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

Muscle Glycogen Availability and Training Adaptation

The first modern-day investigation of the effects of reducing endogenous carbohydrate availability on training adaptation and performance was undertaken by Hansen and colleagues.[15] They studied seven previously untrained male subjects who completed a training program of leg-knee extensor exercise 5 d·wk−1 for 10 wk. The subjects' legs were trained according to a different schedule, but the total amount of work undertaken by each leg was the same: one leg was trained twice a day, every second day in which the second training session was commenced with low glycogen content (LOW), whereas the contralateral leg trained daily under conditions of high glycogen availability (HIGH). On the first day of each 5-day training cycle, both legs trained simultaneously for 1 h at 75% of (one leg) maximal power output. After 2 h of recovery, during which subjects refrained from carbohydrate intake, the LOW leg trained again for a further 1 h at 75% of single-leg peak power output. On the second day, only the HIGH leg trained. Muscle biopsies were taken from both legs before and after 5- and 10-wk of training. Submaximal and maximal exercise testing was performed before and after training. Resting muscle glycogen content before training was similar for both groups, but was increased only in LOW after training (P < 0.05). There was a training-induced increase in the maximal activity of citrate synthase in both legs (P < 0.05), with the magnitude of increase being significantly greater in LOW than in HIGH (P < 0.05). Exercise performance (measured as the time to exhaustion at 90% of posttraining maximal power output) was similar for both legs before training (5.0 ± 0.7 vs 5.6 ± 1.2 min for LOW and HIGH, respectively). Noticeably, the magnitude of increase in posttraining exercise time to exhaustion was twice as great for LOW as HIGH (19.7 ± 2.4 vs 11.9 ± 1.3 min; P < 0.05). These results clearly demonstrate that adaptation and endurance performance are augmented by lack of substrate (i.e., muscle glycogen) availability, at least for previously untrained subjects undergoing a short-term training intervention.[15]

To investigate whether well-trained individuals might attain the same benefit as untrained, less fit individuals who undertake a training regimen with lowered glycogen availability, we recruited male cyclists or triathletes who had a history (>3 yr) of endurance training and who were riding 300 to 500 km·wk−1 in the months before study participation.[35] The athletes were divided into two groups (matched for age, peak oxygen uptake [V·O2peak], and training history) and undertook supervised laboratory training sessions during a 3-wk intervention. The control group (HIGH) trained 6 d·wk−1 with 1 rest day (day 7), alternating between 100-min steady-state aerobic training (AT; ~70% V·O2peak) on the first day and high-intensity interval training (HIT; 8 × 5-min work bouts at maximal effort with 1-min recovery in between work bouts at ~100 W) the next day. The AT and HIT session were deliberately chosen, as these workouts deplete ~50% of resting muscle glycogen stores in the fed state in well-nourished, trained subjects (J.A. Hawley, unpublished observations, 2008). The experimental group (LOW) trained twice per day, every second day, performing the AT in the morning to decrease muscle glycogen content, followed by 2 h of rest without carbohydrate intake, and then HIT. During the time between these two training sessions, subjects rested in the laboratory and were given ad libitum access to water. Accordingly, HIGH completed all HIT sessions at a time when muscle glycogen levels were restored, whereas LOW commenced this interval set when muscle glycogen stores were depleted by ~50% of resting values.

The novel findings from the study of Yeo et al.[35] were that in skeletal muscle of already well-trained individuals, resting glycogen content, the maximal activity of citrate synthase, the content of the electron transport chain component cytochrome-oxidase subunit IV (COX-IV), and rates of whole-body fat oxidation during submaximal exercise were all enhanced to a greater extent by training twice every second day (LOW) compared with training daily (HIGH) after the 3-wk intervention (P < 0.05). Although power output during a 60-min time-trial significantly improved by ~11% (P < 0.05) after both training regimens, there was no difference between HIGH and LOW. A notable observation was that self-selected maximal power output was significantly lower (P < 0.05) for the first six interval training sessions for athletes who commenced these workouts with low muscle glycogen content (i.e., the first 2 wk of the training program), but by the third week of the study, there were no differences in average power output whether subjects commenced the workouts with low or normal glycogen stores (Figure).

Figure.

Training intensity (expressed as a percentage of peak sustained power output (ppo)) sustained during high-intensity interval training (hit) sessions undertaken on three occasions per wk during a 3-wk intervention period. Each HIT session consisted of eight repetitions of 5-min work bouts separated by 1 min of active recovery (100 W). The 5-min work bouts were performed at the subjects maximal self-selected power output. (See text for further details of nutrient-exercise manipulation.) Values are reported as mean ± standard error. *Significantly different between train high (HIGH) and train low (LOW). (Reprinted from Yeo WK, Paton CD, Garnham AP, Burke LM, Carey AL, and Hawley JA. Skeletal muscle adaptation and performance responses to once a day versus twice every second day endurance training regimens. J. Appl. Physiol. 2008;105:1462–70. Copyright © 2008 The American Physiological Society. Used with permission.)

Thus, despite a compromised (i.e., lower power output) training capacity (Figure), the twice-every-second-day regimen elicited a comparable increase in endurance performance to that attained after training every day. Yeo et al.[35] proposed that for an athlete unable to train daily but who can perform two workouts in close proximity, with the second session performed under conditions of low starting muscle glycogen, this nutrient-exercise protocol may offer a time-efficient method of maintaining training adaptations and performance.

Using an identical protocol to that of Yeo et al.,[35] Hulston et al.[19] also showed that power output was compromised when trained cyclists commenced HIT sessions with low versus normal glycogen stores. In addition, they reported that tracer-derived measures of fat oxidation during submaximal cycling were greater after low-glycogen training (26 ± 2 compared with 34 ± 2 μmol·kg−1·min−1; P < 0.01), with the majority of this training-induced increase being derived from muscle triacylglycerol oxidation (from 16 ± 1 to 23 ± 1 μmol·kg−1·min−1; P < 0.05). Commencing selected training sessions with low muscle glycogen levels also increased the protein content of β-hydroxyacyl-CoA-dehydrogenase (β-HAD; P < 0.01), but in agreement with the findings of Yeo et al.,[35] these metabolic changes failed to improve cycling time-trial performance. Taken collectively, the results from these studies[15,19,35] clearly demonstrate that, independent of prior training status, short-term (3–10 wk) interventions in which approximately 50% of the number of sessions are commenced with low muscle glycogen levels promote training adaptations (i.e., increases the activities of enzymes involved in energy metabolism and mitochondrial biogenesis, increases rates of whole-body and muscle-derived triacylglycerol oxidation) to a greater extent than when all workouts are undertaken with normal or elevated glycogen stores. However, despite creating conditions that should, in theory, enhance exercise capacity, the effects of this train-low strategy on a range of performance measures are equivocal (discussed subsequently).

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