Carbohydrate availability and training adaptation: Effects on cell metabolism (A review)

 
Journal Title (Medline/Pubmed accepted abbreviation): Exerc. Sport Sci. Rev.
Year: 2010
Volume: 38
Number: 4
Page numbers: 152-160
doi (if applicable):

Summary of Background and Research Design

Background:Historically, athletes have been advised to consume high carbohydrate diets during both training and competition to ultimately improve performance. However, recent evidence indicates that training in a carbohydrate-depleted state results in a compensatory increase in the production and/or activity of several key enzymes involved in both carbohydrate and fat metabolism. A new theory, the “train low, compete high” theory, has been developed on the premise that performance can be improved by first elevating the activity of carbohydrate and fat metabolism (via carbohydrate depletion) and then consuming a high carbohydrate diet during competition to take advantage of these metabolic adaptations. The authors of this article review the evidence for these metabolic adaptations and if these adaptations have the potential to improve physical performance.

Summary of research findings:
The authors described a key study from another research group in which 7 untrained subjects underwent a 10-week knee-extensor exercise training program. The subjects exercised 5 days per week. One leg was exercised 1 h per day every day at 75% of max power. This was the condition of high carbohydrate availability (HIGH). The other leg was exercised twice per day, every other day, at 75% max power with no carbohydrate intake between training sessions. Thus, the second exercise session was always performed with glycogen stores somewhat depleted (low carbohydrate availability, LOW). The total workload was the same for each leg over the 10-week period. The LOW training pattern was associated with higher muscle glycogen levels after training, higher activity of citrate synthase (an enzyme in the citric acid cycle), and almost 2-fold longer exercise time to exhaustion vs. the HIGH pattern. Similar studies from other groups, this time in trained individuals, also reported greater metabolic adaptation under low carbohydrate availability, including the following: increased citrate synthase activity, increased content of the electron transport chain component cytochrome-oxidase subunit IV, increased fat oxidation, and increased content of ß-hydroxyacyl-CoA-dehydrogenase (the initial enzyme in beta-oxidation of fatty acids). However, these studies and several others did not observe improvements in performance associated with these types of metabolic adaptations.

There are several important questions that remain to be answered before “train low, compete high” can become an accepted training technique. First, there has to be an established link between these metabolic changes and actual, real-world performance increases. Second, it is not clear how much carbohydrate deprivation during training is necessary to achieve the optimal metabolic adaptations. Third, it is known from the studies described above that power output and/or training intensity goes down while training in the carbohydrate-depleted state. This would not be advantageous to highly competitive athletes who believe that their intensity during training is a direct reflection of the intensity at which they will be able to compete. It is unclear if the beneficial metabolic adaptations induced by training low would outweigh the potential negative effects such an approach would have on the intensity of day-to-day training.

Interpretation of findings/Key practice applications:

The “train low, compete high” hypothesis has some physiological support and does not appear to be dangerous, although it might negatively impact training quality. The evidence for metabolic adaptations to training low has been demonstrated pretty consistently in a number of studies. However, these metabolic adaptations, in general, have not resulted in increased physical performance.

Limitations of the research:

There are some potential explanations for the disconnect between clear metabolic adaptation to training low and actual measured improvements in physical performance. The authors suggest that one possibility is that factors other than metabolic adaptation (e.g., the function of the central nervous system and mental attitude) may be more important for improving performance. It is also likely that current research tools for evaluating performance increases in the laboratory may not be sensitive enough to capture changes that, while not statistically significant, could have a real impact on whether or not an athlete wins a race. A time improvement of 5 seconds might not show up as statistically significant, but that same 5 seconds could separate the winners and losers in a race. Given the present state of the science, however, there is not yet enough evidence to suggest that the positive metabolic adaptations to training low will counteract potentially adverse outcomes of training low and, ultimately, improve performance.
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