Carbohydrate Availability and Training Adaptation: Effects on Cell Metabolism

John A. Hawley; Louise M. Burke


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

In This Article

Exogenous Glucose Availability and Training Adaptation

Another strategy to alter carbohydrate availability is to alter the exogenous supply of glucose. Glucose supplementation during exercise inhibits whole-body fat oxidation by suppressing plasma free fatty acid (FFA) levels while concomitantly reducing the entry of long-chain fatty acids into the mitochondrion, an effect that persists for several hours after ingestion. Glucose ingestion also has been reported to attenuate the activation of the AMPK during exercise in some,[2] but not all,[21] studies. If AMPK activation is reduced by increasing glucose availability, then a chronic downregulation of the typical exercise-induced rise in AMPK may attenuate the training response-adaptation process. This is because AMPK activation has a putative role in promoting metabolic and mitochondrial enzyme content in skeletal muscle.[17,18]

Akerstrom et al.[1] studied the effects of altered exogenous glucose availability in healthy males during a 10-wk program of leg-knee extensor training. Subjects trained one leg while ingesting a glucose solution (6% weight-volume for an intake of 0.7 g carbohydrate·kg−1 BM·h−1), and ingested a placebo when training the other leg. Training consisted of 2 h of submaximal "kicking," and each leg was trained on alternate days. Although there were training-induced increases in the maximal activities of both oxidative and lipolytic enzymes (citrate synthase and β-HAD), tracer-derived measures of palmitate turnover, and exercise capacity in both legs, the magnitude of improvement was similar, independent of exogenous carbohydrate availability.

De Bock et al.[13] also have investigated whether muscle adaptation is affected by the nutritional status during training sessions. They recruited moderately active males who performed 6 wk of training (3 d·wk−1 for 1–2 h at 75% of V·O2peak) during which workouts were commenced in either a fasted state or 90 min after a carbohydrate-rich breakfast and additional carbohydrate supplementation (1 g·kg−1 BM·h−1) throughout the exercise bout. In agreement with the results of Akerstrom et al.,[1] a variety of metabolic markers (including succinate dehydrogenase (SDH) activity, GLUT-4, and hexokinase II content) were increased by a similar extent with or without carbohydrate supplementation. Despite a significant increase in fatty acid-binding protein after "fasted" training (P < 0.05), rates of fat oxidation during submaximal exercise were not altered by either training intervention. The results from these studies[1,13] suggest that the major adaptations to endurance training are not augmented by reduced exogenous carbohydrate availability.

Contrasting results were reported by Nybo and colleagues,[25] who determined the effects of 8-wk endurance training in previously untrained males who were allocated into either a group that consumed a sweetened placebo during workouts (low carbohydrate availability) or a cohort who received a 10% carbohydrate solution (high carbohydrate availability). They found that undertaking training without exogenous carbohydrate support produced greater enhancement of the increases in resting muscle glycogen, GLUT-4, and β-HAD. Yet despite these metabolic changes, there was an unclear effect on time-trial performance undertaken after 2 h of submaximal cycling, even when this performance session was undertaken without carbohydrate intake. Both intervention groups achieved similar benefits in fat loss, increases in aerobic capacity, loss of intramyocellular lipid, and improved blood lipid profile, whereas only the carbohydrate-supported training group achieved an increase in lean BM.[25] These results suggest that in previously unconditioned subjects, there may be an impact of altering the exogenous glucose supply during training sessions on selected muscular adaptations, but these are without a functional transfer to the many of benefits of training on health and performance parameters.

Recently, we determined the chronic effects of undertaking daily endurance training with either high or low carbohydrate availability during workouts.[10] During a 28-d intervention period, 16 endurance-trained subjects were all fed a standard diet consisting of 5 g·kg−1 BM. Eight subjects were randomly allocated to a high-carbohydrate-intake group (HICHO) and consumed a carbohydrate nutritional supplement (a 10% glucose solution that provided an additional 25 kJ·kg−1 BM of carbohydrate for every hour of training). The other eight subjects (LOCHO) were fed a placebo during training and ingested energy-matched, fat- and protein-rich snacks after training sessions. There were no clear effects of the dietary intervention on resting muscle glycogen or GLUT-4 protein content. However, the maximal activity of citrate synthase increased to a greater extent in LOCHO than HICHO (P < 0.05), whereas tracer-derived estimates of exogenous glucose oxidation were increased only in HICHO (14% vs 1%; P < 0.05). Cycling performance (a 7-kJ·kg−1 time-trial lasting approximately 30 min undertaken after 100 min of steady-state submaximal cycling and performed after the intake of a carbohydrate-rich meal) was improved to a similar extent in both groups after the diet-training intervention, regardless of whether the bout was undertaken with or without carbohydrate intake during the bout. These results suggest that, although there were some differences in the training adaptations arising from altering carbohydrate availability during training sessions, these did not transfer into clear performance differences under the specific conditions of the cycling trials.[10]


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