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Insulin and glucose regulation
Vet Clin Equine 18 (2002) 295–304
Insulin and glucose regulation Sarah L. Ralston, VMD, PhD* Department of Animal Science, Cook College, Rutgers—the State University of New Jersey, 84 Lipman Drive, New Brunswick, NJ 08901, USA
Blood concentrations of insulin and glucose can reflect endocrine abnormalities such as pituitary dysfunction and polysaccharide storage myopathies, especially if measured after a standardized challenge. Blood glucose and insulin concentrations in horses, however, are influenced by a myriad of factors. A single blood sample taken without consideration of these factors is virtually useless as a diagnostic tool. Length of fasts prior to glucose challenges tests, time since feeding, diurnal variations in cortisol, feed type, excitement or stress, reproductive status, illness, genetics, and obesity all affect blood glucose and insulin concentrations. This article discusses factors that influence glucose and insulin metabolism with respect to the impact they may have on the interpretation of standardized test results.
Normal glucose and insulin metabolism in horses Glucose Fasting blood glucose concentrations in horses are usually between 60 and 90 mg/dL. These concentrations are maintained by gluconeogenesis, primarily in the liver, and are regulated by glucagon, cortisol, and other counter-regulatory hormones [1]. Counter-regulatory responses of corticotropin-releasing hormone, vasopressin, and adrenocorticotropin to an insulin challenge are similar to those reported in humans and increase the rate of glucose synthesis if the glucose concentration starts to decrease [2,3]. Critical blood glucose concentrations for the activation of the counter-regulatory hormonal systems have not been established. Blood glucose concentrations after fasts of 12 hours or longer are remarkably constant in normal horses [4,5]. Fasting plasma concentrations of glucose and insulin did not differ if horses were adapted to only hay and barley or barley/beet pulp and * E-mail address: [email protected] (S.L. Ralston). 0749-0739/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 7 4 9 - 0 7 3 9 ( 0 2 ) 0 0 0 1 4 - 7
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molasses that provided either 2.8 or 3.1 kg neutral detergent fiber (NDF) per day [6] or one of three feeds varying significantly in fat/NDF/nonstructural carbohydrate (NSC) content [7]. Even after moderate, prolonged, or intense short-term exercise, horses fasted for 12 to 24 hours maintained relatively constant blood glucose concentrations [8,9]. Abnormally high or low blood glucose levels in horses fasted for more than 12 hours may be indicative of metabolic disease or pathology; however, glucose responses up to 5 to 6 hours after feeding vary significantly according to the type of feed given (glycemic index), feeds to which the horse was adapted, physiologic status, and fitness. These are be addressed individually below. Insulin The regulation of insulin secretion in horses adapted to traditional hay plus concentrate types of rations appears to be similar to that of humans [10,11]. Fasting insulin concentrations are usually between \5 and 20 lIU/ mL. Insulin is secreted when blood glucose concentrations increase and serves to enhance cellular uptake and lipogenesis. Horses also respond to insulin secretagogues such as arginine and lysine in a fashion similar to that reported in humans [10–12]. As with glucose, fasting concentrations of insulin are relatively constant, regardless of diet, physiologic status, or conditioning. Significant deviations from normal may be considered diagnostic of abnormalities in insulin metabolism; however, in ‘‘normal’’ horses, insulin secretion and clearance in response to a glucose challenge or feeding are dramatically influenced by the time since the horse was last fed, rations fed, body condition, physiologic status, and circulating cortisol concentrations. Modulation of glycemic responses Glycemic responses are the increases in blood glucose and insulin after ingestion of a meal or test nutrient. The glycemic response to a meal depends on the composition of the feed, rates of consumption, gastric emptying, digestion of NSC, glucose absorption, and utilization after absorption [7]. Lower insulin responses to a standardized carbohydrate challenge are reported in animals adapted to only forage versus those fed higher carbohydrate grains [4,10,11,13]. Blood glucose and insulin show minimal variations after a meal of hay [14], whereas after 1.5 to 2 kg of high starch concentrate, blood glucose peaks at 100 to 200 mg/dL within 60 to 90 minutes, with insulin peaks of up to 190 lIU/mL appearing at 90 to 120 minutes [7,14–18]. Concentrates containing high fat (6%–10%), fiber (>20% NDF), or both result in lower (P\0.05) glycemic responses [7]. Horses adapted to pelleted feeds that differed only in the SCHO content (30% versus 80%) did not differ significantly in their postprandial (after
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eating) glucose profiles, although insulin secretion was marginally higher (P\0.05 at 180 minutes after feeding) in the 30% soluble carbohydrate (SCHO) group [15]. Exercise Exercise conditioning improves glucose tolerance, resulting in lower glycemic responses after a standardized meal as the animals become more ‘‘fit’’ [19]. When evaluating the results of glucose tolerance tests, the relative fitness of the animal must be taken into consideration. Acute bouts of exercise increase the utilization of glucose and can result in decreases in blood concentrations, especially if blood glucose was elevated before the initiation of exercise [8,9,20]. If fasted for 12 hours or fed only hay before exercise, horses experienced minimal fluctuations in blood glucose, although there was a tendency for glucose to decline somewhat after prolonged exercise [9]. There were precipitous drops in glucose during exercise, however, in horses that had relatively high glucose concentrations at the start of the exercise due to ingestion of high carbohydrate feeds [8]. Within 15 to 30 minutes of the initiation of exercise, blood glucose was back to fasting concentrations. If the exercise continues, it may drop below baseline [8,20]. Feeding a high fat diet (7.5%–10% in the total ration) appears to have a glucose sparing effect in horses subjected to prolonged exercise, resulting in higher concentrations of glucose during and after exercise than in horses fed lower fat (2.5%–3%) rations [20]. Exercise conditioning also affects insulin responses. Yearling horses conditioned with 20 minutes of trotting on a treadmill three times a week had lower insulin responses to feeding than did nonconditioned control horses, suggesting improved insulin sensitivity with conditioning [19]. Hyperinsulinemic obese ponies subjected to exercise training had remarkably improved insulin sensitivity after only 2 weeks of exercise conditioning [21], although the alterations observed may have resulted from the concomitant weight loss rather than physical fitness per se. Physiologic status Pregnant mares apparently experience a period of insulin resistance up to 270 days of gestation similar to gestational diabetes in humans [22,23]. Fasting blood glucose and insulin concentrations are not affected by the insulin resistance, but they exhibit hyperinsulinemic responses to dextrose challenges and feeding during the first 270 days of gestation. Glucose and insulin responses return to normal during the final trimester and apparently are not altered by lactation. Diurnal variation in glucose tolerance and the effects of stress and fasting In most studies of glycemic responses in horses, the animals were usually fed their test meals in the morning [10,11,14–18,24,25] after a 12-to 24-hour
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fast. In an early investigation of the 24-hour variation in blood glucose and insulin in which no significant variations were found [26], however, the feeding activity of the horses being investigated was not recorded and they had free access to a concentrate feed [26]. Fasting horses showed no diurnal changes in glucose or insulin, although the normal cycle of cortisol (high in the morning, significantly lower in the late afternoon and evening) was observed [14]. In a 24-hour study of glucose and insulin concentrations, horses were fed 4 to 5 kg hay and 1 kg of grain concentrate at 8:00 AM and 4:00 PM daily and their feeding activities were recorded. Blood samples were taken hourly. The horses had lower (P\0.005) insulin responses in the afternoon than in the morning (Ralston, Hintz and Divers, 1998, unpublished data), although glucose concentrations did not differ, suggesting a diurnal variation in insulin sensitivity. It is important to note, however, that although the horses ate the concentrate meals within 30 minutes of feeding, it took them 5 to 6 hours to consume the hay completely. This resulted in them having no feed available for more than 8 hours overnight but fasting only 1 to 2 hours before the afternoon feeding. The investigators hypothesized at the time that the length of fast was more of a factor in the variation in glucose and insulin responses than was the actual time of day. In follow-up studies in which horses were fasted for 0 (continuous access), 8, or 10 hours between feedings, it was found that the length of fast and plasma cortisol were more highly correlated with the concomitant glucose and insulin responses to a meal of grain than the actual time of day (Ralston, 1999, unpublished data). The group of horses used in this study was not adapted to being confined to stalls during the day; however, for the study, they were fasted in stalls for the allotted amount of time during the day before the initiation of sampling, starting at either 8:00 AM (10-hour fast) or 10:00 AM (8-hour fast) before the first feeding at 6:00 PM. When on continuous access, the horses were presented with fresh feed at 4-hour intervals and not fasted prior to sampling. The continuously fed horses exhibited the same diurnal variation in insulin response pattern observed previously, with higher insulin and cortisol concentrations after feeding in the morning than in the late afternoon. When fasted for 8 or 10 hours in stalls before the afternoon feeding, however, the horses did not exhibit diurnal variations in insulin release and cortisol was higher (P\0.05) in the evening than in the continuously fed horses. It is hypothesized that the animals were stressed by the daytime fast. When the horses were kept in for a second afternoon feeding, the cortisol concentrations were lower, as were insulin responses to the concentrate feed. These results are comparable to the effects of stress and time of day on insulin sensitivity in humans [27–30]. Investigators need to adapt subjects to the experimental conditions before collecting data and clinicians need to consider possible effects of diurnal or stress induced variations in cortisol, length of fast, or both when interpreting glucose and insulin data.
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Growth hormone Daily injections of recombinant growth hormone result in an apparent insulin resistance that is manifested by hyperinsulinemic responses to carbohydrate challenges. Daily growth hormone injections in aged mares were investigated as part of a large collaborative study [31,32]. Some of the mares in the study were already hyperinsulinemic secondary to pituitary dysfunction or obesity (Ralston, 1996, unpublished data); however, the growth hormone treatments resulted in significant hyperinsulinemia that persisted for at least 4 weeks after cessation of the treatment in all of the treated animals [32]. Similarly, horses 4 to 12 months of age, which have high circulating concentrations of endogenous growth hormone, also tend to be hyperinsulinemic relative to their responses to similar challenges later in life [8,33]. Growth hormone status of a horse should be considered when evaluating the results of a glucose tolerance test.
Mineral modulation of insulin sensitivity Chromium Chromium picolinate supplementation reportedly enhances insulin action [1], especially in aged animals [34]. Chromium L-methionine (0.02 mg/kg) decreased (P\0.05) insulin secretion by 30% to 57% after a standardized meal of grain in normal aged mares [25] without influencing the changes in plasma glucose. Similar changes, however, were not observed after an intravenous glucose tolerance test nor were there statistically different changes in hyperinsulinemic aged mares (Ralston, unpublished data). Yearlings supplemented with 210 or 420 lg chromium tripicolinate/kg feed had lower glucose responses and faster clearance after an intravenous glucose tolerance test [35] than did unsupplemented controls. Fasting glucose and insulin concentrations were numerically lower in the supplemented horses but the differences were not statistically significant [35].
Tests of glucose and insulin metabolism Intravenous dextrose challenge Intravenous injection of a concentrated high dose of glucose (dextrose) tests the animal’s secretion of insulin and the sensitivity of its tissues to insulin action [13]. It is considered to be the ‘‘gold standard’’ of glucose tolerance tests [13,36]. The standard intravenous glucose tolerance test is the administration of 0.5 g dextrose/kg body weight as a 50% solution by rapid injection via an intravenous catheter, traditionally after a 10- to 12-hour fast. Blood samples are drawn from the contralateral jugular vein before injection and
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at 15 minutes and 30 minutes and then hourly for 5 to 6 hours after injection. Tubes treated with fluoride citrate should be used for glucose samples and heparinized tubes should be used for the insulin samples. Samples should be chilled immediately and the plasma drawn off as soon as possible and frozen pending glucose/ insulin analysis. Glucose concentrations should peak in \15 minutes (242^17 mg/dL), whereas insulin peaks are usually seen at 30 minutes (157^11 lU/mL). Concentrations are back to baseline within 1 hour in normal horses. For practical purposes it is probably not necessary to continue sampling past 2 hours for diagnostic purposes. Excessively high or prolonged responses may be due to obesity [37], pituitary dysfunction [13,36,38], horses adapted to only dry hay rations [4,11], pregnancy [22,23], stress or genetic or age related insulin resistance [16,17,24,39]. Oral glucose tolerance Oral glucose tolerance tests reveal not only the ability of the animal to secrete insulin and the sensitivity of its tissues to the action of insulin but also its intestinal ability to absorb the glucose challenge [40,41]. Traditionally a dose of 1.0 g dextrose/kg body weight is administered by nasogastric tube as a 20% solution after a 12- to16-hour fast. Blood samples are drawn before administration and then either hourly or at 0.5-hour intervals for 6 hours. Blood samples are handled and analyzed as with the intravenous test. The glucose response curve tends to be lower and more prolonged than in the intravenous test, with peak glucose observed between 1 and 2 hours (120–180 mg/ dL) with higher peak insulin at 1.5 to 2.5 hours (60–150 lIU/mL). Concentrations return to baseline within 4 to 5 hours. A lower dose of 0.25 g/kg body weight administered by oral dose syringe as a paste or in applesauce gives a glucose/insulin response that is virtually identical to the responses seen after a meal of 1- to 2-kg of concentrate feed (Ralston, 1999, unpublished data). Glucose normally peaks at 90 to 120 mg/dL in 60 to 90 minutes; after 60 and 90 minutes, insulin should be \20 lIU/mL in foals younger than 6 months old and \100 lIU/mL in older horses (Ralston, 1999–2000, unpublished data). The lower dose test can serve as a more physiologic test in horses in which nasogastric intubation is a problem or when there is concern about the larger dose of dextrose. Abnormally low glucose/insulin responses suggest delayed gastric emptying, reduced absorption [41], or increased insulin sensitivity secondary to polysaccharide storage myopathy [42–45]. Abnormally high or prolonged responses are seen in obese horses and horses with genetic- or age-induced insulin resistance [16,17,33], pituitary dysfunction [36,38] or high levels of stress (Ralston, 1998, unpublished data). Grain challenges In a study of the effects of chromium supplementation in aged mares, it was found that a test meal of grain was a more sensitive test for abnormal
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glucose or insulin responses in aged horses than the traditional intravenous dextrose test [25]. It is hypothesized that the standard challenge of 0.15 g dextrose per kg body weight, which drives blood glucose to greater than 250 g/dL and stimulates insulin concentrations as high as 200 lIU/mL within 5 to 10 minutes, may obscure more subtle differences in insulin sensitivity. After a horse consumes 1.5 to 2 kg of texturized sweet feed or high carbohydrate pellets, its blood glucose normally peaks at 90 to 120 mg/dL within 90 minutes and its blood insulin normally peaks at 60 to 120 lIU/mL within 90 to 120 minutes [15,25,14,46]. Blood samples should be handled in the same fashion as in the previous tests. Glucose readings greater than 250 mg/dL or insulin levels higher than 200 lIU/mL should be considered abnormal responses and warrant further investigation.
Abnormal glucose and insulin metabolism in horses Hyperinsulinemia Hyperinsulinemia, defined as plasma insulin concentrations greater than 200 lIU/mL in adult animals [16], is common in horses. Most studies of glucose and insulin metabolism in horses have reported at least one hyperinsulinemic individual [13,15–18,21,25,26,37–39]. In studies in which adult horses or ponies were used, pituitary adenomas or obesity were most commonly associated with the reported hyperinsulinemia [15,21,25,26,38]. In other studies the hyperinsulinemia was frequently a serendipitous finding and appeared to be genetic or breed related [16–18,37,39] or was unexplained by the authors [11,26]. It is important to note in that all of the studies in which hyperinsulinemia was a serendipitous finding, less than 20 horses had been used, an indication of the high prevalence of hyperinsulinemia or insulin resistance in the general equine population. Hyperinsulinemia associated with pituitary dysfunction is also common, especially in aged mares [36,38]. Abnormally high insulin responses to either a standardized glucose challenge or a meal of grain often precede the gross clinical signs of pituitary dysfunction, such as hirsutism, polyuria/polydipsia, and abnormal cortisol secretion [36,38]. Hypoglycemia Naturally occurring postprandial hypoglycemia has recently been reported in horses with polysaccharide storage disease [42–45,47,48]. In these animals insulin secretion is within normal limits but they suffer profound hypoglycemia after a meal of starch, supposedly due to an enhanced sensitivity to the action of insulin [47]. As in humans, dietary control of the disease consists of a high protein and fat diet with limited starch intake [44,45,48]. The counter-regulatory hormonal responses to hypoglycemia have not been described in these animals, nor is it known if bone metabolism is affected.
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Hyperinsulinemia and abnormal bone growth in young horses In two separate studies the author has found a strong correlation between postprandial hyperinsulinemia and the radiographic presence of osteochondrosis lesions in Standardbred horses younger than 12 months of age [16–18]. Horses between 3 and 12 months of age were also documented to have higher insulin responses to meals than those older than18 months of age [17]. Krusic and colleagues [33] reported similar apparent insulin resistance in Lipizzaner horses between 3 and 12 months of age. This led to the hypothesis that abnormally high insulin, whether due to an inherited insensitivity, high levels of growth hormone, or in response to large amounts of carbohydrate being fed, would alter bone metabolism, which, under conditions that have yet to be elucidated, result in osteochondrosis or other growth abnormalities. References [1] Anderson RA. Nutritional factors influencing the glucose/insulin system. Chromium. J Am Coll Nutr 1997;16:404–10. [2] Alexander I, Roud HK, Irvine CHG. Effect of insulin-induced hypoglycaemia on secretion patterns and rates of corticotrophin-releasing hormone, arginine vasopressin and adrenocorticotrophin in horses. J Endocrinol 1997;154:401–9. [3] Mengozzi G, Giovanelli L, Rastelli S, Demi S, Intorre L, Soldani G. Adrenergic regulation of blood glucose levels and insulin release in horses: role of alpha 2 agonists and antagonists. Acta Vet Scand 1991;(Suppl. 87):336–7. [4] Jacobs KA, Bolton JR. The effect of diet on the oral glucose tolerance test in horses. J Am Vet Assoc 1982;180:884–6. [5] Nadal MR, Thompson DL, Kincaid LA. Effect of feeding and feed deprivation on plasma concentrations of prolactin, insulin, growth hormone and metabolites in horses. J Anim Sci 1997;75:736–44. [6] Lindberg JE, Jacobsson K-G. Effects of barley and sugar beet pulp on digestibility, purine excretion and blood parameters in horses. Pferdeheilkunde 1992;September:116–18. [7] Williams CA, Kronfeld DS, Staniar WB, Harris PA. Plasma glucose and insulin responses of Thoroughbred mares fed a meal high in starch and sugar or fat and fiber. J Anim Sci 2001;79:2196–201. [8] Lawrence LM, Williams J, Soderholm LV, Roberts AM, Hintz HF. Effect of feeding state on the response of horses to repeated bouts of intense exercise. Equine Vet J 1995; 27:27–30. [9] Pagan JD, Harris PA, Kennedy MAP, Davidson N, Hoekstra KE. Feed type and intake affects on glycemic responses in Thoroughbred horses. In: Proceedings of the 16th Equine Nutrition and Physiology Symposium, Raleigh, NC. Lexington (KY): ENPS. 1999. p. 149–50. [10] Argenzio RA, Hintz HF. Volatile fatty acid tolerance and the effect of glucose and VFA on plasma insulin levels in ponies. J Nutr 1971;101:723–30. [11] Argenzio RA, Hintz HF. Effect of diet on glucose entry and oxidation rates in ponies. J Nutr 1972;102:879–903. [12] Sticker LS, Thompson DL, Smith LA, Leise BS, Gentry LR. Pituitary hormone and insulin responses to infusion of amino acids and N-methyl-D, L-aspartate (NMA) in horses. Proceeding of the 16th Equine Nutrition and Physiology Symposium. Raleigh, NC. Lexington (KY): ENPS. 1999. p. 90–1.
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[13] Garcia M, Beech J. Equine intravenous glucose tolerance test: glucose and insulin responses of healthy horses fed grain and hay and of horses with pituitary adenoma. Am J Vet Res 1986;47:570–2. [14] Stull CL, Rodiek AV. Responses of blood glucose, insulin and cortisol concentrations to common equine diets. J Nutr 1988;118:206–13. [15] Ralston SL. Effect of soluble carbohydrate content of pelleted diets on postprandial glucose and insulin profiles in horses. Pferdeheilkunde 1992;September:112–15. [16] Ralston SL. Postprandial hyperglycemia/insulinemia in young horses with osteochondritis dissecans lesions. J Anim Sci 1995;73:184. [17] Ralston SL. Hyperglycemia/hyperinsulinemia after feeding a meal of grain to young horses with osteochondritis dessicans (OCD) lesions. Pferdeheilkunde 1996;May:320–2. [18] Ralston SL, Black A, Suslak-Brown L, Schoknecht PA. Postprandial insulin resistance associated with osteochondrosis in weanling fillies. J Anim Sci 1998;76(Suppl. 1):176. [19] Black A, Ralston SL, Shapses SA, Schoknecht PA. Skeletal development in weanling horses in response to high dietary energy and exercise. J Anim Sci 1997;75(Suppl. 1):170. [20] Potter GD, Hughes SL, Julen TR, Swinney DL. A review of research on digestion and utilization of fat by the equine. Pferdeheilkunde 1992;September:119–23. [21] Freestone JF, Beadle R, Shoemaker K, Bessin RT, Wolfsheimer KJ, Church C. Improved insulin sensitivity in hyperinsulinaemic ponies through physical conditioning and controlled feed intake. Equine Vet 1992;24:184–6. [22] Evans JW. Effect of fasting, gestation, lactation and exercise on glucose turnover in horses. J Anim Sci 1971;33:1001–4. [23] Fowden AL, Comline RS, Silver R. Insulin secretion and carbohydrate metabolism during pregnancy in the mare. Equine Vet J 1984;16:239–46. [24] Ralston SL, Baile CA. Plasma glucose and insulin concentrations and feeding behavior in ponies. J Anim Sci 1982;54:1132–7. [25] Ralston SL, Dimock AN, Socha M. Glucose/insulin responses to IV dextrose versus oral concentrate challenges following chromium supplementation in geriatric mares. In: Proceedings of the 16th Equine Nutrition and Physiology Symposium. Raleigh, NC. Lexington (KY): ENPS. 1999. p. 90–1. [26] Evans JW, Thompson PG, Winget CM. Glucose and insulin biorhythms in the horse. J S Afr Vet Assoc 1974;45:317–29. [27] Frape DL, Williams NR, Scriven AJ, Palmer CR, O’Sullivan K, Fletcher RJ. Diurnal trends in responses of blood plasma concentrations of glucose, insulin, and C-peptide following high- and low-fat meals and their relation to fat metabolism in healthy middleaged volunteers. Br J Nutr 1997;77:523–35. [28] Moan A, Hoieggen A, Nordby G, Os I, Eide I, Kjeldsen SE. Mental stress increases glucose uptake during hyperinsulinemia: associations with sympathetic and cardiovascular responsiveness. Metabolism 1995;44:1303–7. [29] Van Cauter E, Desir D, Decoster C, Frey F, Balasse O. Nocturnal decrease in glucose tolerance during constant glucose infusion. J Clin Endocrinol Metab 1989;69:604–11. [30] Van Cauter E, Shapiro ET, Tillel H, Polonsky KS. Circadian modulation of insulin responses to meals: relationship to cortisol rhythm. Am J Physiol 1992;262: E467–75. [31] Christensen RA, Malinowski K, Ralston SL, Scanes CG, Hafs HD. Chronic effects of equine growth hormone (eGH) on postprandial changes in plasma glucose, nonesterified fatty acids and urea nitrogen in aged mares. J Anim Sci 1996;74(Suppl 1):226. [32] Christensen RA, Malinowski K, Ralston SL, Scanes CG, Hafs HD. Chronic effects of equine growth hormone (eGH) on plasma insulin, insulin-like growth factor-I and thyroid hormones in aged mares. J Anim Sci 1996;74(Suppl 1):226. [33] Krusic L, Krusic-Kaplja A, Cestnik V. Insulin response after oral glucose application in growing Lipizzaner foals. In: Proceedings of the 15th Equine Nutrition and Physiology Symposium. Fort Worth, TX. Lexington (KY): ENPS 1997. p. 397–403.
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[34] Preuss HG. Effects of glucose/insulin perturbations on aging and chronic disorders of aging: the evidence. J Am Coll Nutr 1997;16:395–403. [35] Ott EA, Kivipelto J. Influence of chromium tripicolinate on growth and glucose metabolism in growing horses. J Anim Sci 1999;77:3022–30. [36] Beech J. Tumors of the pituitary gland (pars intermedia). In: Robinson NE, editor. Current therapy in equine medicine. 2nd edition. Philadelphia: W.B. Saunders Co.; 1987. p. 182–5. [37] Jeffcott LB, Field JR, McClean JG, O’dea K. Glucose tolerance and insulin sensitivity in ponies and Standardbred horses. Equine Vet J 1986;18:97–101. [38] Ralston SL, Nockels CF, Squires EL. Differences in diagnostic test results and hematologic data between aged and young horses. Am J Vet Res 1988;49:1387–92. [39] June V, Soderholm V, Hintz HF, Butler WR. Glucose tolerance in the horse, pony and donkey. J Equine Vet Sci 1992;12:103–5. [40] Klein J, Schlze E, Deegen E, Giese W. Metabolism of naturally occurring [13C] glucose given orally to horses. Am J Vet Res 1988;49:1259–62. [41] Roberts MC, Hill FWG. The oral glucose tolerance test in the horse. Equine Vet J 1973; 5:171–3. [42] Valberg SJ, MacLeay JM, Mickelson JR. Exertional rhabdomyolysis associated with polysaccharide storage myopathy in the horse. Comp Contin Ed Prac Vet 1998;19:1077–86. [43] Valberg SJ, Townsend D, Mickelson JR. Skeletal muscle glycolytic capacity and phosphofructokinase regulation in horses with polysaccharide storage myopathy. Am J Vet Res 1998;59:782–5. [44] Valentine BA, Divers TJ, Lavoie JP. Severe equine polysaccharide storage myopathy in draft horses: clinical signs and response to dietary therapy. In: Proceedings of the 42nd Annual Meeting of the American Association of Equine Practice, Denver, CO. 1996. p. 294–6. [45] Valentine BA, Hintz HF, Freels KM, Reynolds AJ, Thompson KN. Dietary control of exert ional rhabdomyolysis in horses. J Am Vet Med Assoc 1998;212:1588–93. [46] Ralston SL, Van den Broek G, Baile CA. Feed intake patterns and associated blood glucose, free fatty acid and insulin changes in ponies. J Anim Sci 1979;49:838–45. [47] De La Corte FD, Valberg SJ, Williamson SE, MacLeay JM, Mickelson JR. Glucose uptake in horses with polysaccharide storage myopathy. Am J Vet Res 1999;60:458–61. [48] De La Corte FD, Valberg SJ, MacLeay JM, Billstrom J. The effect of feeding a fat supplement to horses with polysaccharide storage myopathy. World Eq Vet Rev 1999; 4:12–9.
Insulin and glucose regulation Sarah L. Ralston, VMD, PhD* Department of Animal Science, Cook College, Rutgers—the State University of New Jersey, 84 Lipman Drive, New Brunswick, NJ 08901, USA
Blood concentrations of insulin and glucose can reflect endocrine abnormalities such as pituitary dysfunction and polysaccharide storage myopathies, especially if measured after a standardized challenge. Blood glucose and insulin concentrations in horses, however, are influenced by a myriad of factors. A single blood sample taken without consideration of these factors is virtually useless as a diagnostic tool. Length of fasts prior to glucose challenges tests, time since feeding, diurnal variations in cortisol, feed type, excitement or stress, reproductive status, illness, genetics, and obesity all affect blood glucose and insulin concentrations. This article discusses factors that influence glucose and insulin metabolism with respect to the impact they may have on the interpretation of standardized test results.
Normal glucose and insulin metabolism in horses Glucose Fasting blood glucose concentrations in horses are usually between 60 and 90 mg/dL. These concentrations are maintained by gluconeogenesis, primarily in the liver, and are regulated by glucagon, cortisol, and other counter-regulatory hormones [1]. Counter-regulatory responses of corticotropin-releasing hormone, vasopressin, and adrenocorticotropin to an insulin challenge are similar to those reported in humans and increase the rate of glucose synthesis if the glucose concentration starts to decrease [2,3]. Critical blood glucose concentrations for the activation of the counter-regulatory hormonal systems have not been established. Blood glucose concentrations after fasts of 12 hours or longer are remarkably constant in normal horses [4,5]. Fasting plasma concentrations of glucose and insulin did not differ if horses were adapted to only hay and barley or barley/beet pulp and * E-mail address: [email protected] (S.L. Ralston). 0749-0739/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 7 4 9 - 0 7 3 9 ( 0 2 ) 0 0 0 1 4 - 7
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molasses that provided either 2.8 or 3.1 kg neutral detergent fiber (NDF) per day [6] or one of three feeds varying significantly in fat/NDF/nonstructural carbohydrate (NSC) content [7]. Even after moderate, prolonged, or intense short-term exercise, horses fasted for 12 to 24 hours maintained relatively constant blood glucose concentrations [8,9]. Abnormally high or low blood glucose levels in horses fasted for more than 12 hours may be indicative of metabolic disease or pathology; however, glucose responses up to 5 to 6 hours after feeding vary significantly according to the type of feed given (glycemic index), feeds to which the horse was adapted, physiologic status, and fitness. These are be addressed individually below. Insulin The regulation of insulin secretion in horses adapted to traditional hay plus concentrate types of rations appears to be similar to that of humans [10,11]. Fasting insulin concentrations are usually between \5 and 20 lIU/ mL. Insulin is secreted when blood glucose concentrations increase and serves to enhance cellular uptake and lipogenesis. Horses also respond to insulin secretagogues such as arginine and lysine in a fashion similar to that reported in humans [10–12]. As with glucose, fasting concentrations of insulin are relatively constant, regardless of diet, physiologic status, or conditioning. Significant deviations from normal may be considered diagnostic of abnormalities in insulin metabolism; however, in ‘‘normal’’ horses, insulin secretion and clearance in response to a glucose challenge or feeding are dramatically influenced by the time since the horse was last fed, rations fed, body condition, physiologic status, and circulating cortisol concentrations. Modulation of glycemic responses Glycemic responses are the increases in blood glucose and insulin after ingestion of a meal or test nutrient. The glycemic response to a meal depends on the composition of the feed, rates of consumption, gastric emptying, digestion of NSC, glucose absorption, and utilization after absorption [7]. Lower insulin responses to a standardized carbohydrate challenge are reported in animals adapted to only forage versus those fed higher carbohydrate grains [4,10,11,13]. Blood glucose and insulin show minimal variations after a meal of hay [14], whereas after 1.5 to 2 kg of high starch concentrate, blood glucose peaks at 100 to 200 mg/dL within 60 to 90 minutes, with insulin peaks of up to 190 lIU/mL appearing at 90 to 120 minutes [7,14–18]. Concentrates containing high fat (6%–10%), fiber (>20% NDF), or both result in lower (P\0.05) glycemic responses [7]. Horses adapted to pelleted feeds that differed only in the SCHO content (30% versus 80%) did not differ significantly in their postprandial (after
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eating) glucose profiles, although insulin secretion was marginally higher (P\0.05 at 180 minutes after feeding) in the 30% soluble carbohydrate (SCHO) group [15]. Exercise Exercise conditioning improves glucose tolerance, resulting in lower glycemic responses after a standardized meal as the animals become more ‘‘fit’’ [19]. When evaluating the results of glucose tolerance tests, the relative fitness of the animal must be taken into consideration. Acute bouts of exercise increase the utilization of glucose and can result in decreases in blood concentrations, especially if blood glucose was elevated before the initiation of exercise [8,9,20]. If fasted for 12 hours or fed only hay before exercise, horses experienced minimal fluctuations in blood glucose, although there was a tendency for glucose to decline somewhat after prolonged exercise [9]. There were precipitous drops in glucose during exercise, however, in horses that had relatively high glucose concentrations at the start of the exercise due to ingestion of high carbohydrate feeds [8]. Within 15 to 30 minutes of the initiation of exercise, blood glucose was back to fasting concentrations. If the exercise continues, it may drop below baseline [8,20]. Feeding a high fat diet (7.5%–10% in the total ration) appears to have a glucose sparing effect in horses subjected to prolonged exercise, resulting in higher concentrations of glucose during and after exercise than in horses fed lower fat (2.5%–3%) rations [20]. Exercise conditioning also affects insulin responses. Yearling horses conditioned with 20 minutes of trotting on a treadmill three times a week had lower insulin responses to feeding than did nonconditioned control horses, suggesting improved insulin sensitivity with conditioning [19]. Hyperinsulinemic obese ponies subjected to exercise training had remarkably improved insulin sensitivity after only 2 weeks of exercise conditioning [21], although the alterations observed may have resulted from the concomitant weight loss rather than physical fitness per se. Physiologic status Pregnant mares apparently experience a period of insulin resistance up to 270 days of gestation similar to gestational diabetes in humans [22,23]. Fasting blood glucose and insulin concentrations are not affected by the insulin resistance, but they exhibit hyperinsulinemic responses to dextrose challenges and feeding during the first 270 days of gestation. Glucose and insulin responses return to normal during the final trimester and apparently are not altered by lactation. Diurnal variation in glucose tolerance and the effects of stress and fasting In most studies of glycemic responses in horses, the animals were usually fed their test meals in the morning [10,11,14–18,24,25] after a 12-to 24-hour
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fast. In an early investigation of the 24-hour variation in blood glucose and insulin in which no significant variations were found [26], however, the feeding activity of the horses being investigated was not recorded and they had free access to a concentrate feed [26]. Fasting horses showed no diurnal changes in glucose or insulin, although the normal cycle of cortisol (high in the morning, significantly lower in the late afternoon and evening) was observed [14]. In a 24-hour study of glucose and insulin concentrations, horses were fed 4 to 5 kg hay and 1 kg of grain concentrate at 8:00 AM and 4:00 PM daily and their feeding activities were recorded. Blood samples were taken hourly. The horses had lower (P\0.005) insulin responses in the afternoon than in the morning (Ralston, Hintz and Divers, 1998, unpublished data), although glucose concentrations did not differ, suggesting a diurnal variation in insulin sensitivity. It is important to note, however, that although the horses ate the concentrate meals within 30 minutes of feeding, it took them 5 to 6 hours to consume the hay completely. This resulted in them having no feed available for more than 8 hours overnight but fasting only 1 to 2 hours before the afternoon feeding. The investigators hypothesized at the time that the length of fast was more of a factor in the variation in glucose and insulin responses than was the actual time of day. In follow-up studies in which horses were fasted for 0 (continuous access), 8, or 10 hours between feedings, it was found that the length of fast and plasma cortisol were more highly correlated with the concomitant glucose and insulin responses to a meal of grain than the actual time of day (Ralston, 1999, unpublished data). The group of horses used in this study was not adapted to being confined to stalls during the day; however, for the study, they were fasted in stalls for the allotted amount of time during the day before the initiation of sampling, starting at either 8:00 AM (10-hour fast) or 10:00 AM (8-hour fast) before the first feeding at 6:00 PM. When on continuous access, the horses were presented with fresh feed at 4-hour intervals and not fasted prior to sampling. The continuously fed horses exhibited the same diurnal variation in insulin response pattern observed previously, with higher insulin and cortisol concentrations after feeding in the morning than in the late afternoon. When fasted for 8 or 10 hours in stalls before the afternoon feeding, however, the horses did not exhibit diurnal variations in insulin release and cortisol was higher (P\0.05) in the evening than in the continuously fed horses. It is hypothesized that the animals were stressed by the daytime fast. When the horses were kept in for a second afternoon feeding, the cortisol concentrations were lower, as were insulin responses to the concentrate feed. These results are comparable to the effects of stress and time of day on insulin sensitivity in humans [27–30]. Investigators need to adapt subjects to the experimental conditions before collecting data and clinicians need to consider possible effects of diurnal or stress induced variations in cortisol, length of fast, or both when interpreting glucose and insulin data.
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Growth hormone Daily injections of recombinant growth hormone result in an apparent insulin resistance that is manifested by hyperinsulinemic responses to carbohydrate challenges. Daily growth hormone injections in aged mares were investigated as part of a large collaborative study [31,32]. Some of the mares in the study were already hyperinsulinemic secondary to pituitary dysfunction or obesity (Ralston, 1996, unpublished data); however, the growth hormone treatments resulted in significant hyperinsulinemia that persisted for at least 4 weeks after cessation of the treatment in all of the treated animals [32]. Similarly, horses 4 to 12 months of age, which have high circulating concentrations of endogenous growth hormone, also tend to be hyperinsulinemic relative to their responses to similar challenges later in life [8,33]. Growth hormone status of a horse should be considered when evaluating the results of a glucose tolerance test.
Mineral modulation of insulin sensitivity Chromium Chromium picolinate supplementation reportedly enhances insulin action [1], especially in aged animals [34]. Chromium L-methionine (0.02 mg/kg) decreased (P\0.05) insulin secretion by 30% to 57% after a standardized meal of grain in normal aged mares [25] without influencing the changes in plasma glucose. Similar changes, however, were not observed after an intravenous glucose tolerance test nor were there statistically different changes in hyperinsulinemic aged mares (Ralston, unpublished data). Yearlings supplemented with 210 or 420 lg chromium tripicolinate/kg feed had lower glucose responses and faster clearance after an intravenous glucose tolerance test [35] than did unsupplemented controls. Fasting glucose and insulin concentrations were numerically lower in the supplemented horses but the differences were not statistically significant [35].
Tests of glucose and insulin metabolism Intravenous dextrose challenge Intravenous injection of a concentrated high dose of glucose (dextrose) tests the animal’s secretion of insulin and the sensitivity of its tissues to insulin action [13]. It is considered to be the ‘‘gold standard’’ of glucose tolerance tests [13,36]. The standard intravenous glucose tolerance test is the administration of 0.5 g dextrose/kg body weight as a 50% solution by rapid injection via an intravenous catheter, traditionally after a 10- to 12-hour fast. Blood samples are drawn from the contralateral jugular vein before injection and
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at 15 minutes and 30 minutes and then hourly for 5 to 6 hours after injection. Tubes treated with fluoride citrate should be used for glucose samples and heparinized tubes should be used for the insulin samples. Samples should be chilled immediately and the plasma drawn off as soon as possible and frozen pending glucose/ insulin analysis. Glucose concentrations should peak in \15 minutes (242^17 mg/dL), whereas insulin peaks are usually seen at 30 minutes (157^11 lU/mL). Concentrations are back to baseline within 1 hour in normal horses. For practical purposes it is probably not necessary to continue sampling past 2 hours for diagnostic purposes. Excessively high or prolonged responses may be due to obesity [37], pituitary dysfunction [13,36,38], horses adapted to only dry hay rations [4,11], pregnancy [22,23], stress or genetic or age related insulin resistance [16,17,24,39]. Oral glucose tolerance Oral glucose tolerance tests reveal not only the ability of the animal to secrete insulin and the sensitivity of its tissues to the action of insulin but also its intestinal ability to absorb the glucose challenge [40,41]. Traditionally a dose of 1.0 g dextrose/kg body weight is administered by nasogastric tube as a 20% solution after a 12- to16-hour fast. Blood samples are drawn before administration and then either hourly or at 0.5-hour intervals for 6 hours. Blood samples are handled and analyzed as with the intravenous test. The glucose response curve tends to be lower and more prolonged than in the intravenous test, with peak glucose observed between 1 and 2 hours (120–180 mg/ dL) with higher peak insulin at 1.5 to 2.5 hours (60–150 lIU/mL). Concentrations return to baseline within 4 to 5 hours. A lower dose of 0.25 g/kg body weight administered by oral dose syringe as a paste or in applesauce gives a glucose/insulin response that is virtually identical to the responses seen after a meal of 1- to 2-kg of concentrate feed (Ralston, 1999, unpublished data). Glucose normally peaks at 90 to 120 mg/dL in 60 to 90 minutes; after 60 and 90 minutes, insulin should be \20 lIU/mL in foals younger than 6 months old and \100 lIU/mL in older horses (Ralston, 1999–2000, unpublished data). The lower dose test can serve as a more physiologic test in horses in which nasogastric intubation is a problem or when there is concern about the larger dose of dextrose. Abnormally low glucose/insulin responses suggest delayed gastric emptying, reduced absorption [41], or increased insulin sensitivity secondary to polysaccharide storage myopathy [42–45]. Abnormally high or prolonged responses are seen in obese horses and horses with genetic- or age-induced insulin resistance [16,17,33], pituitary dysfunction [36,38] or high levels of stress (Ralston, 1998, unpublished data). Grain challenges In a study of the effects of chromium supplementation in aged mares, it was found that a test meal of grain was a more sensitive test for abnormal
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glucose or insulin responses in aged horses than the traditional intravenous dextrose test [25]. It is hypothesized that the standard challenge of 0.15 g dextrose per kg body weight, which drives blood glucose to greater than 250 g/dL and stimulates insulin concentrations as high as 200 lIU/mL within 5 to 10 minutes, may obscure more subtle differences in insulin sensitivity. After a horse consumes 1.5 to 2 kg of texturized sweet feed or high carbohydrate pellets, its blood glucose normally peaks at 90 to 120 mg/dL within 90 minutes and its blood insulin normally peaks at 60 to 120 lIU/mL within 90 to 120 minutes [15,25,14,46]. Blood samples should be handled in the same fashion as in the previous tests. Glucose readings greater than 250 mg/dL or insulin levels higher than 200 lIU/mL should be considered abnormal responses and warrant further investigation.
Abnormal glucose and insulin metabolism in horses Hyperinsulinemia Hyperinsulinemia, defined as plasma insulin concentrations greater than 200 lIU/mL in adult animals [16], is common in horses. Most studies of glucose and insulin metabolism in horses have reported at least one hyperinsulinemic individual [13,15–18,21,25,26,37–39]. In studies in which adult horses or ponies were used, pituitary adenomas or obesity were most commonly associated with the reported hyperinsulinemia [15,21,25,26,38]. In other studies the hyperinsulinemia was frequently a serendipitous finding and appeared to be genetic or breed related [16–18,37,39] or was unexplained by the authors [11,26]. It is important to note in that all of the studies in which hyperinsulinemia was a serendipitous finding, less than 20 horses had been used, an indication of the high prevalence of hyperinsulinemia or insulin resistance in the general equine population. Hyperinsulinemia associated with pituitary dysfunction is also common, especially in aged mares [36,38]. Abnormally high insulin responses to either a standardized glucose challenge or a meal of grain often precede the gross clinical signs of pituitary dysfunction, such as hirsutism, polyuria/polydipsia, and abnormal cortisol secretion [36,38]. Hypoglycemia Naturally occurring postprandial hypoglycemia has recently been reported in horses with polysaccharide storage disease [42–45,47,48]. In these animals insulin secretion is within normal limits but they suffer profound hypoglycemia after a meal of starch, supposedly due to an enhanced sensitivity to the action of insulin [47]. As in humans, dietary control of the disease consists of a high protein and fat diet with limited starch intake [44,45,48]. The counter-regulatory hormonal responses to hypoglycemia have not been described in these animals, nor is it known if bone metabolism is affected.
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Hyperinsulinemia and abnormal bone growth in young horses In two separate studies the author has found a strong correlation between postprandial hyperinsulinemia and the radiographic presence of osteochondrosis lesions in Standardbred horses younger than 12 months of age [16–18]. Horses between 3 and 12 months of age were also documented to have higher insulin responses to meals than those older than18 months of age [17]. Krusic and colleagues [33] reported similar apparent insulin resistance in Lipizzaner horses between 3 and 12 months of age. This led to the hypothesis that abnormally high insulin, whether due to an inherited insensitivity, high levels of growth hormone, or in response to large amounts of carbohydrate being fed, would alter bone metabolism, which, under conditions that have yet to be elucidated, result in osteochondrosis or other growth abnormalities. References [1] Anderson RA. Nutritional factors influencing the glucose/insulin system. Chromium. J Am Coll Nutr 1997;16:404–10. [2] Alexander I, Roud HK, Irvine CHG. Effect of insulin-induced hypoglycaemia on secretion patterns and rates of corticotrophin-releasing hormone, arginine vasopressin and adrenocorticotrophin in horses. J Endocrinol 1997;154:401–9. [3] Mengozzi G, Giovanelli L, Rastelli S, Demi S, Intorre L, Soldani G. Adrenergic regulation of blood glucose levels and insulin release in horses: role of alpha 2 agonists and antagonists. Acta Vet Scand 1991;(Suppl. 87):336–7. [4] Jacobs KA, Bolton JR. The effect of diet on the oral glucose tolerance test in horses. J Am Vet Assoc 1982;180:884–6. [5] Nadal MR, Thompson DL, Kincaid LA. Effect of feeding and feed deprivation on plasma concentrations of prolactin, insulin, growth hormone and metabolites in horses. J Anim Sci 1997;75:736–44. [6] Lindberg JE, Jacobsson K-G. Effects of barley and sugar beet pulp on digestibility, purine excretion and blood parameters in horses. Pferdeheilkunde 1992;September:116–18. [7] Williams CA, Kronfeld DS, Staniar WB, Harris PA. Plasma glucose and insulin responses of Thoroughbred mares fed a meal high in starch and sugar or fat and fiber. J Anim Sci 2001;79:2196–201. [8] Lawrence LM, Williams J, Soderholm LV, Roberts AM, Hintz HF. Effect of feeding state on the response of horses to repeated bouts of intense exercise. Equine Vet J 1995; 27:27–30. [9] Pagan JD, Harris PA, Kennedy MAP, Davidson N, Hoekstra KE. Feed type and intake affects on glycemic responses in Thoroughbred horses. In: Proceedings of the 16th Equine Nutrition and Physiology Symposium, Raleigh, NC. Lexington (KY): ENPS. 1999. p. 149–50. [10] Argenzio RA, Hintz HF. Volatile fatty acid tolerance and the effect of glucose and VFA on plasma insulin levels in ponies. J Nutr 1971;101:723–30. [11] Argenzio RA, Hintz HF. Effect of diet on glucose entry and oxidation rates in ponies. J Nutr 1972;102:879–903. [12] Sticker LS, Thompson DL, Smith LA, Leise BS, Gentry LR. Pituitary hormone and insulin responses to infusion of amino acids and N-methyl-D, L-aspartate (NMA) in horses. Proceeding of the 16th Equine Nutrition and Physiology Symposium. Raleigh, NC. Lexington (KY): ENPS. 1999. p. 90–1.
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[13] Garcia M, Beech J. Equine intravenous glucose tolerance test: glucose and insulin responses of healthy horses fed grain and hay and of horses with pituitary adenoma. Am J Vet Res 1986;47:570–2. [14] Stull CL, Rodiek AV. Responses of blood glucose, insulin and cortisol concentrations to common equine diets. J Nutr 1988;118:206–13. [15] Ralston SL. Effect of soluble carbohydrate content of pelleted diets on postprandial glucose and insulin profiles in horses. Pferdeheilkunde 1992;September:112–15. [16] Ralston SL. Postprandial hyperglycemia/insulinemia in young horses with osteochondritis dissecans lesions. J Anim Sci 1995;73:184. [17] Ralston SL. Hyperglycemia/hyperinsulinemia after feeding a meal of grain to young horses with osteochondritis dessicans (OCD) lesions. Pferdeheilkunde 1996;May:320–2. [18] Ralston SL, Black A, Suslak-Brown L, Schoknecht PA. Postprandial insulin resistance associated with osteochondrosis in weanling fillies. J Anim Sci 1998;76(Suppl. 1):176. [19] Black A, Ralston SL, Shapses SA, Schoknecht PA. Skeletal development in weanling horses in response to high dietary energy and exercise. J Anim Sci 1997;75(Suppl. 1):170. [20] Potter GD, Hughes SL, Julen TR, Swinney DL. A review of research on digestion and utilization of fat by the equine. Pferdeheilkunde 1992;September:119–23. [21] Freestone JF, Beadle R, Shoemaker K, Bessin RT, Wolfsheimer KJ, Church C. Improved insulin sensitivity in hyperinsulinaemic ponies through physical conditioning and controlled feed intake. Equine Vet 1992;24:184–6. [22] Evans JW. Effect of fasting, gestation, lactation and exercise on glucose turnover in horses. J Anim Sci 1971;33:1001–4. [23] Fowden AL, Comline RS, Silver R. Insulin secretion and carbohydrate metabolism during pregnancy in the mare. Equine Vet J 1984;16:239–46. [24] Ralston SL, Baile CA. Plasma glucose and insulin concentrations and feeding behavior in ponies. J Anim Sci 1982;54:1132–7. [25] Ralston SL, Dimock AN, Socha M. Glucose/insulin responses to IV dextrose versus oral concentrate challenges following chromium supplementation in geriatric mares. In: Proceedings of the 16th Equine Nutrition and Physiology Symposium. Raleigh, NC. Lexington (KY): ENPS. 1999. p. 90–1. [26] Evans JW, Thompson PG, Winget CM. Glucose and insulin biorhythms in the horse. J S Afr Vet Assoc 1974;45:317–29. [27] Frape DL, Williams NR, Scriven AJ, Palmer CR, O’Sullivan K, Fletcher RJ. Diurnal trends in responses of blood plasma concentrations of glucose, insulin, and C-peptide following high- and low-fat meals and their relation to fat metabolism in healthy middleaged volunteers. Br J Nutr 1997;77:523–35. [28] Moan A, Hoieggen A, Nordby G, Os I, Eide I, Kjeldsen SE. Mental stress increases glucose uptake during hyperinsulinemia: associations with sympathetic and cardiovascular responsiveness. Metabolism 1995;44:1303–7. [29] Van Cauter E, Desir D, Decoster C, Frey F, Balasse O. Nocturnal decrease in glucose tolerance during constant glucose infusion. J Clin Endocrinol Metab 1989;69:604–11. [30] Van Cauter E, Shapiro ET, Tillel H, Polonsky KS. Circadian modulation of insulin responses to meals: relationship to cortisol rhythm. Am J Physiol 1992;262: E467–75. [31] Christensen RA, Malinowski K, Ralston SL, Scanes CG, Hafs HD. Chronic effects of equine growth hormone (eGH) on postprandial changes in plasma glucose, nonesterified fatty acids and urea nitrogen in aged mares. J Anim Sci 1996;74(Suppl 1):226. [32] Christensen RA, Malinowski K, Ralston SL, Scanes CG, Hafs HD. Chronic effects of equine growth hormone (eGH) on plasma insulin, insulin-like growth factor-I and thyroid hormones in aged mares. J Anim Sci 1996;74(Suppl 1):226. [33] Krusic L, Krusic-Kaplja A, Cestnik V. Insulin response after oral glucose application in growing Lipizzaner foals. In: Proceedings of the 15th Equine Nutrition and Physiology Symposium. Fort Worth, TX. Lexington (KY): ENPS 1997. p. 397–403.
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[34] Preuss HG. Effects of glucose/insulin perturbations on aging and chronic disorders of aging: the evidence. J Am Coll Nutr 1997;16:395–403. [35] Ott EA, Kivipelto J. Influence of chromium tripicolinate on growth and glucose metabolism in growing horses. J Anim Sci 1999;77:3022–30. [36] Beech J. Tumors of the pituitary gland (pars intermedia). In: Robinson NE, editor. Current therapy in equine medicine. 2nd edition. Philadelphia: W.B. Saunders Co.; 1987. p. 182–5. [37] Jeffcott LB, Field JR, McClean JG, O’dea K. Glucose tolerance and insulin sensitivity in ponies and Standardbred horses. Equine Vet J 1986;18:97–101. [38] Ralston SL, Nockels CF, Squires EL. Differences in diagnostic test results and hematologic data between aged and young horses. Am J Vet Res 1988;49:1387–92. [39] June V, Soderholm V, Hintz HF, Butler WR. Glucose tolerance in the horse, pony and donkey. J Equine Vet Sci 1992;12:103–5. [40] Klein J, Schlze E, Deegen E, Giese W. Metabolism of naturally occurring [13C] glucose given orally to horses. Am J Vet Res 1988;49:1259–62. [41] Roberts MC, Hill FWG. The oral glucose tolerance test in the horse. Equine Vet J 1973; 5:171–3. [42] Valberg SJ, MacLeay JM, Mickelson JR. Exertional rhabdomyolysis associated with polysaccharide storage myopathy in the horse. Comp Contin Ed Prac Vet 1998;19:1077–86. [43] Valberg SJ, Townsend D, Mickelson JR. Skeletal muscle glycolytic capacity and phosphofructokinase regulation in horses with polysaccharide storage myopathy. Am J Vet Res 1998;59:782–5. [44] Valentine BA, Divers TJ, Lavoie JP. Severe equine polysaccharide storage myopathy in draft horses: clinical signs and response to dietary therapy. In: Proceedings of the 42nd Annual Meeting of the American Association of Equine Practice, Denver, CO. 1996. p. 294–6. [45] Valentine BA, Hintz HF, Freels KM, Reynolds AJ, Thompson KN. Dietary control of exert ional rhabdomyolysis in horses. J Am Vet Med Assoc 1998;212:1588–93. [46] Ralston SL, Van den Broek G, Baile CA. Feed intake patterns and associated blood glucose, free fatty acid and insulin changes in ponies. J Anim Sci 1979;49:838–45. [47] De La Corte FD, Valberg SJ, Williamson SE, MacLeay JM, Mickelson JR. Glucose uptake in horses with polysaccharide storage myopathy. Am J Vet Res 1999;60:458–61. [48] De La Corte FD, Valberg SJ, MacLeay JM, Billstrom J. The effect of feeding a fat supplement to horses with polysaccharide storage myopathy. World Eq Vet Rev 1999; 4:12–9.