CATI - Effects of Postprandial Interval and Feed Type on Substrate Availability During Exercice

- Research Bulletin -

Effects of Postprandial Interval and Feed Type on Substrate Availability During Exercice
by
Carolyn Stull and Anne Rodiek

CATI Publication #941004
© Copyright October 1994, all rights reserved

ABSTRACT

      The glycemic and lipemic effects of 3 feeding regimes and two postprandial intervals were determined at rest and during exercise using 6 Thoroughbred geldings on 6 testing days according to a Latin square design. Three feeding regimes consisted of a fasting regime, and isoenergetic meals (4.1 Mcal) of either cracked corn or alfalfa. During the time period of 1 or 4 h postprandially, a standardized exercise test (SET) was imposed on fed and fasted horses. Blood was sampled via jugular catheters every 15 min from 0630 to 1500 h, except during the 55 min SET in which 7 samples were drawn. The SET consisted of three 10 min bouts of saddling, walking to the arena, and warmup (walking and trotting), followed by three bouts of progressively more intense cantering at heart rates of 130-140 (10 min), 150-160 (10 min), and 170-180 (5 min) beats per minute. There were no dietary differences in prefeeding levels of glucose, insulin, lactate, glycerol or free fatty acids (FFA). However, glucose in corn-fed horses fell below prefeeding concentration during the SET but rebounded at the termination of cantering. Glucose in alfalfa-fed and fasted horses gradually rose throughout the SET. Insulin levels closely followed glucose response. During the SET, corn-fed horses exhibited higher insulin concentrations (P < 0.05) than fasted or alfalfa-fed horses. During the onset of the SET 1 or 4 h postprandially, FFA levels in corn-fed were less (P < 0.05) than alfalfa-fed or fasted horses. Glycerol levels gradually increased during the SET; however, no differences were found between dietary treatments.

KEY WORDS: Horses, exercise, glucose, free fatty acids, insulin, glycerol

INTRODUCTION

      Energy substrate availability is critical to the muscle cells of the exercising horse. Ultimately, the energy utilized in the metabolic process of muscular contraction is derived from foodstuffs in the diet. Nutritional manipulation including the composition, size and timing of a meal prior to exercise may potentiate or inhibit an optimal performance. Maintenance of energy metabolites including carbohydrates and fats is important in offsetting fatigue, especially in endurance-type activities. Previous studies in horses have demonstrated glycemic, lipemic, and associated hormone differences between common diets in resting (Stull and Rodiek, 1988) and exercising horses (Duren et al., 1987; Rodiek, et al., 1991; Zimmerman, et al., 1992; Lawrence, et al., 1993). However, the influence of interval length between meal ingestion and subsequent exercise has not been studied in the performance horse. The aim of this study was to investigate the influences of diet composition and postprandial interval on glycemic and lipemic responses during moderate exercise. Two postprandial intervals were selected to impose exercise while glucose levels were ascending following meal ingestion, and at the return of glucose to prefeeding levels after a meal. Dietary treatments consisted of isoenergetic meals of corn or alfalfa, while a fasting regime was also included to examine responses in the absence of dietary influences.

MATERIALS AND METHODS

General procedures
      Six mature Thoroughbred geldings (mean body weight of 513 kg) previously utilized in an equitation program were used in a Latin square design to determine the effects of 3 feeding regimes and 2 postprandial intervals on the availability of energy metabolites during exercise. Each horse was randomly assigned to one of 6 treatments on 6 test days. Test days were approximately 7 days apart. Feeding regimes consisted of cracked corn (1.2 kg), alfalfa hay (2.0 kg), or a fasting regime in which no feed was offered. Isoenergetic meals (4.1 Mcal) of corn or alfalfa were fed to provide 25% of the daily digestible energy requirement (NRC, 1989). Two postprandial intervals (1 and 4 h) were imposed between feeding and a standard exercise test (SET). Both fed and fasted horses performed a SET at 1 and 4 h postprandially. Between test days, horses were fed a diet of alfalfa hay and mixed grains to maintain body weight. Horses were fasted overnight on test days, and fed their meal at 0700 h. Any remaining feed was removed at 0800 h. One horse refused 50% of the corn meal during the 1 hour feeding period, while three horses refused alfalfa hay at 70%, two horses at 30%, and two horses at 20% of the amount offered.

      Blood samples (20 ml) were drawn from indwelling catheters placed in the jugular vein at least 1 h prior to the first sample collection. Samples were drawn every 15 min from 0630 to 1500 h, except during the 55 min SET in which 7 samples were drawn. All samples were placed on ice immediately after collection. Blood samples intended for glucose and lactate measurements were collected in evacuated tubes containing potassium oxalate and sodium fluoride, while samples for free fatty acids (FFA) and glycerol were collected in EDTA tubes. Plasma was stored frozen for analyses. Blood samples collected for insulin were allowed to clot and the serum was harvested and frozen.

Standard exercise test
      All horses were ridden during the SET in a sand arena with similar tack by riders of similar weight and experience. The SET was imposed on each horse either 1 or 4 h after the start of meal ingestion. The SET consisted of 10 min periods each of saddling, walking to the arena, and warmup (walking and trotting), followed by three cantering bouts of increasing workloads as measured by heart rate (HR). HR was monitored and recorded by an onboard telemetric meter (V-MAX, Equine Performance Technology, Seven Valleys, PA). HR was targeted at 130-140 and 150-160 beats per minute (bpm) for the first two 10 min cantering periods, followed by a third 5 min cantering period with targeted HR of 170-180 bpm. Horses were halted between each SET period (less than 30 seconds) to obtain a blood sample and flush catheters with heparinized saline. A total of 7 samples was collected during SET.

Blood analysis
      Glucose and lactate concentrations were determined using a autoanalyzer (YSI 2300 STAT Plus, Yellow Springs, OH). Insulin was analyzed using a commercially available radioimmunoassay kit (Kit No. D1804, Micromedic Systems, Horsham, PA) validated for equine blood by Reimers, et al., (1982). A total of 10 plasma samples from each sampling period (including all 7 samples collected during the SET) were analyzed in duplicate for glycerol and FFA using enzymatic colorimetric assays (Procedure 337, Sigma Diagnostics, St. Louis, MO; Cat. No. 1383 175, Boehringer Manneim, Indianapolis, IN; respectively).

Statistical analysis
      The ordering of six treatments (3 feeding regimes and 2 postprandial intervals) was assigned according to a Latin Square design. Since the order effect was not significant, it was suppressed in subsequent analyses. The statistical significance of diet and temporal effect within each postprandial interval group were tested using repeated measures analyses of variance using two within factors (diet and time within a given day). Statistical significance is claimed at P < 0.05. Post hoc comparisons among diets were based on pairwise comparisons among the diet treatments. The significance of diet x time interactions was assessed using contrast tests. Data were analyzed alternately in raw form and after log transformation. The two sets of results were qualitatively identical and only analysis of raw data is present ed (SAS, 1989).

      No differences were shown for sampling days or individual horse variation in any of the parameters. For comparisons between and within dietary treatments, the mean values for glucose, insulin and lactate were analyzed by periods designated as prefeeding (two samples prior to the meal), postprandial (4 or 16 samples after the meal and prior to SET in the 1 or 4 h protocol, respectively), SET (7 samples during the SET), and recovery (24 or 12 samples after the SET in the 1 or 4 h protocol, respectively).

RESULTS

Glucose
      Differences in glucose response to the three dietary treatments were observed during the postprandial and SET periods (Table 1 and 2). No differences between dietary treatments were observed during prefeeding or recovery periods in either the 1 or 4 h protocols. Mean plasma glucose concentrations were higher (P < 0.05) during the postprandial period in corn-fed horses than alfalfa-fed or fasted horses in both protocols. During the SET of the 4 h protocol, fasted horses maintained a higher mean glucose concentration than corn-fed horses (Table 2). During the SET of the 1 h protocol, no dietary treatment differences were observed.

      Within dietary treatments, plasma glucose concentrations varied significantly within response to the SET and time interval of the SET after eating. Glucose concentration in corn-fed horses exhibited a biphasic response curve in both the 1 and 4 h SET protocols (Fig. 1). In both protocols, glucose concentration fell precipitously below prefeeding concentrations during cantering in the SET but rebounded at the termination of cantering. In the 1 h protocol, the peak concentration during SET (6.4 mmol/L) was similar to the rebound peak (6.4 mmol/L) occurring after 30 min of recovery. However in the 4 h protocol, the peak elevation (7.4 mmol/L) during the postprandial period which occurred 90 min after eating began, and was greater than the rebound peak (5.8 mmol/L) observed 15 min into the recovery period.

      In alfalfa-fed horses, glucose peaks occurred at the onset of SET (4.8 mmol/L) in the 1 h protocol, and approximately 105 min after eating began (5.3 mmol/L) in the 4 h protocol (Figure 1). A decline in glucose concentration below prefeeding levels during the SET occurred in neither the 1 or 4 h protocol in alfalfa-fed or fasted horses. In alfalfa-fed horses, glucose reached peak concentration (5.8 mmol/L) 30 min following the termination of the SET in the 1 h protocol, and peaked (5.9 mmol/L) 15 min post SET in the 4 h protocol.
      Mean glucose concentration in fasted horses during the SET and recovery was higher (P < 0.05) than prefeeding and postprandial values during the 1 h protocol (Table1). During the 4 h protocol, only recovery glucose values were larger (P < 0.05) than prefeeding and postprandial. In both the 1 and 4 h protocols, peak glucose concentrations (5.8 and 6.6 mmol/L, respectively) of fasted horses occurred 15 min following the termination of SET. Differences in glucose responses between diets occurred throughout the sampling period (Table 1 and 2).

Insulin
      Insulin response closely followed glucose response for each protocol. During 1 h protocol, corn and alfalfa-fed horses exhibited a biphasic insulin response curve, with the first peak occurring at the onset of the SET and the second elevation following SET. Fasting horses showed a single peak following SET. A single peak in insulin concentration was observed in 4 h protocol for corn and alfalfa-fed horses at approximately 2 h postprandially, corresponding to the postprandial glucose peak. No dietary differences were observed in mean insulin concentration during prefeeding or recovery periods in either protocol; however, corn-fed horses showed an elevated (P < 0.05) mean insulin concentration during postprandial and SET periods in both protocols (Table 1 and 2).

Lactate
      Serum lactate concentrations rose sharply in both protocols during SET, peaked with the final SET sample, and returned to prefeeding levels in recovery (Fig.1). All dietary treatments in both protocols showed increases in lactate concentrations during the SET as compared to prefeeding and postprandial values. No effects due to diet were found within the prefeeding, postprandial, SET or recovery periods in either 1 or 4 h protocols (Table 1 and 2). Although not significant, the lactate peaks for the corn-fed horses (5.6 ± 4.7 and 8.1 ± 1.9 mmol/L) were greater than the alfalfa-fed (4.2 ± 1.0 and 6.5 ± 1.8 mmol/L) or the fasted (3.7 ± 0.6 and 5.9 ± 3.7 mmol/L) horses in both the 1 and 4 h protocols, respectively.

FFA
      No differences in FFA concentrations (0.25 ± 0.14 and 0.24 ± 0.09 mmol/L, respectively) were observed in prefeeding levels in either 1 or 4 h protocols. During the SET of the 1 h protocol, FFA concentrations in corn-fed horses were lower than ( P < 0.05) fasted horses at each sampling with the exception of the final sample in which no differences between diets were observed (Fig. 2). During recovery, corn-fed horses were found to have lower FFA concentration than fasted horses. A similar trend was found in FFA concentration during the SET of the 4 h protocol with corn-fed horses showing lower levels (P < 0.05) of FFA compared to fasting horses during the first 4 samples. No differences between diets were found during the remaining SET and recovery samples.

Glycerol
      No effect on glycerol concentration due to diet was shown in either protocol throughout the entire sampling period. Glycerol concentration gradually increased during SET of the 1 and 4 h protocol (Fig. 2). During the 1 h protocol, glycerol was elevated ( P < 0.05) in the corn and alfalfa-fed horses during the final sample of SET over prefeeding concentration. Glycerol was elevated (P < 0.05) in the final SET sample over prefeeding concentration in all three dietary treatments during the 4 h protocol.

DISCUSSION

      Corn is a common equine feed consisting of 66% starch, a soluble carbohydrate. Alfalfa is low in starch (2%) and high (25%) in the insoluble carbohydrate, cellulose (NRC, 1971). In resting horses, a previous study examined the postprandial glycemic response of glucose and insulin to isoenergetic meals of corn or alfalfa (Stull and Rodiek, 1988). Glucose showed peak response 2 h after the start of meal ingestion in both diets, whereas insulin peaked approximately 3 hours postprandially. Corn diets showed a larger insulin response than alfalfa.
      In subsequent investigations, fit and unfit horses were fed the isoenergetic diets 2 h prior to a submaximal exercise test. Both fit and unfit horses showed similar responses. The corn-fed horses showed a sharper decline from peak levels of insulin and glucose with the onset of the exercise test and a larger rebound effect at the cessation of exercise than did the alfalfa-fed horses (Rodiek, et al., 1991). During exercise, FFA concentrations were significantly higher in alfalfa-fed than corn-fed horses (Zimmerman, et al., 1992). The postprandial intervals of 1 and 4 h in the present study were selected to impose the SET while glucose levels were ascending during the postprandial period and at the return of glucose to prefeeding levels after a meal. The fasting regime was included to examine the glycemic and lipemic responses in the absence of dietary influences.
      The length (25 min of cantering), intensity (HR between 130 and 180 bpm) of the SET, and the rise in lactate indicate that the SET predominately utilized aerobic metabolic pathways with minor contributions of net energy from anaerobic metabolism. Heart rates from 120 to 210 bpm in exercising horses have been correlated with increasing workloads (Persson, 1983). Endurance horses have shown a 2 to 3 fold increase in lactate following a competition at submaximal exercise intensity (Lucke and Hall, 1980; Essen-Gustavsson, et al., 1984); whereas an approximate 40-fold increase has been reported following maximal exercise in standardbred race horses (Lindholm and Saltin, 1974). In the present study, peak lactate increased 10 to 14 fold over resting (prefeeding) levels. Both heart rate and lactate concentrations indicate a submaximal, moderate intensity effort during SET. Although not significant, lactate concentration during the SET was highest in corn-fed horses.
      Fatigue may be due to the depletion of glycogen stores within muscle fibers rather than a decline in circulating blood glucose (Lindholm, et al., 1974; Essen-Gustavsson, et al., 1984; Valberg, 1986). However, the elevated glucose following a meal may change the dynamics of substrate availability. In general, both the 1 and 4 h protocols showed similar response with respect to glucose and insulin in all 3 dietary treatments, whereby, glucose and insulin concentrations rebound following exercise. However, a 4 h interval between eating and exercise dampened the declines of glucose and insulin concentrations observed when corn-fed horses performed the SET only 1 h postprandially. The dramatic decline in glucose concentration during the SET may be due to a sm all uptake by the muscles for energy along with the continued action of insulin released following the meal, most pronouncedly after a corn meal. The half-life of insulin in horses is 33 min which has been shown to be slower than humans in responding to a decline in glucose (Madigan and Evans, 1973). By the termination of the SET with 30 min of cantering 1 h postprandially, insulin levels were similar in all three dietary treatments.
Lipids in the form of FFA are an important source of energy for submaximal exercise in the horse. The primary source of circulating FFA is from lipolysis in the adipose tissue and secondly from the triglyceride depots within the muscle (Snow, 1975). Glycerol and FFA are released into the circulation with mobilization of triglycerides from adipose tissue. Both glycerol and FFA concentrations increased throughout the SET indicating mobilization of FFA as an energy source from adipose tissue. Lipid oxidation has been shown to increase in horses with increasing duration (Rose, et al., 1980) and intensity of exercise (Lindholm, et al., 1974).
      In the SET, exercise intensity increased during each interval of cantering (as measured by HR). Thus, the concentration of circulating lipid metabolites increased throughout the SET. Lower levels of FFA were observed in corn-fed horses compared to alfalfa-fed horses which is in agreement with previous results (Zimmermann, et al., 1992). Insulin has the general effect of inhibiting lipolysis and promoting glycogenesis in adipose tissue and liver (Keele and Neil, 1971). Thus, the lower circulating FFA concentration may be due to the influence of elevated insulin concentration postprandially in corn-fed horses.
      Energy metabolites circulating in the blood are in a constant state of flux dependent on tissue uptake and the mobilization of lipids and glycogen from extramuscular sources. These results indicate that the composition of the diet and the timing of the meal prior to exercise can be manipulated to influence glycemic and lipemic metabolites available in the blood during moderate exercise. Further studies are necessary to investigate the effects of manipulating common equine diets on performance indices in exercising horses.

ACKNOWLEDGEMENTS

      This project was supported in part by the Equine Research Laboratory with funds provided by the Oak Tree Racing Association, the State of California satellite wagering fund, and contributions by private donors. This work was also supported by California Agricultural Technology Institute. Technical assistance of Marsha Feldman, Bill Swan, and Kelly Weaver along with statistical consultation of Neil Willits is gratefully acknowledged.

REFERENCES

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Essen-Gustavsson, B., K. Karlstrom, and A. Lindholm. (1984) Fibre types, enzyme activities and substrate utilisation in skeletal muscles of horses competing in endurance rides. Equine Vet. J., 16(3):197-202.

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Rodiek, A., S. Bonvicin, C. Stull, and M. Arana. (1991). Glycemic and endocrine responses to corn or alfalfa fed prior to exercise. In: Equine Exercise Physiology, Persson, S.G.B., Lindholm, A., and Jeffcott, L.B. (eds.), ICEEP Publications, Davis, Califo rnia, pp. 323-330.

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Snow, D. H. (1975). Biochemical changes in blood and muscle associated with exercise. Proc. of the 1st International Symposium on Equine Hematology. pp. 427-434.

Stull, C. L., and A. V. Rodiek. (1988). Responses of blood glucose, insulin and cortisol concentrations to common equine diets. J. Nutr. 118:206-213.

Valberg, S. (1986). Glycogen depletion patterns in the muscle of Standardbred Trotters after exercise of varying intensities and durations. Equine Vet. J. 18(6):479-484.

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