Carbohydrate Availability and Training Adaptation: Effects on Cell Metabolism

John A. Hawley; Louise M. Burke

Disclosures

Exerc Sport Sci Rev. 2010;38(4):152-160. 

In This Article

The Training Response-adaptation Process

From a cellular perspective, (endurance) training adaptation can be viewed as a consequence of the accumulation of specific proteins required for sustaining energy metabolism during and after exercise. Thus, the training-induced increase in gene expression that allows for subsequent changes in protein abundance is crucial to the adaptation process.[15] Although exercise alone is a powerful stimulus for the transcription of multiple "metabolic" genes, nutrition - in particular, altered carbohydrate availability (i.e., nutrient exercise interaction) - also is a potent modulator of this transcriptional response. For example, the rate of translation of postexercise skeletal muscle interleukin 6 (IL-6) messenger ribonucleic acid (mRNA) is reduced by feeding glucose during exercise, whereas the transcriptional rate of IL-6 from the nuclei of contracting skeletal muscle fibers also is influenced by muscle glycogen content.[20] An acute bout of endurance exercise commenced with low muscle glycogen stores also results in a greater transcriptional activation of enzymes involved in carbohydrate metabolism (i.e., the adenosine monophosphate-activated protein kinase [AMPK], glucose transporter 4 [GLUT-4], hexokinase, and the pyruvate dehydrogenase [PDH] complex) compared with when glycogen is normal or elevated before exercise.[27,28,32,33] Such information underpins the recent postulate that a "cycling" of muscle substrate stores is required to obtain the optimal adaptations to exercise training and provides the impetus for the hypothesis that training with low muscle glycogen availability may enhance training adaptation to a greater extent than training with normal or elevated glycogen stores.[15] Extending this paradigm, Baar and McGee[4] have proposed that the classic principles of training incorporating systematic progressive overload are no longer adequate for optimal performance, and based on our increasing knowledge of the role of nutrition and training, this century-old principle is in need of revision. Specifically, these workers recommend athletes deliberately train in a glycogen-depleted state to maximize the physiological adaptation to endurance exercise. Others[29] also have noted that training-nutrient "periodization" is necessary to optimize phenotypic adaptation and performance. We now examine the scientific evidence for the hypothesis that training undertaken with low carbohydrate availability promotes endurance-training adaptation to a greater extent than when training undertaken with high carbohydrate availability.

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