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Carbohydrate Intake for Endurance Training: Redefining Traditional Views
Roger Vaughan, M.S. & Len Kravitz, Ph.D.

Article Reviewed:
Burke, L.M., Hawley, J.A., Wong, S.H.S., and Jenkendrup, A.E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(Supp 1), S17-S27.

Nutrition for athletic performance is emerging as an essential preparation component for all levels of athletes, including the novice exerciser to the Olympic competitor. There are several event-specific methods to adjust the diet of a recreational athlete to enhance performance, of which carbohydrate manipulation is the most common. Carbohydrate intake and availability within the body plays a central role in athletic performance, especially for endurance athletes. Carbohydrate consumption, type, timing and amount are decisive factors for glycogen (stored form of glucose in muscle and liver) replenishment and utilization in exercise performance, and are explored further in this column

Introducing the Concept of “Carbohydrate Availability”
Estimating carbohydrate needs of a recreational athlete can prove complicated because it is dependent on the individual's training schedule, volume of training, and type of competition (triathlon, half marathon, marathon, iron man, etc). The exerciser's food preferences and carbohydrate tolerability should also be considered. Currently, Burke et al. (2011) believe that maintaining ample “carbohydrate availability” in the body for the demands of exercise is most important when designing recommendations. Recreational athletes who just consume “high carbohydrate” diets defined by the percent of carbohydrate calories to total energy intake do not consistently meet carbohydrate needs. Burke and colleagues continue that “carbohydrate availability” suggests that recreational athletes should focus on the amount of carbohydrate within the body 'available' for the specific type of exercise session as well as the post-exercise carbohydrates (to be stored as glycogen) needed during recovery. General carbohydrate fueling and refueling guidelines to meet exercise demands are provided in Table 1.

Table 1. Daily Recommendations for Carbohydrate Consumption for Fuel and Recovery*
Moderate-to-High intensity 1-3 hr/day
6-10 grams/kilogram/day
Include a mixture of protein and carbohydrate meal combinations immediately post-workout to assure optimal glycogen storage and cell maintenance/repair

Moderate intensity up to 1 hour/day
5-7 grams/kilogram/day
As long as total energy fuel needs are being met, the pattern of intake may be by client choice and convenience

Low intensity under 1 hour/day
3-5 grams/kilogram/day
Goal is for carbohydrate intake to maintain blood glucose levels during exercise and replace depleted muscle glycogen stores for faster recovery
*Fine-tune these recommendations individually with clients.
Adapted from Burke et al. (2011)

Acute Manipulation of Carbohydrate Status
To promote sufficient carbohydrate availability during competition, many recreational athletes traditionally have chosen a 'carbohydrate loading' strategy. Common carbohydrate loading practices include a 3-4 day tapering of carbohydrate restriction followed by increasing carbohydrate intake for 2-3 days prior to the event. It now appears that the initial 3-4 days of depletion is NOT required for increased glycogen storage and carbohydrate availability (Burke et al., 2011). In fact, high glycogen storage concentrations have been achieved following as little as 24 hours of rest with high carbohydrate intake (Burke et al., 2011). It is also important for the exercise professional to inform her/his client that carbohydrate loading doesn't always translate into an increase in athletic performance. Competition performance is multi-factorial involving physiological, metabolic, nutritional and psychological components, and thus a compilation, integration and manipulation of several factors.

Carbohydrate Types for Performance: Using The Glycemic Index May Help
Rather than carbohydrate loading, some recreational athletes focus on choosing carbohydrate type and timing of intake. For example, many recreational athletes may choose carbohydrates based on their glycemic index (GI) (See Side Bar 1), such as low glycemic foods/drinks prior to exercise and high glycemic foods/drinks during and following exercise. The purported main benefit of such consideration is to achieve better maintenance of plasma glucose concentrations during and following exercise (Burke et al., 2011). However, there is inconsistent evidence that consumption of low GI foods prior to exercise results in improved athletic performance (Burke et al.). And, Burke et al note that carbohydrate intake during exercise may offset the effect of the pre-exercise low glycemic carbohydrate intake.

Side Bar 1. What is the Glycemic Index?
The GI is a numerical ranking system used to measure the rate of digestion and absorption of foods and their resultant effect on blood glucose. A food ranking high on the GI produces a large, momentary spike in glucose after it is has been consumed. By contrast, a food with a low GI causes a slower, sustained rise in blood glucose. Jenkins and colleagues (1981) established the GI as a way to classify carbohydrate-containing foods for improvement of glucose control for diabetics. They had subjects consume 50 grams of food and monitored blood glucose response for 2 hours. This response was then compared to 50 grams of a reference food, either glucose or white bread. GI scores are classified as either low (below 55), medium (56-69), or high (greater than 70). More recently, extensive GI tables have been developed (Atkinson et al., 2008). It should also be noted that the GI response for any food may vary significantly between individuals, so it's important for personal trainers to have clients test foods for themselves to determine their effect.

Carbohydrates During Exercise
Carbohydrate consumption during exercise competitions (>1 hour) has become a common practice to increase performance. Indeed, consistent carbohydrate intake during sustained exercise maintains blood glucose while sparing glycogen and muscle catabolism (breakdown), however there is no one size fits all approach to carbohydrate intake during exercise. During events lasting longer than 60 minutes, carbohydrate intake during exercise consisting of 30-60 g/hour is recommended, accompanied by adequate hydration. Consumption of greater than 60 g/hour may lead to gastrointestinal upset. Presently, there are numerous sports drinks, energy gels and bars that are designed to meet competitor's needs during exercise, many of which encompass multiple types of carbohydrates. Concurrent ingestion of a product containing a variety of carbohydrates (i.e., sucrose, fructose, glucose, maltodexin) may lead to optimal carbohydrate fuel utilization during exercise (Burke et al, 2011).

Avant-Garde New Endurance Training Concept: Train low and Compete High
Some of the adaptations to muscle include increased mitochondrial (ATP synthesis factory) size and number, enzyme activity (to breakdown foodstuffs), and structure of some muscular contractile components (which improve contractility). Interestingly, decreased cellular energy results in a muscular response to increase mitochondrial size and number, which eventually translates into increased aerobic performance (Burke et al., 2011). Burke and colleagues continue that subjects exercised with lower glycogen stores or under fasting conditions often experience greater increases in mitochondria and enzymes controlling fatty acid and carbohydrate catabolism following similar exercise. These observations have led to a new concept being developed known as train low/compete high. Competitive athletes following this new paradigm of consumption train with lower carbohydrate availability, and switch to higher carbohydrate availability prior to competition. However, it is a misconception that in order for train low/compete high to work athletes must adhere to a low carbohydrate diet; rather, it is encouraged that athletes simply train under depleted circumstance and then meet carbohydrate needs throughout the remainder of the day. Importantly, more research is needed in this area so as to eventually provide healthy guidelines and recommendations for serious client athletes.

Summary Thoughts: Complexity of Carbohydrate Consumption
The complexity of nutrient type and timing for exercise performance is vast and dependent on an enormous amount of variables. There is much that remains unknown about ideal training strategies. For competitive recreational athletes, it is best for them to always remember their needs vary from others because of their body size, age, gender, level of competition, and type of athletic event. Indeed, because there are such great differences between various sports and competitions, it is constructive for recreational client athletes to experiment with carbohydrate intake strategies and develop personalized preferences based on food likes and tolerance. It is also encouraged that client athletes consider working with Registered Dietitians and/or Certified Specialists in Sports Dietetics to best design dietary plans that meet both performance goals and nutrient needs throughout the day.

Additional References:
Jenkins, D., Wolever, T., Taylor, R. Barker, H., Fielden, H., Baldwin, J., Bowling, A., Newman, H., Jenkins, A., and Goff, D. (1981). Glycemic index of foods: a physiological basis for carbohydrate exchange. American Journal of Clinical Nutrition, 34: 362-366

Atkinson, F.S., Foster-Powell, K., Brand-Miller, J.C. (2008). International tables of glycemic index and glycemic load values: 2008; Diabetes Care, 31(12), 2281-2283.


@bio:Roger Vaughan, M.S., is a doctoral candidate in the Exercise Science program at the University of New Mexico. His research interests include chemical and behavioral stimulators of mitochondrial biosynthesis, biochemical dynamics and detection of metastatic prostate cancer, and exercise in sports performance and disease prevention. He loves his new wife, their dogs and recreational bodybuilding.

Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, where he won the Outstanding Teacher of the Year award. He has received the prestigious Can-Fit-Pro Lifetime Achievement Award and was chosen as the American Council on Exercise 2006 Fitness Educator of the Year.