By Matt McNamara
As we marvel at the capacity of the elite athlete to ingest and use such storied levels of energy day in and day out, it is also worthwhile to try and understand some of the adaptations that must occur in order to sustain such incredible work volumes. Specifically, what is the role of a high versus low carbohydrate based dietary regimen in facilitating these metabolic changes? Luckily, there are a couple of divergent views on this.
The Low Carb Argument
Over the past fifteen or so years the idea of carbohydrate restricted diets has become mainstream. From Barry Sears and his “Zone” Diet that took hold in the early to mid nineties, to the work of Dr Robert Atkins, and his Atkins diet, cutting carbohydrate intake has been shown time and again to help the general population to better manage weight. Unfortunately, this methodology rarely includes the athletic population, and as such misses a critical question. What is the demand of the individual in question? Sedentary individuals may indeed benefit from a dietary modification that lowers total carbohydrate, but do athletes?
Several researchers and physiologists have looked into the fitness gains seen in a low carbohydrate diet. Dr Phillip Maffetone has written extensively on a low carbohydrate approach to performance training. Specifically he espouses a 40/30/30 diet that is built on percentages of carbohydrate, fat and protein. In general his discussion centers on the insulin response rather than the deep metabolic adaptations that lead to better performance.
In 2009 Nybo et al looked at chronic training responses in two groups during an 8-week managed training program. One group was given additional carbohydrate supplementation, the other was given a placebo. At the conclusion of the study it was shown that while peak power and maximal oxygen uptake increased approximately 17% in both groups, actual resting muscle glycogen increased in only the placebo (no supplemental CHO) group. Nybo et al hypothesized that while CHO supplementation during exercise influences several markers of performance, actual cardiorespiratory endurance and peak power were not positively affected. Unfortunately, their population was untrained.
Similarly, in 2008 DeBock et al investigated the effects on metabolic response in athletes who were fasted prior to an exercise bout. Two groups of moderately trained individuals were trained in 1-2 hour bouts at approximately 75% of peak VO2. Training was undertaken 3 days per week for 6 weeks and groups were broken into either a fasted (no food for 11 hours prior to exercise) or carbohydrate fed state (regular diet of approximately 65% CHO). In addition the CHO group received a high carbohydrate meal 90 minutes before their exercise bout, and approximately 1 gram per kilogram of body weight of CHO solution each hour. Nutrition was managed for each group throughout the study.
Results showed that as a result of the training stimulus both groups improved exercise time to exhaustion and peak VO2 approximately 10% and power at lactate threshold by about 8%. Pre-exercise muscle glycogen content increased in the CHO group, but not in the fasted group. Interestingly the breakdown of muscle glycogen during exercise bout was blunted more in the fasted group than in the CHO group at the conclusion of the 6-weeks. The authors implied that this showed a positive effect in muscle glycogen retention for the fasted group.
Unfortunately, their assertion that exercise in a fasted state promoted free fatty acid oxidation wasn’t established. The authors hypothesized that this was because each group was fed a high carbohydrate diet (approx 65% CHO) in order to maintain body mass (given that the fasted group didn’t ingest CHO in the 11 hours pre exercise), and that this may have masked any potential gains in fatty acid oxidation. This claim was made despite their acknowledgement that in every situation where CHO is more readily available, muscles prefer it to fatty acids. Perplexing.
The High Carb Retort
Gregory Cox, Sally Clark, et al of the Australian Institute of Sport, sought to establish the definitive answer as to the efficacy of daily training with high carbohydrate availability, at elite level volume and intensity. Their recently published study closes an important loop in the quest to understand the role of carbohydrate availability on performance.
In their trial sixteen elite level endurance athletes (cyclists and triathletes) were divided into either a high carbohydrate or low carbohydrate group and then put through a 28 day training protocol designed to mimic real world training volume of between 15 and 20 hours per week. Both groups were matched for total caloric intake, and underwent 5-day testing protocols pre/post training to determine outcomes. A variety of measures were taken to try and get a comprehensive picture of the exercising athlete’s metabolic adaptations.
The high carbohydrate group (High CHO) ingested approximately 60% more daily CHO than the low CHO group (max of 8.5g/Kg and 5.3g/Kg respectively). Both groups averaged a nearly identical volume of training throughout at ~16 hours per week. Pre and post test resting glycogen stores were unchanged between the two groups, but citrate synthase (a marker of metabolic efficiency) levels increased in both groups, but only significantly in the High CHO group (p<0.001).
Substrate oxidation was a core investigative priority. Recall that one tenet of the low carbohydrate approach was an anticipated increase in fatty acid oxidation. While this ultimately proved transient in low carbohydrate research, the Cox et al study showed an increase in fat oxidation in both low and high CHO groups, though only the high CHO group showed a significant difference. Exogenous carbohydrate utilization was much higher in the high CHO group, meaning they were better at oxidizing the CHO they ate during the exercise bout than the low CHO group.
All of these findings ultimately coalesce into a couple of key areas The high volume, highly trained population of athletes given a ready supply of carbohydrate showed marked increases in citrate synthase activity and in the oxidation of glucose consumed during exercise in contrast with a matched, low carbohydrate group. Despite this trend, no clear benefit in endurance performance was established. No difference was found in resting glycogen levels between groups either.
There are generally two trains of thought with regard to carbohydrate availability and the necessary training adaptations that improve performance. One group believes that a low CHO regimen leads to increased fat oxidation and a stunting of carbohydrate use during exercise, thereby retaining said carbohydrate for use later in the exercise bout. The other camp believes that having CHO readily available in ones diet, and supplementing with exogenous carbohydrate during exercise leads to better performance.
In a recent study from the Journal of Applied Physiology, Cox, Clark, et al demonstrated that a high intensity training bout of 28 days that included readily available carbohydrate for daily nutrition and for uptake during the exercise bout proved to be more beneficial with regards to citrate synthase activity and, most importantly, exogenous carbohydrate oxidation. So, in short, elite level athletes should consume a high carbohydrate diet and supplement their daily exercise regime with additional carbohydrate stores to try and maximize performance
Gregory R. Cox, Sally A. Clark, Amanda J. Cox, Shona L. Halson, Mark Hargreaves, John A. Hawley, Nikki Jeacocke,1 Rodney J. Snow, Wee Kian Yeo, and Louise M. Burke; Daily training with high carbohydrate availability increases exogenous
carbohydrate oxidation during endurance cycling. J Appl Physiol 109:126-134, 2010.
Nybo L, Pedersen K, Christensen B, Aagaard P, Brandt N, Kiens, B. Impact of Carbohydrate Supplementation During Endurance Training On Glycogen Storage and Performance. Institute of Exercise and Sport Sciences, University of Copenhagen, Copenhagen Denmark. Acta Physiology 197:117-27, 2009.
K. De Bock, W. Derave, B. O. Eijnde, M. K. Hesselink, E. Koninckx, A. J. Rose, P. Schrauwen, A. Bonen, E. A. Richter, and P. Hespel. Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake
J Appl Physiol 104:1045–55, 2008.
About Matt McNamara: Matt is a USA Cycling Level 1 coach with over 20 years of racing, coaching and team management experience. He is the founder and president of Sterling Sports Group, a performance coaching company focused on creating a seamless interface between athlete and coach, technology and personal attention. Learn more by visiting his website at www.sterlingwins.com.