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Carbohydrates for Endurance Reviewed and Updated by ERB member Bob Seebohar, MS, RD, CSSD, CSCS, Sport Dietitian for the US Olympic Committee. Intro: Energy for endurance exercise is fueled primarily by fats and carbohydrates, with carbohydrate utilization increasing as the intensity of the exercise increases. Thus, carbohydrates are crucial to competitive endurance exercise performance. In addition to food based carbohydrates, there are many different energy supplements marketed for sport, which are available in a variety of forms. These carbohydrate supplements are available as a result of demand based upon experimental and research based evidence, but understanding when your body needs carbohydrate in which amount depends on three key areas: capacity, conversion and type. Capacity: At rest, the human body typically has enough carbohydrates to fuel 3 hrs of exercise at a rate of 10-12 kcal/minute (600-700 kcal/hour) which includes blood, muscle, and liver glycogen stores totaling 1,520 to 2,020kcal. The conversion of carbohydrates to energy is highly efficient compared to fats and protein. Thus, carbohydrates are a great fuel source, but our storage capacity, even with training, is generally insufficient to meet the demands of competitive endurance sports. Conversion: The ability to rapidly replenish carbohydrate stores after training, and the ability to consume and convert ingested carbohydrates into a usable form of carbohydrate is important in allowing you to train and compete at the your best. Ingesting the wrong carbohydrates at the wrong time, or ingesting too little carbohydrate can impair performance both in the short term and long term. Consuming a slowly digested carbohydrate during times where the body is at, or above threshold can lead to disaster. During times where you exercise or race at and above your threshold, your blood circulation is focused on the working muscles and away from the stomach. This makes digestion of foods difficult. In fact, consuming a slowly absorbed sugar during these times will slow gastric emptying (the emptying of fluids and foods from the stomach to the blood stream) and in essence block fluids from being absorbed. This can actually cause dehydration. The amount of dietary carbohydrate needed to fuel daily training is equally important to the type of carbohydrate that an athlete eats. Unfortunately glycogen stores in both the liver and muscle as well as blood glucose are limited. These stores are often substantially lower than the fuel requirements of many athletes daily training programs. Therefore eating adequate amounts of carbohydrate throughout the day and surrounding endurance training is critical to performance. IE: You are in the middle of a long (3 hour) workout and feel good because you are consuming plenty of carbohydrates and fluids during this time. In the latter half of your workout you notice the urge to constantly urinate. You think this is good as it must mean you are properly hydrated. Soon you begin to cramp and your performance dramatically goes down-hill. What actually happened is that your body was not able to properly absorb the slower digested carbohydrates which decreased gastric emptying and caused dehydration and electrolyte imbalance. Eating the right carbohydrates at the right time is critical. Note: during workouts performed well below threshold there is still enough blood going to the stomach where absorption is considerably easier. Type: The biochemical structure of the carbohydrate, the absorption process, the size of the food particle, the degree of processing, the variety/origin of the food, the contents and timing of the previous meal, and the co-ingestion of fat, fiber, or protein affect a carbohydrates absorption and glycemic index. (Guezennec, 1995). Trial and error is the only real way to test a product or foods digestibility. Below are some guidelines to further educate you. What is the glycemic index and glycemic load?Based on a 50g portion size, the glycemic index (GI) of a carbohydrate represents the magnitude of the increase in blood sugar that occurs after ingestion of the carbohydrate. Glycemic index does not define the portion size of the carbohydrate meal ingested (whether the portion size is 5g or 500g GI is not affected), but portion size can affect blood sugar. Carbohydrates with a higher GI are easily digested and cause a higher rush of sugar into your blood than carbohydrates with a low GI. Elevated blood sugar causes insulin to be secreted to help modulate the sugar and a subsequent sugar crash follows. When using the GI, it is important to remember that it is a measure of single carbohydrate sources. A complete list of GI for foods can be found at http://diabetes.about.com/library/mendosagi/ngilists.htm Glycemic load (GL) is the measure of the GI multiplied by the total carbohydrate content. This provides a more practical and accurate determination of blood sugar response. Glycemic Load is the glycemic index numerical value divided by 100 and multiplied by its available carbohydrate content (in grams). GL takes the glycemic index into account, but is based on how much carbohydrate is in the food or drink tested. Glycemic load is numerically lower than the glycemic index of a food or drink. To illustrate what I just explained, here is an example.
In many cases, GL is not based on a typical amount of food eaten, so GL does not provide realistic information, unless the food is weighed prior to consuming it. The important thing to take from GL is that it provides an understanding of the relationship between a specific amount of food and its biochemical response. Here are the criteria that classify food into their respective GI and GL values:
Although the glycemic index may have implications for the athlete surrounding training, it is not a universal system to rank carbohydrate rich foods and it becomes more confusing once glycemic load is taken into consideration. A number of other attributes of foods may be of value to the athlete like nutritional content, gastric comfort, or palatability. In addition, a variety of factors affect the GI and GL of different foods such as degree of processing, presence of fructose or lactose, presence of fat or protein, and the amount of food eaten. Furthermore, individuals can have a different glucose response to the same food. Recent data demonstrates that the there is more to the metabolic response of common breakfast cereals than just the glycemic index (Schenk et al., 2003). It will be interesting to see further research on this topic and its application to athletes. Why is glycemic index important? From the corresponding graph (during rest, not exercise) it is clearly evident that the GI can affect your primary fuel supply quite dramatically. Consuming a high GI carbohydrate immediately before a race or workout can cause your blood sugar to spike, then quickly fall below optimal levels, while a low GI carbohydrate can help stabilize your energy release. Your body’s physiological response to carbohydrates differs considerably before, during and after training, making it critical to choose the appropriate fuel for the appropriate response.  How do sugars differ? Conventional wisdom says that since all carbohydrates are eventually digested and absorbed as glucose, the original food source of the sugar, whether a bean or a candy bar, matters little. Sugar is sugar. Sucrose is sucrose. Not exactly! Fructose has a GI of 20±5 and is a simple sugar (monosaccharide) like glucose and galactose. Natural sources of fructose include honey and fruits. Fructose is 75% sweeter than glucose and is generally found in honey and fruits in addition to its many uses as a food-sweetening additive. It is absorbed more slowly into the bloodstream than straight glucose and sucrose and, therefore, at rest it has a less erratic effect on blood sugar levels. Individuals with diabetes or those that are very sensitive to changes in blood sugar find fructose to be advantageous. But, as a result of its slow absorption, beverages that contain fructose can cause gastric upset and slow gastric emptying. Research suggests that fructose is more tolerable when combined with sucrose and glucose and at lower amounts (fructose is typically the secondary sugar). Avoid beverages that list "high fructose corn syrup" as primary ingredients as they will slow fluid uptake and not provide optimal sugars to support exercise energy requirements. As a pre-exercise meal, or between workouts, fructose may be a good source of carbohydrates. Galactose, a simple sugar (monosaccharide) and the main carbohydrate in milk, was made popular in the athletic market with its introduction into a sports drink, G-Push. This drink no longer exists but it did claim that galactose was more quickly absorbed without an increase in insulin release. However, this is not confirmed with clinical research. Lactose is the primary sugar in dairy products and is composed of one molecule of glucose and one of galactose. Because of its galactose content, it is more slowly absorbed into the bloodstream than pure glucose and is therefore more blood-sugar-friendly. Glucose: In terms of immediate use of carbohydrate within the body, glucose (a monosaccharide) with a GI of 99±3 is the most important. Glucose can be directly absorbed by the small intestine and directly transported to the cells to be metabolized. Glucose can also be stored as glycogen (chains of glucose) within muscles and the liver, and can also be converted to fat for energy storage. During exercise, consumption of glucose allows the body to maintain an adequate supply of carbohydrate for metabolism. Glucose is often called dextrose when it is added to foods. The body eventually breaks down all sugars and carbohydrates into glucose, which is the form in which sugar enters cells to be used for energy. During times of exercise at or above threshold, glucose can be easily digested. Sucrose, a disaccharide commonly known as table sugar, has a GI of 68±5 and is composed of one molecule of glucose and one molecule of fructose. It is usually made from refining extracts of sugar beets or sugar cane. Maltodextrin, otherwise known as glucose polymers or complex carbohydrates, has a GI of 85±15 and is composed long chains of simple sugars. Much research has focused on maltodextrin as a superior fuel during endurance exercise, which is true, only under certain criteria. Maltodextrin has superior oxidation rates when compared to any simple sugar. Research by Asker Jeukendrup clearly shows that mixing maltodextrin with simple sugars promotes even higher oxidation rates than any single source (see recommendations). Lactose GI=46±2 Maltose GI=105±12 Honey GI=55±5 Gatorade® GI=78±13 Research updates: The maximal absorption rate for glucose in the intestine has been estimated to range from 1.2-1.7 grams per minute (72-102 grams per hour) at rest. It has been suggested that intestinal carbohydrate absorption is a limiting factor for exogenous carbohydrate oxidation when large amounts of single carbohydrate are consumed during exercise. Recent research indicates that consuming a mixture of sugars (glucose and fructose) as opposed to a single sugar source at a rate of 2.4 grams per minute (144 grams per hour) produces a higher oxidation rate, reaching peak levels of approximately 1.75 grams per minute (105 grams per hour). This data provides support that if high exogenous carbohydrate oxidation rates are needed during exercise, the body may be able to oxidize a greater amount of carbohydrates when the carbohydrates consumed are in a higher amount and from combined sources of sugars as opposed to single sugars. (Jentjens et. al. 2004, 2005, 2006; Currell & Juekendrop, 2008). Much research has focused on carbohydrate drinks and foods during exercise to slow the depletion of the body’s carbohydrate stores and thus delay the onset of fatigue. Exercise-induced elevation in epinephrine depresses the release of insulin from the pancreas. Thus, concerns about carbohydrate feedings increasing insulin and depressing fatty acid availability is less likely to occur when carbohydrate is fed during exercise. Exercising at or above threshold can dramatically reduce your body’s ability to properly digest foods (due to the distribution of blood to the exercising muscle). During these times it is best to consume carbohydrates and foods that are easily digested (typically high GI carbohydrate foods). Liquid calorie sources are typically easier to tolerate and digest. One research study has indicated that during a time trial effort, a carbohydrate drink mouth rinse (not consumption) actually improved performance during a 1-hour cycle TT. The authors feel that additional motivation that occurred when having a mouth rinse with a carbohydrate drink might have provided the benefit compared to a water only rinse. This might be most important for athletes performing a very high intensity effort where ingesting drinks may be challenging. Recommendations: The depletion of carbohydrate stores within the body leads to bonking which hinders your ability to race and/or train hard. The ability to rapidly replenish carbohydrates after training, and the ability to consume and convert ingested carbohydrates into a usable form of carbohydrate is important in allowing you to train and compete at your best. Consuming the wrong carbohydrates at the wrong time, or ingesting too little carbohydrate can impair performance both in the short term and long term. Before: Focus on low to moderate GI carbohydrate sources. The most important pre-exercise consideration is to make sure you have topped off your carbohydrate stores. The second thing to consider is making certain you DO NOT consume high GI foods or drinks just prior to racing. Consumption of lower GI foods 30-60 min prior to an endurance exercise bout tends to promote positive effects during exercise including: 1) Minimizing the hypoglycemic response that occurs at the start of exercise; 2) Increasing the concentration of fatty acids in the blood; and 3) Increasing fat oxidation and reducing reliance on carbohydrate as fuel. This carbohydrate sparing prolongs your endurance and helps prevent the bonk. Adding fats, fiber and protein to a food or meal can help reduce the GI of that meal but ensure that you have tried this many times in race simulation training prior to your race. Adding these nutrients to your pre-race meal for the first time at a race could result in digestive issues. During: Focus on a mixture of carbohydrate sources. The GI of a food consumed during exercise is less important than at other times because the insulin response to carbohydrate ingestion is suppressed during exercise. Three recent studies done by Dr. Jentjens and Dr. Jeukendrup have clearly shown that mixing carbohydrate sources improves absorption and oxidation rates over any single source. Look for products which mix maltodextrin or glucose with fructose. Because of multiple transport mechanisms in the small intestine, specifically a glucose and fructose transporter, adding these two sugars to your during race nutrition plan with maximize your body’s ability to absorption rate. After: Consuming high GI carbohydrate sources to replenish lost glycogen stores is the focus after training or a race. The ability to replenish these stores determines how ready you will be the next day for another workout. It is at this time where a high GI carbohydrate has the ability to shuttle glycogen into the cell quicker and more efficiently than low or moderate GI carbohydrates. If you have access to a high GI carbohydrate, then grab it, if not grab any carbohydrate you can get your hands on and swallow it down with water and a source of sodium. Glucose is the highest GI sugar available. Low molecular weight proteins and high levels of glutamine have been shown to improve glycogen re-synthesis more than carbohydrates alone.
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