Sugar & Health

The role of carbohydrates in athletes

  • Body carbohydrate stores provide an important fuel source for the brain and muscle during exercise, and can be manipulated by exercise and dietary intake.
  • A key strategy in promoting optimal performance in competitive events or training is modifying the timing, amount and type of carbohydrate food and drinks according to the demands of the session and the individual needs of the athlete. 
  • Different dietary approaches for optimal sporting performance, for example a ‘low carbohydrate high fat’ diet and carbohydrate ‘periodization’, continue to be explored. However, there is a need for additional and improved evidence to support their widespread use.

Maintaining an optimal diet has many benefits to athletes, including improved and consistent performance, enhanced recovery, maintaining ideal body weight and composition, and a reduced risk of injury. Whether a weekend warrior or professional athlete, carbohydrate rightfully receives a great deal of attention in sports nutrition.

Importance of carbohydrates for exercise:

Key energy source

Carbohydrates provide the body with its first option for energy and are a key fuel for the brain and central nervous system (1). During any type of activity, muscles use glucose from carbohydrate for fuel (1).

Easily digestible

Carbohydrate foods are an easy option prior to exercise. They are generally well tolerated and preferred by athletes, with the ability to be more easily digested compared to fat or protein foods (2). 

Versatile

Carbohydrates can support exercise over a range of intensities due to its use by both anaerobic and oxidative pathways (2). For short and high intensity exercise, muscle and liver stores of glycogen provide the main source of energy (2), which need to be replaced post training sessions. For longer exercise, the extent of carbohydrate utilisation varies depending on intensity, type of training and overall diet (2).

Strong evidence to support carbohydrate availability improves performance

Performance of prolonged, sustained or intermittent high-intensity exercise is enhanced by strategies that maintain high carbohydrate availability (i.e., matching glycogen stores and blood glucose to the fuel demands of exercise) (2).  In contrast, depletion of these stores is associated with fatigue in the form of reduced work rates, impaired skill and concentration, and increased perception of effort (2).

Easily manipulated

The body’s carbohydrate stores are relatively limited and can be acutely manipulated on a daily basis by dietary intake or even a single session of exercise to match the requirements of exercise (2).


Due to the above factors, the low availability of carbohydrate stores can play a major limiting factor for exercise performance. A key strategy in promoting optimal performance in competitive events or training is matching an athlete’s carbohydrate stores with the fuel demands of the session, with a focus on dietary carbohydrate pre, during and post exercise (2). The amount of carbohydrate needs to be sufficient to fuel athletes’ training programmes and optimise the recovery of muscle glycogen stores between workouts.

Carbohydrate requirements for activity:*

Intensity level

Activity Example

Carbohydrate requirements

Timings

Light

Low intensity or skills-based activities – technical

3-5g/kg of athlete’s body weight/d

  • Timing of intake of carbohydrate over the day may be manipulated to promote high carbohydrate availability for a specific session by consuming carbohydrate before or during a session, or during recovery of a previous session.
  • As long as the total fuel needs are provided, the pattern of intake may simply be guided by convenience, individual choice and sport rules.

Moderate

Moderate exercise programme (eg: 1h/d)

5-7g/kg/d

High

Endurance program (e.g. 1-3h/d moderate to high intensity exercise).

6-10g/kg/d

Very High

Extreme commitment (e.g. > 4-5h/d moderate to high intensity exercise).

8-12g/kg/d

*Table adapted from Burke et al (2011): Carbohydrates for training and competition, Journal of Sports Sciences (3). 

Pre-exercise – Fueling 

A carbohydrate rich meal or snack 1-2 hours prior to exercise can provide a good fuel source to meet the demands of general exercise. However, more rigorous fueling strategies are necessary prior to a competition or training session for some exercise modes, in order to provide high carbohydrate availability to maximise the training session and ensure fatigue does not reduce/shorten the session. For prolonged exercise (greater than 60 minutes), there is evidence consuming 1 to 4 g/kg carbohydrate in the 1-4 hours before exercise can enhance endurance and performance (2). In some circumstances there may be additional benefit from higher glycogen stores, achieved by carbohydrate loading. This protocol creates super compensation of muscle glycogen stores by following a higher carbohydrate diet (10-12g/kg every 24 hours) for 2-3 days leading up to an endurance event (2).

In general, foods with a low-fat, low fibre and low-moderate protein content are preferred for a pre-event meal as these are less likely to cause gastrointestinal issues. In athletes who experience pre-event nerves or those with uncertain timetables, a quickly digestible option may be beneficial, such as liquid meal supplements (2).

During Exercise- Performance

During shorter periods of exercise (less than 45 minutes), carbohydrate intake during exercise is not considered to be necessary. However, for longer periods there can be a number of performance benefits including glycogen sparing, provision of an exogenous muscle substrate, prevention of hypoglycaemia, delay hunger and activation of reward centres in the central nervous system (2). In addition, there is new evidence to suggest that “mouth sensing” (or mouth rinsing), which is the practice of providing frequent contact of carbohydrate to the mouth and oral cavity, can stimulate parts of the brain and central nervous system to enhance work output and pacing (2).
The amount, type, and timing of carbohydrate depend on a number of factors including type of event, exercise and the athlete’s preparation. A range of everyday foods, fluids and formulated sports products that include sports beverages may be used. However, it is important to ensure foods and fluids have been well tested by the athlete during training to avoid adverse gastrointestinal discomfort issues (2). In addition, ease of eating may be important in some sporting competitions. Removing foods from hard to open packaging may be beneficial to save time and avoid distraction.

Post exercise – Recovery

The primary goal of post exercise carbohydrate recovery is glycogen restoration. Replacing carbohydrate is important between frequent bouts of exercise. Time plays an important factor in recovery as the rate of glycogen resynthesis is only 5% per hour, therefore an athlete should consume carbohydrate as soon as practical after the first workout (4). Approximately 1 to 1.2 g carbohydrate/kg/h during the first 4 - 6 hours has been proven to be effective in maximising refuelling time between workouts (4). 

Carbohydrate-rich foods with a moderate to high glycemic index is recommended for a post-exercise meal as this provides the most readily available source of carbohydrate for muscle glycogen synthesis (4). A Post exercise meal containing a range of nutrients may assist in other recovery processes and the addition of protein can be beneficial particularly when carbohydrate intake is suboptimal or when frequent snacking is not possible, by promoting additional glycogen recovery (4).

Is there any difference between sucrose and glucose on exercise performance? 
Research to date suggests sucrose appears to be as effective as other highly metabolisable carbohydrates (e.g., glucose, glucose polymers) in providing an exogenous fuel source during endurance exercise, stimulating the synthesis of liver and muscle glycogen during exercise recovery and improving endurance exercise performance (5). While gaps exist in the understanding of the metabolic and performance consequences of sucrose ingestion before, during and after exercise relative to other carbohydrate types or blends, sucrose should continue to be regarded as one of a variety of options available to help athletes achieve their specific carbohydrate intake goals (5). To learn more about the sources and types of carbohydrates and sugars, click here.
 

 

Is there any benefit of a Low Carbohydrate High Fat diet for athletes? 
Low Carbohydrate High Fat (LCHF) diets have received a great deal of attention globally in recent years. Most of the discussion in LCHF diets for sports performance is based on enthusiastic claims and testimonials rather than a strong evidence base (6). Although adaptation to a LCHF (whether ketogenic or not) increases the muscle’s capacity to utilise fat as an exercise substrate, there is no long-term evidence this leads to a clear performance advantage (6). In fact, there is a risk of impairing the capacity for high intensity exercise (6).

 

How does “periodization” of carbohydrate intake effect performance?
There has been increasing interest in a “periodized” approach to carbohydrate availability in the training program, where sessions undertaken to promote adaptation are integrated with others focused on high quality performance outcomes (7). One such sequence of this periodized approach is the “sleep-low” strategy which consists of three stages; 1. A late afternoon high intensity session with high glycogen stores, 2. Withholding carbohydrate to maintain glycogen depletion during overnight recovery; and 3. A low-moderate intensity steady-state exercise session the following morning (7). One week of exposure to this strategy has been shown to be successful in improving performance in trained endurance athletes and could be implemented during the weeks preceding a competition before the taper period (7). Current research on this strategy has inherent limitations and as such, further studies are required to contribute to the evidence base.
 


The ability of diet to improve performance is an area of great interest, with continual emerging research. There is a need for ongoing research and practice to identify a range of approaches to optimal training and competition diets according to the specific requirements of an event and the experience of the individual. The provision of sufficient carbohydrate intake pre-, during and post exercise plays a crucial role in performance (in both training and competition).

For more information, a full set of current guidelines for carbohydrate intakes in athletes during exercise fuelling can be found in the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic performance position paper.

FOR MORE ON: The digestion, absorption and transport of carbohydrates

References:

1. Edited by Mann, J and Truswell A.S. (2002). Essentials of Human Nutrition. Oxford; New York :Oxford University Press.
2. Academy of Nutrition and Dietetics, American College of Sports Medicine, and Dietitians of Canada (2016). Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics. Retrieved from http://www.eatrightpro.org/~/media/eatrightpro%20files/practice/position%20and%20practice%20papers/position%20papers/nutritionathleticperf.ashx
3. Louise M. Burke, John A. Hawley, Stephen H. S. Wong & Asker E. Jeukendrup (2011): Carbohydrates for training and competition, Journal of Sports Sciences, 29:sup1, S17-S27
4. Burke, L.M, Kiens, B & Ivy, J.L. (2004). Carbohydrates and fat for training and recovery. Journal of Sports Sciences, 22, 15–30.
5. Wallis, G.A, Wittekind, A. (2013). Is There a Specific Role for Sucrose in Sports and Exercise Performance? International Journal of Sport Nutrition and Exercise Metabolism, 23, 571-583.
6. Burke, L.M. (2015). Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon? Sports Med. Retrieved from DOI 10.1007/s40279-015-0393-9
7. Marquet, L.A, Hauswirth, et al., (2016) Periodization of Carbohydrate Intake: Short-Term Effect on Performance. Nutrients, 8, 755.

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