Research Article: Repeated post-exercise administration with a mixture of leucine and glucose alters the plasma amino acid profile in Standardbred trotters

Date Published: February 1, 2012

Publisher: BioMed Central

Author(s): Katarina EA Nostell, Birgitta Essén-Gustavsson, Johan T Bröjer.


The branched chain amino acid leucine is a potent stimulator of insulin secretion. Used in combination with glucose it can increase the insulin response and the post exercise re-synthesis of glycogen in man. Decreased plasma amino acid concentrations have been reported after intravenous or per oral administration of leucine in man as well as after a single per oral dose in horses. In man, a negative correlation between the insulin response and the concentrations of isoleucine, valine and methionine have been shown but results from horses are lacking. This study aims to determine the effect of repeated per oral administration with a mixture of glucose and leucine on the free amino acid profile and the insulin response in horses after glycogen-depleting exercise.

In a crossover design, after a glycogen depleting exercise, twelve Standardbred trotters received either repeated oral boluses of glucose, 1 g/kg body weight (BW) at 0, 2 and 4 h with addition of leucine 0.1 g/kg BW at 0 and 4 h (GLU+LEU), or repeated boluses of water at 0, 2 and 4 h (CON). Blood samples for analysis of glucose, insulin and amino acid concentrations were collected prior to exercise and over a 6 h post-exercise period. A mixed model approach was used for the statistical analyses.

Plasma leucine, isoleucine, valine, tyrosine and phenylalanine concentrations increased after exercise. Post-exercise serum glucose and plasma insulin response were significantly higher in the GLU+LEU treatment compared to the CON treatment. Plasma leucine concentrations increased after supplementation. During the post-exercise period isoleucine, valine and methionine concentrations decreased in both treatments but were significantly lower in the GLU+LEU treatment. There was no correlation between the insulin response and the response in plasma leucine, isoleucine, valine and methionine.

Repeated post-exercise administration with a mixture of leucine and glucose caused a marked insulin response and altered the plasma amino acid profile in horses in a similar manner as described in man. However, the decreases seen in plasma amino acids in horses seem to be related more to an effect of leucine and not to the insulin response as seen in man.

Partial Text

Leucine is one of three branched chain amino acids (BCAA) and a potent stimulator of insulin secretion, which is mediated by oxidative decarboxylation and allosteric activation of glutamate dehydrogenase [1-3]. The metabolically linked secondary signals that lead to insulin release have not yet been established. The combination of carbohydrates and protein or amino acids has been shown to potentiate the insulin response and increase the post exercise re-synthesis of muscle glycogen in man [4,5]. In horses, per oral administration of leucine and glucose post exercise has been shown to induce an insulin response equal to the one seen after administration of intravenous glucose [6,7]. Contrary to man, the post-exercise re-synthesis of muscle glycogen is not increased in horses [7].

Enrolment in this study was done in parallel with a study with the purpose to study the insulin response and glycogen resynthesis after post-exercise supplementation with leucine and glucose [7]. For more detailed information regarding horses, diet and exercise, see [7]. The study (C338/8) was sanctioned by the Ethical Committee for Animal Experiments, Uppsala, Sweden.

One horse became lame during the exercise test and was excluded leaving 11 horses in the study. The overall mean heart rate at the end of the intervals for the two experimental periods was 210 ± 2 and 211 ± 1 beats/min respectively.

The results showed that repeated post exercise administration with a mixture of leucine and glucose caused a marked increase in plasma leucine concentration and decreased levels of the other BCAA and methionine. This agrees with the results observed post-exercise in humans after repeated supplementation with glucose together with a mixture of whey, leucine and phenylalanine [17]. A recent study on horses reported a similar effect on the BCAA profile after a single gastric gavage of leucine and glucose in the early recovery period after glycogen depleting treadmill exercise [6]. In the present study, plasma leucine concentrations increased 4-5 fold after supplementation with leucine, which is less than in the study by Urschel et al. [6] where plasma leucine concentrations increased 8-10 fold. In that study, the dose was higher (0.3 g/kg BW) and given as a single per oral dose whereas the present study used a lower dose (0.1 g/kg BW) given twice. The administration of leucine at 0 and 4 h post-exercise was chosen since oral administration of leucine stimulates the insulin response for 4 h in horses [21]. Despite this, the decreases in the plasma isoleucine and valine concentrations were in parity to those seen by Urschel et al. [6].

Repeated post-exercise administration of leucine and glucose caused decreased concentrations of isoleucine and valine as well as methionine in a similar pattern as reported in humans. Contrary to man, the changes in these amino acid concentrations did not seem to be related to the insulin response. Further studies are needed to evaluate the mechanism behind the effect of leucine on amino acid metabolism in horses.

The authors declare that they have no competing interests.

JBR and KNO took equal responsibility for designing the study as well as coordinating the project and apply for funding and drafted the manuscript. The practical experiment was conducted with equal contribution from KNO and JBR with support from BEG. BEG was involved in the sampling of the horses and commented the manuscript. All authors read and approved the final manuscript.