at 180 min (Figure 2Go) after oral administration of glucose.



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  		Figure 1. Plasma glucose (a) and insulin (b) concentrations in response to a nasogastric glucose dose (0.2 g/kg of BW) during late gestation (solid line), early lactation (heavy dashed line), and late lactation (light dashed line). Mares were adapted to pasture and a supplement rich in either starch and sugar or fat and fiber. Data represent main effects of reproductive stage across diet and are summarized as means ± SE.

 


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  		Figure 2. Plasma glucose (a) and insulin (b) concentrations in response to a nasogastric glucose dose (0.2 g/kg of BW) in mares adapted to pasture and a supplement rich in starch and sugar (SS, solid line) or fat and fiber (FF, dashed line). Data represent main effects of diet across late gestation, early lactation, and late lactation and are summarized as means ± SE.

 
The glucose AUC was influenced by reproductive stage and diet (Figures 1Go and 2Go; Table 5Go). Across reproductive stage, glucose AUC was higher in FF than SS mares (P = 0.022). The glucose AUC was higher in FF than SS mares in late gestation (P = 0.044) and tended to be higher in FF than SS in late lactation (P = 0.090), but not in early lactation (P = 0.28). For both diets, mares tended to have a higher glucose AUC in late gestation than early lactation (P = 0.057). There was no effect (P = 0.21) of reproductive stage on SS mares, but in FF mares, glucose AUC was lower in early lactation than late gestation (P = 0.036) and tended to be lower in early lactation than in late lactation (P = 0.076). These effects may be attributed to similar effects on glucose clearance.


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  		Table 5. Plasma glucose incremental area under the curve (AUC), glucose clearance, insulin incremental AUC and ratio of incremental insulin AUC:glucose AUC, in response to a nasogastric glucose dose (0.2 g/kg of BW) in Thoroughbred mares. Effects were physiological stage and diets rich in starch and sugar (SS) or fiber and fat (FF)a
 
Glucose clearance (Table 5Go) was more rapid across all reproductive stages in SS than FF mares (P = 0.007). Glucose clearance was more rapid in SS vs. FF mares in late gestation (P = 0.049) and early lactation (P = 0.043), and tended to be more rapid in SS than FF in late lactation (P = 0.098). There was no effect (P = 0.36) of reproductive stage in FF mares, but in SS mares, glucose clearance tended to be more rapid in early lactation than late gestation (P = 0.065) and late lactation (P = 0.096).

Insulin
Basal insulin concentrations (Table 4Go) were not different (P > 0.14) between SS and FF mares during any reproductive stage. For both diets, basal insulin was higher in late gestation than early (P = 0.001) or late lactation (P = 0.001), and was lower in early lactation than late lactation (P = 0.001). In FF mares, basal insulin was higher in late gestation than early (P = 0.019) or late (P = 0.027) lactation. In SS mares, basal insulin was higher in late gestation than in early (P = 0.001) or late lactation (P = 0.001) and was lower in early lactation than late lactation (P = 0.001).

After oral glucose administration, the highest plasma insulin concentrations were measured at 30 min in the SS mares and 60 min in the FF mares, regardless of reproductive stage (Figure 2Go). Plasma insulin concentrations were not different (P > 0.10) from baseline concentrations at 90 min in SS mares and 240 min in FF mares, and at 120 min in late gestation and early lactation, and 180 min in late lactation after oral administration of glucose.

The insulin AUC was influenced primarily by reproductive stage (Figure 1Go; Table 5Go). For both diets, insulin AUC was lower in early lactation than in late gestation (P = 0.002) or late lactation (P = 0.004). Insulin AUC was higher in FF vs. SS mares in late gestation (P = 0.045), but not in early or late lactation (P > 0.57). In SS mares, insulin AUC was lower in early lactation than in late gestation (P = 0.039) or late lactation (P = 0.010), and in FF mares, insulin AUC was lower in early lactation than in late gestation (P = 0.002) or late lactation (P = 0.013).

Insulin:Glucose Ratio
The log10 of the insulin AUC:glucose AUC ratio (Table 5Go) was not influenced by reproductive stage or diet (P > 0.12).

Cortisol
Basal cortisol concentrations (Table 4Go) were lower in SS vs. FF mares during early lactation (P = 0.013), but not (P = 0.81) during late gestation or late lactation. For both diets, basal cortisol was lower in late gestation than early lactation (P < 0.001) or late lactation (P = 0.016). In FF mares, basal cortisol was higher in early lactation than in late gestation (P = 0.001) or late lactation (P = 0.012), and higher in late lactation than in late gestation (P = 0.026). In SS mares, basal cortisol was lower in late gestation than in early (P = 0.005) or late lactation (P = 0.048). Plasma cortisol concentrations were not consistently influenced by oral administration of glucose.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Implications
 Literature Cited
 
The results show that plasma glucose clearance was more rapid in mares fed SS compared with FF, especially during late gestation and early lactation. The more rapid glucose clearance in SS vs. FF mares indicates an adaptation to meals rich in hydrolyzable carbohydrate, and an accommodation to feeding—fasting cycle changes, as indicated by their ability to manage glucose in doses. Conversely, the more sluggish glucose clearance of the FF mares more closely mimics the natural state of horses grazing pasture and not acclimated to meal feeding—fasting cycle changes.

Because of previous confounding of glucose tolerance and glycemic index in equine studies, it should be emphasized that the present investigation evaluated chronic adaptations of mares to feed energy sources by means of the glucose tolerance or clearance test (rather than the glycemic effect of a meal). Previous reports have used a grain meal to which the horse is adapted as the stimulus to elicit a glucose and insulin response, in effect a glycemic index. The glycemic response to a grain meal may be influenced by the amount of hydrolyzable carbohydrate in the meal, mastication, intake time, and digestibility of the feed. The oral glucose tolerance test used in this study removed the variability associated with a grain meal because the specific glucose dose was known, and variation associated with intake was not a factor. The oral glucose dose used provided glucose equivalent to that expected in a proportionately small grain meal, an amount to which the SS mares would be adapted but which would be unique to the FF mares. The higher glucose AUC in the FF mares indicated that the FF mares were unaccustomed to twice-daily glucose and insulin perturbations associated with meal feeding of SS.

Previous work in our laboratory (Williams et al., 2001) using SS and FF supplements similar to those used in this study indicated a higher feed glycemic index, assessed as higher glucose and insulin AUC, in the SS vs. FF supplement. A larger glycemic response to an oral glucose tolerance test would be expected in horses adapted to diets with a low glycemic index, such as FF or pasture, and a lower glycemic response to an oral glucose tolerance test would be expected in horses chronically perturbed twice daily by meals with a high glycemic index, such as SS or traditional sweet feeds. Similarly, horses adapted to pasture only, compared with a stable diet of hay and commercial feed, had a higher response to an oral glucose dose and 1.8 times as much glucose AUC (Jacobs and Bolton, 1982). Compared with pasture-fed horses, the horses fed the stable diet had approximately 3.5 times as much hydrolyzable carbohydrate, so their diet likely had a higher glycemic index, which influenced their response to oral glucose. Adaptation to meals with a high glycemic index may have enhanced the ability of these stabled horses, as well as the SS mares in this study, to clear glucose at a rate faster than horses accustomed to their inherent nature of grazing pasture.

The metabolic adaptation of the mares to pregnancy was reflected in larger glucose and insulin AUC and slower glucose clearance during late gestation compared with early lactation. This was more evident in the FF mares, whose adaptation to fatty acid utilization may have enhanced their ability to reserve glucose for fetal needs and to meet maternal energy requirements by peripheral metabolism of fatty acids. Adaptation to dietary fat in the horse has been found to enhance oxidation of long-chain fatty acids and reduce utilization of glucose and glycogen (Potter et al., 1992). In contrast to the FF mares, the adaptation to the high-glycemic index SS feed and a twice-daily need to accommodate hyperglycemia associated with meal feeding in the SS mares improved glucose clearance but could possibly affect glucose conservation for fetal needs, as alluded by the negligible effect of reproductive stage in SS mares. Further research would be required for verification.

Glucose metabolism adapts to pregnancy and lactation in many species, including humans (Boden, 1996), laboratory animals (Leturque et al., 1987), sheep (Petterson et al., 1993), cattle (Bell, 1995), pigs (Père et al., 2000), and horses (Evans, 1971; Fowden et al., 1984). The adaptation of glucose metabolism to pregnancy includes progressive development of insulin resistance, which allows for improved placental transfer of glucose in order to meet increasing demands of the fetus (Leturque et al., 1987; Petterson et al., 1993; Père et al., 2000). The insulin resistance facilitates the supply of glucose to the fetus at the expense of maternal tissues, through a shift in substrate utilization—from carbohydrates to fatty acids, and decreased glucose utilization in peripheral tissues (Bell, 1995; Boden, 1996). In horses, changes in pancreatic ß-cell function during pregnancy may contribute to insulin resistance (Fowden et al., 1984), which would elicit effects on glucose metabolism similar to those found in this study. In humans, progressive insulin resistance may trigger gestational diabetes, which increases risks of perinatal complications and subsequent development of maternal noninsulin-dependent diabetes mellitus.

One might consider that the effect of reproductive stage may have been confounded to some extent by adaptation to seasonal changes in pasture nutrients. Mares in late pregnancy consumed hay in combination with pasture, which may have increased their intake of ADF and NDF and lowered intake of hydrolyzable carbohydrate. The primary seasonal difference in pasture was higher NDF and lower nonstructural carbohydrate and hydrolyzable carbohydrate in early lactation compared with late gestation or late lactation. If this seasonal variation in carbohydrate composition were the primary influence on glucose metabolism, the expected outcome would have been a larger glucose and insulin AUC and slower glucose clearance during early lactation. Our results were not in agreement with this expected seasonal effect, so clearly the effect of reproductive stage dominated any effect of seasonal variation in forage nutrients.

The trend toward more rapid glucose clearance in SS mares during early lactation and the negligible effect of diet may reflect increased demands of the mammary gland for glucose as needed for milk synthesis compared with the demands of the gravid uterus. In dairy cattle, glucose uptake by the gravid uterus accounts for approximately one half of the maternal glucose supply, whereas the lactating mammary glucose requirement is estimated at three times that of the gravid uterus (Bell, 1995). Considering the substantial increase in glucose requirement for lactation, mostly for lactose synthesis, which is in greater concentration in mare milk than in cow milk (Ullrey et al., 1966; Naylor and Bell, 1985), it is likely that the increased utilization of glucose during early lactation was influenced by mammary demands.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
Literature Cited
 
The response to a standard oral glucose dose is influenced by adaptation to supplemental dietary energy source (fiber and fat or sugar and starch) and reproductive stage (pregnancy or lactation). The glucose response to oral glucose was lower and glucose clearance was more rapid in mares adapted to a diet rich in sugar and starch compared with those high in fiber and fat. The metabolic adaptation of the mare to pregnancy was reflected by reduced insulin sensitivity in late pregnancy, as evidenced by higher glucose and insulin response to oral glucose. Feeding grain meals rich in sugar and starch twice daily influenced glucose metabolism in horses to such an extent that the natural adaptation of glucose metabolism to pregnancy and lactation was moderated. Feeding a diet rich in fiber and fat more closely mimics the natural grazing state of pasture and allows for adaptation of glucose metabolism to pregnancy and lactation.


   Footnotes
 
1 We appreciate the support of the John Lee Pratt Graduate Fellowship Program in Animal Nutrition at Virginia Tech, the late Paul Mellon, Upperville, VA, and the WALTHAM Centre for Pet Nutrition, Melton-Mowbray, U.K. The technical assistance and encouragement of the staff at the Virginia Tech Middleburg Agricultural Research and Extension Center is gratefully acknowledged. Back

Received for publication August 8, 2002. Accepted for publication March 6, 2003.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 


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Evans, J. W. 1971. Effect of fasting, gestation, lactation and exercise on glucose turnover in horses. J. Anim. Sci. 33:1001–1004.

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Petterson, J. A., F. R. Dunshea, R. A. Ehrhardt, and A. W. Bell. 1993. Pregnancy and undernutrition alter glucose metabolic responses to insulin in sheep. J. Nutr. 124:1286–1295.

Potter, G. D., S. L. Hughes, T. R. Julen, and D. L. Swinney. 1992. A review of research on digestion and utilization of fat by the equine. Pferdeheilkunde 1:119–123.

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Rosner, B. 1995. Guidelines for judging the significance of a P-value. Page 200 in Fundamentals of Biostatistics. B. Rosner, ed. Wadsworth Publishing Co., Belmont, CA.

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