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Carbohydrates and Athletic Performance

Carbohydrates and Athletic Performance

oes it seem like every new recipe you see is low carb? Carbohydrates have taken on a negative reputation lately reminiscent of when fat was demonized because “eating fat makes you fat”. Just as with fats, carbs don’t make you fat. In fact, we need carbohydrates for our brain and body just to function normally. For athletes, carbohydrates can make the difference between average performance and great performance. This (semi) brief overview aims to provide insight on how carbohydrates impact athletic performance and how to use them to support your own performance.
First, it is important to understand a little bit about carbohydrate metabolism and utilization. When carbohydrates are consumed, the body breaks them down to the smallest molecular form, glucose. Glucose is transported by the blood stream to organs and muscles to provide fuel for your body. The brain is the highest glucose-consuming organ and is essentially powered exclusively by glucose. Glucose that is not used for energy is stored as glycogen in the liver and muscles. After glycogen stores have been filled, any remaining glucose is converted to triglycerides, or fat, as energy reserves. The body stores much more energy in the form of fat than glycogen, with about 1,500 calories from glycogen and >80,000 calories stored as fat on average. The blood stream is another place that you will find glucose in the body, and there it is referred to as blood sugar or blood glucose. However, there is only a very small amount normally present in the blood. 

One of the main purposes of carbohydrates in an athlete’s diet is to maintain and restore glycogen stores, as glycogen is extremely important when it comes to athletic performance. Energy from glycogen is a major fuel source for activity, generally with more glycogen used with higher intensity work. If glycogen levels are low or suboptimal, performance can suffer. It has been well established that decreased glycogen availability can result in quicker time to fatigue in both endurance and high intensity exercise. Reduction in work rate, concentration abilities, and skill, and an increase in perceived effort given are some other ways in which inadequate glycogen stores may negatively impact athletic performance. 
There are a number of ways in which an athlete can use nutrition, namely carbohydrate intake, to maximize glycogen stores to support high quality performance. Daily carbohydrate consumption is the first thing to consider. Intensity, frequency, and duration of exercise are important factors when determining how much carbohydrate an athlete needs in their diet to support their training and competitions. Expressing carbohydrate needs in terms of g/kg bodyweight is favored over the traditional % of total calories consumed when working with an athletic population.

 Daily Carbohydrate Intake Recommendations for Athletes Adapted from  Burke et al. 2011  

Daily Carbohydrate Intake Recommendations for Athletes
Adapted from Burke et al. 2011 

hese guidelines for carbohydrate intake make it clear that athletes of different sports will have very different carbohydrate needs to fuel their training. For example, a 75 kg (165 lb.) golfer consuming 3-5 g/kg carbs per day would have a daily carb intake of 225-375 g carbohydrates. A 100 kg (220 lb.) football player participating in two-a-days and aiming for 6-10 g/kg carbs would consume 600-1000 g carbohydrates per day.
To determine the appropriate amount of carbohydrate within these large ranges, the athlete, or even better a RD, will need to consider a couple factors. A very high carb intake may prevent an athlete from having enough room in the diet for protein and fats, so the other macronutrients needs must be factored in. The schedule or preferences of an athlete can effect how many grams of carbohydrates are actually realistic for them to consume. The need to lose or gain weight can influence how much carbohydrate an athlete can afford to fit into their diet. If there is space in the diet to be flexible with carbohydrate intake, an athlete can see how various amounts of carbs within the guideline ranges effect energy levels and performance.

Carbohydrates Around Training
After establishing how much carbohydrate an athlete should have throughout the day, focusing on manipulating carbohydrate intake in the time around a training session or event is the next step in enhancing performance, as well as recovery. Nutrition around activity is generally divided into before, during, and after the event. About 1-4 hours prior to activity is considered the before timeframe. Around 3-4 hours out from exercise, a carbohydrate rich meal is advised. The meal should also contain a protein source, and should not be too high in fiber or fat due to the slow digestion of fat and fiber that could possibly lead to gastrointestinal discomfort. Around 1 hour from the event, a carb rich snack can be consumed to “top off” muscle glycogen. Daily fueling and carbohydrate intake before exercise aims to ensure that glycogen stores are as full as possible leading into physical activity.
While pre-workout nutrition is similar for most athletes, carbohydrate guidelines during exercise vary greatly. Glycogen availability is one of the most influential factors affecting the need for carbs during activity. Muscle and liver glycogen stores can supply enough energy for about 90-120 minutes of continuous, strenuous activity. When glycogen runs low or runs out, carbohydrates must be supplemented to avoid a decrease in performance. If carbs are not consumed, the body will begin to rely heavily on fatty acid oxidation to supply energy for activity. Because the oxidation of fat produces energy (ATP) at a slower rate than the oxidation of carbohydrate, exercise intensity must decrease to match the energy supply rate when the body runs out of glycogen/carbs to burn. Providing carbs during exercise can help to maintain a high carbohydrate oxidation rate allowing an athlete to sustain a higher intensity of activity.
So, how much carbohydrate should an athlete consume to support performance when they are exercising long enough to deplete glycogen? The amount of carb that can be utilized and burned for energy is limited by how much carbohydrate can be intestinally absorbed. About 60 g glucose per hour is thought to be the maximum amount that the body can absorb from food in the intestines. However, intestinal absorption rates can be increased about 75% when multiple types of carbohydrates are consumed. A 2008 study by Jeukendrup et al. determined that when carbohydrates are consumed in the ratio of 2:1 glucose:fructose, 80-90 g total carbohydrates per hour can be absorbed. Will eating that much carbohydrate translate to optimally enhancing performance though?

2010 study had 51 athletes complete cycling activities while consuming beverages containing various concentrations of carbohydrates in the form of 2:1 glucose:fructose. The athletes cycled for 2 hours and then immediately performed a 20 km timed trial. Performance enhancement was determined to be greatest with consumption of 60-80 g carbs per hour. This amount of carbohydrate can be considered appropriate for athletes of any size as carbohydrate oxidation is not related to body mass.
Carbohydrate has been well established as an effective ergogenic aid during endurance exercise, but can carbs benefit performance of shorter duration and/or higher intensities? Based on what has been discussed so far, it would seem that exogenous (consumed) carbohydrate would not have much effect in these situations where glycogen is not necessarily being depleted. However, research has shown otherwise. Conflicting data and theory led researchers to hypothesize that a non-metabolic mechanism may be at play. A group of researchers conducted 2 studies to validate their hypothesis. One study determined that a glucose infusion (IV) was unable to increase exercise performance during a 1 hour timed cycling trial. The other study showed performance enhancement with a carbohydrate mouth rinse during the same cycling trial.
These and several subsequent studies have confirmed that there are oral carbohydrate receptors that influence parts of the brain associated with reward and motor control. Although this mechanism is not yet fully understood, it is clear that carbohydrates can benefit performance through effects on the central nervous system. This knowledge has lead to guidelines advising that a carbohydrate mouth rinse or ingestion of small amounts of a carbohydrate containing food or beverage can enhance high intensity athletic performance during events lasting approximately 30-75 minutes. Research has failed to show any performance benefit of carbohydrate ingestion during activity lasting less than 30 minutes, and therefore carbs are not necessary in these shorter events. 

thletic performance is certainly not limited to speed or distance achieved. The mental capacity of an athlete is of great importance as well. This is especially true in team sports where cognitive and motor skill performances are crucial to overall performance. Carbohydrate consumption before and during intermittent exercise representative of team sports has generally been shown to improve such abilities as motor skill, agility, and coordination.
Carbohydrate supplementation is able to improve athletic performance during exercise through multiple mechanisms. Consuming carbs during exercise of at least moderate intensity and duration can decrease the rate of glycogen utilization, provide a fuel source when glycogen has been depleted, maintain high carbohydrate oxidation rates, supply glucose to prevent low blood glucose levels, and influence the central nervous system to improve mental and physical performance. The chart below provides guidelines for carbohydrate ingestion for optimal performance of moderate to high intensity exercise of various durations.

 Guidelines for Carbohydrate Intake During Exercise Adapted from  Jeukendrup 2011  and  Burke et al. 2011

Guidelines for Carbohydrate Intake During Exercise
Adapted from Jeukendrup 2011 and Burke et al. 2011

fter an exercise session has finished, carbohydrates are again of great interest to athletes now looking to recover effectively. Depending on the intensity and duration of the activity, an athlete may have experienced partial to complete glycogen depletion. Consuming carbs soon after exercise allows athletes to maximize their time for glycogen replenishment and recovery. Glycogen storage rates are highest within 1-2 hours after activity, so carb intake during this time is essential for efficient muscle recovery. This is especially important for athletes who have another competition or training session within 8 hours. Failure to consume carbohydrate right after the first event prevents the muscle from optimally restoring glycogen levels and can lead to impaired performance in the second event. When there are 8-24 hours between exercise it is not as essential to eat carbs in the 1-2 hour post-event window because the extended recovery time allows the body to replenish glycogen stores as long as the diet contains an adequate amount of carbohydrates.
How much carbohydrate should you have to replenish glycogen after strenuous activity? To maximally stimulate glycogen storage within a few hours of exercise, about 1.2 g/kg carbs per hour should be consumed. This can be done through larger meals, or a few snacks to meet these guidelines. However, when protein is ingested along with the carbs, less carbohydrate needs to be consumed to reach peak glycogen storage rates. A combination of .4 g protein/kg and .8 g carbohydrates/kg was shown to be as effective as just 1.2g/kg carbohydrate. For more in depth info on carbs and recovery nutrition, see here.

e have been talking lots about how moderate to high carbohydrate intake can support optimum performance and recovery, but are there any times when low carb intake may be of benefit to an athlete? In some circumstances, yes. Research over the last several years has presented a novel carbohydrate centered approach focused on adaptation to exercise rather than optimizing performance of exercise. Studies examining training with low glycogen or carbohydrate availability have found resultant increases in enzymes involved with carbohydrate metabolism and an increase in metabolic adaptations that could boost fat metabolism during exercise. This “train low” concept suggests an athlete can enhance the cellular response and adaptation to exercise by performing activity with decreased carbohydrate or glycogen stores, but still advises adequate carbohydrate and glycogen availability when high quality performance is required. This type of carbohydrate periodization, while clearly beneficial on a cellular level, has so far not translated to any enhancements in performance compared to a regular, adequate carbohydrate diet. “Train low” presents a new, intriguing approach to gaining benefits from exercise. However, until further research elucidates more insight on how this nutritional strategy can positively impact real world performance, “train low” guidelines and recommendations cannot be determined.
While “train low” is based on periodization of low carb intake, some athletes are choosing to go low to no carb all of the time. It may seem counterintuitive that athletes would be able to perform without any carbs to provide fuel, but there is a growing body of research providing support for a low carb diet for some athletes. This topic is a whole other field of study that will be discussed at a later time. If you want to dig in to this subject now, start here. 

arbohydrates and exercise performance is a complicated and extensively studied area of sports nutrition. Although I by no means have covered this subject 100% here, I hope the information presented leaves you with a better understanding of how carbohydrates impact physical activity and how you can use them strategically to enhance your athletic performance. If you want to take a more in depth look at some of the topics discussed, I highly recommend learning more from the links throughout this post. Thanks for reading!

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