Metabolic Responses and Exercise - Essay Example

Effect of training in the fasted on metabolic responses during exercise with carbohydrate intake Critique Introduction Skeletal muscle physiology has been under intense investigation in the past as a horde of variations have been noticed in the growth, physiology and metabolic patterns in muscles of normal and highly trained athletes. The study by De Bocke, et al (2008) is an endeavor to investigate how the skeletal muscle responds to exercise in the fasting stage, when carbohydrate intake is controlled in moderately active persons who are subjected to endurance training. The authors believe that there is no research till date indicating any alteration in muscle adaptation to endurance training when its nutritional status is known during the training process. This is the overall purpose of this research. Skeletal muscle subjected to intense physical stress requires ready availability of energy resources and it is established that carbohydrate intake during exercise delays the onset of fatigue as it is a readily available source of energy. Only in the absence or deficiency of carbohydrates, other reserve sources such as body fat and protein are tapped under normal physiological circumstances. Oxidative metabolism in the skeletal muscle is upregulated by normal physiological mechanisms of gene transcription which increases the mRNA content of several genes in response to the demand imposed as a result of exercise. Carbohydrate intake is known to diminish this activity. Low availability of glycogen resources in endurance training is also known to stimulate gene expression in a manner which can prove beneficial to endurance performance. This physiological adaptation, the authors believe can be tapped for a better training method for athletes. They believe that exercise induced glycogen breakdown is maintained at an optimum level when exercise is done under fasting conditions and this facilitates energy provision from the oxidation of fat reserves in the body. A better understanding of the underlying mechanisms can help develop better training programs for athletes, which was the indirect endeavor in this study. The authors’ hypothesis is pertinent to the currently available knowledge of skeletal muscle physiology as increased interest has been generated in this field due to the increasingly competitive environment in the international arena. Studies on trained athletes are numerous and the authors’ selection of healthy individuals from normal population in favor of actual athletes is an appropriate strategy as it can yield more pertinent data about skeletal muscle activity, which is already in pre conditioned and altered state in established athletes. The authors took adequate precautions in this regard by ensuring that the participants did not indulge in any strenuous activity prior to the commencement of this study. In order to eliminate any errors in the collection of data during the experiment, the authors standardized the exercise stress to be imposed on the participating volunteers in a preliminary study Uniform methods of exercise delivery were used and oxygen uptake rate and lactate threshold measurements were done using standard equipment. Familiarization sessions quantified the workload to exhaustion so that reasonably accurate data could be obtained during the actual experiment, where data from two equal groups of 10 each was to be analyzed. Both groups participated in two experimental sessions separated by 6 weeks duration which was considered as the training period. The degree of exercise, volume of food and water intake, energy source in the form of maltodextrine, method of collection of blood samples and ergospirometry methods were identical in both the pretraining and the posttraining sessions in order to facilitate the yield of accurate data which could be analyzed by appropriate statistical techniques. The measures of the variables in this study were appropriate in relation to the objectives of this study as oxygen uptake rate and lactate thresholds can yield accurate indirect data for the oxidation rates of carbohydrates as well as fat. Muscle biopsy samples can yield important biochemical and physiological data. Analysis of muscle glycogen content by fluorometric assay, muscle fibre type-specific oxidative capacity by assessing succinate dehydrogenase, measurement of optical density of muscle fibres using sophisticated photography and software, and immunological studies including PCR were employed for estimation of mRNA. All the data generated using these techniques can provide vital clues and data for comparison to evaluate the affects of glucose intake in the two groups and infer whether there are any significant differences or not. Human subjects were used in this study. They may not represent the entire human population as variations exist due to ethnicity and geographical location in exercise tolerance. But the physiological, molecular and immunological techniques used in this study are likely to yield significant data from the analyses of samples from the two groups of 10 persons each who were either fasted (F) or carbohydrate fed (CHO) during the experiment and whose parameters were standardized with utmost detail before performing the actual experiments. The one major finding in this study was that fasting blunted exercise induced glycogen breakdown, which did not occur in the carbohydrate fed group. After training, pre-exercise glycogen content was significantly higher in the carbohydrate fed group, as expected. This can point to the fact that this glycogen will be first to be utilized as an energy source during exercise stress in carbohydrate fed individuals. However as expected, the authors did not observe any changes within the two groups as far as fat oxidation as an alternative source of energy was concerned. The hypothesis that the authors put forward as a purpose of this study was therefore defeated as no significant change in fat oxidation was seen between the two groups. The study, though conducted with great caution and scientific detail, failed to focus on its primary purpose as it was too much spread out into evaluation of numerous parameters which leads to confusion. Although the standardization and normalization procedures used were comprehensive, such a study needs to focus on one parameter at a time in a larger population size in numerous isolated studies so that a collective conclusion can be drawn from studies at different locations. Reflection and Analysis Harridge Stephen (2007) has reviewed the factors influencing the regulation of skeletal muscle and elaborated the mechanisms which have an overlapping and rather complex mode of regulation and action. The skeletal muscle as such is a highly adaptable specialized tissue which has to play the vital role of supporting the skeleton and providing mobility to an individual and is subject to great variations in size and composition based upon the physical stress, hormonal influences and pre existing nutritional status. Genetic events such as increased rate in translation of specific mRNA are triggered inside the skeletal muscle as a response to exercise. Studies can therefore be defined by evaluation of measurable physiological and immunological data. Alterations in the metabolic and mitogenic pathways in skeletal muscle of normal persons and highly trained athletes have been studied with increasing intensity in the last few years (Yu Mei, et al, 2003). The influence of pre-existing nutritional status as well as the availability of a steady supply of glucose during exercise increase both exercise endurance and stamina. The authors hypothesized that a fasting state may mobilize energy sources from other quarters than stored glycogen in fasting individuals and thus designed this study to evaluate their opinion. The study was well designed in terms of detail and appropriate sample selection and the period of training of six weeks adequate to elicit altered physiological profiles between the control (carbohydrate fed) and the experimental (fasted) group. The sample size of 10 per group was also adequate as microbiological, hormonal and molecular studies were to be conducted on a number of samples collected from the participating subjects. Intramyo-cellular triglyceride content (IMCL) was considered as a marker for detecting and evaluation of metabolic changes which were expected to be different between the two groups but remained unchanged. After training, pre-exercise glycogen content was higher in the carbohydrate fed group but not in the fasted group. For a given initial glycogen content, exercise-induced glycogen breakdown was blunted in the fasted group as compared to the carbohydrate fed group. Neither IMCL breakdown nor fat oxidation rates during exercise were altered by training. In order to prevent progressive glycogen depletion throughout the training period, the authors administered a weight-maintaining carbohydrate-rich diet in a consistent manner to the subjects during the entire duration of the experiment. The authors believe that this procedure involving daily high-dose carbohydrate intake may have inhibited the training-induced adaptations to facilitate fat oxidation, as the preferential mechanism for energy requirement was oriented towards rapidly replenished glycogen stores. In earlier experiments too, carbohydrate replete states had shown similar unchanged manner of fat oxidation rates in response to endurance training. The authors concluded that regular exercise in the fasted state might stimulate muscle glucose uptake for a given absolute exercise intensity in presence of carbohydrates consumption, which in turn inhibits net glycogenolysis. The glycogen sparing as encountered in the fasting subjects in this experiment prompts the authors to encourage further studies which might be beneficial for those indulging in endurance training. References: De Bock K. , Derave W. , Eijnde B. O. , Hesselink M. K. , Koninckx E. , Rose A. J. , Schrauwen P , Bonen A. , Richter E. A. and Hespel P., (2008), Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake, Appl Physiol 104: 1045–1055. Harridge Stephen D. R.,2007, Plasticity of human skeletal muscle: gene expression to in vivo function, Exp Physiol 92.5 pp 783–797 Yu Mei , Stepto Nigel K. , Chibalin Alexander V. , Fryer Lee G. D. , Carling Dave , Krook Anna , Hawley John A. and Zierath Juleen R. , Metabolic and mitogenic signal transduction in human skeletal muscle after intense cycling exercise, J Physiol (2003), 546.2, pp. 327–335 Read More