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

Abstract and Introduction

Abstract

Several markers of endurance training adaptation are enhanced to a greater extent when individuals undertake selected training sessions with low compared with normal muscle glycogen content or with low exogenous carbohydrate availability. The potential mechanisms underlying the cellular responses arising from such nutrient-exercise interactions are discussed in the context of promoting training adaptation.

Introduction

The relationship between dietary carbohydrate intake, muscle glycogen content, and endurance exercise capacity is well documented, and it has become widely accepted that a high-carbohydrate intake before, combined with carbohydrate supplementation during prolonged, submaximal exercise, can postpone the development of muscular fatigue and enhance performance.[5,11] A common belief arising from this premise is that a high-carbohydrate intake during training will permit an athlete to train harder and longer and thus achieve a superior training response. Accordingly, sport nutritionists and exercise physiologists consistently have recommended that athletes who undertake training that is reliant on muscle glycogen as a primary or limiting fuel source consume a diet that provides high carbohydrate availability.[6]

We purposefully highlight the terminology used here, noting that there has been a subtle and frequently overlooked modification to the current sports nutrition guidelines regarding carbohydrate intake in the athlete's daily diet. Rather than promoting a standardized "high-carbohydrate" intake for all athletes (regardless of whether this is expressed as a percentage of energy intake, total grams, or grams per kilogram of body mass (BM)), the guidelines now promote a sliding scale of intake that is more closely matched to the predicted fuel costs of the athlete's training and recovery.[6] This recommendation is underpinned by the rationale that training sessions should be undertaken with adequate fuel supplies from muscle glycogen and other carbohydrate-based fuels.

However, this standpoint does not consider the question of whether it is a lack or a surplus of substrate that triggers and promotes the training adaptation process. Indeed, the value of high carbohydrate availability for supporting the demands of training has been met by some with skepticism. Such a viewpoint is, no doubt, based on the failure of long-term studies of trained individuals to show clear evidence of superior performance outcomes from high-carbohydrate diets compared with an energy-matched diet low in carbohydrate. However, recent reviews of this surprisingly sparse literature have identified that many of the studies may not have achieved major differences in carbohydrate availability for training needs, certainly in the context of the contemporary definition of recommended carbohydrate intake.[6]

Changes in an athletes' dietary intake and training program that alter the concentration of blood-borne substrates and the hormonal milieu cause large perturbations in the macronutrient storage profile of skeletal muscle and other insulin-sensitive tissues. As such, altering nutrient availability can exert profound effects on both resting energy metabolism and subsequent fuel utilization patterns during training and/or competition, as well as the acute regulatory processes underlying gene expression[3] and cell signaling.[9,32,34] An intriguing study, albeit in untrained individuals, reported superior training adaptation and subsequent exercise capacity after 10 wk of training, incorporating alternate sessions commenced with low carbohydrate availability compared with training with high carbohydrate support.[15] The article (discussed subsequently) coined the term "train low, compete high" to describe this novel training-nutrient approach. It should be emphasized that training with low muscle glycogen content in that study[15] comprised only 50% of the total training load during the intervention period.

"Train low" has now become a catchphrase in athletic circles, as well as in scientific literature; however, we note that this terminology is used to describe both a range of practices other than the original protocol and as a generic or "one-size-fits-all" theme promoted as a replacement to the era of the "high-carbohydrate diet" in sport. We have witnessed firsthand the confusion caused by misunderstood terminology in sports nutrition.[6] Accordingly, we encourage the concept of low and high carbohydrate availability to be promoted. Furthermore, we observe that there are many ways of achieving low carbohydrate availability before, during, and after training sessions that differ in the site of low carbohydrate availability (i.e., endogenous glycogen vs exogenous glucose), in the duration of exposure, the number of tissues affected (i.e., muscle, liver), and the frequency and timing of their incorporation into an athlete's periodized training program (Table). To understand the importance of these subtleties and to examine the support for "training high" or "training low," we provide a brief overview of the results of contemporary studies that have determined the effects of chronically manipulating endogenous (muscle glycogen) and/or exogenous (blood glucose) carbohydrate availability on endurance training adaptation and, where appropriate, performance outcomes. The potential mechanisms underlying the cellular responses that arise from these nutrient-training interactions are discussed in the context of promoting training adaptation.

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