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. Ingestion of 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, this means I am properly hydrated'. Soon you begin to cramp and your performance dramatically goes down-hill. What actually happened is, your body was not able to properly absorb the slow carbohydrates, slowing gastric emptying and causing 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 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. What glycemic index does not define is 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 low GI. Elevated blood sugar causes insulin to be secreted to help modulate the sugar and a subsequent sugar crash follows. Glycemic Load measures the GI multiplied by the total carbohydrate content giving a more practical and accurate determination of blood sugar response. A complete list of GI for foods can be found at http://diabetes.about.com/library/mendosagi/ngilists.htm

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 this example, a high GI food becomes a low GL food. So based on the serving size and quantity of watermelon eaten, it can have a better GL and therefore a lower rise in blood sugar. As you increase the serving size eaten, you increase the amount of carbohydrates eaten which will in turn increase the GL of the food.

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 glycemix index (GI) may have implications for the athlete surrounding training, it is not a universal system to rank carbohydrate rich foods and 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 it is clearly evident that the GI can affect your primary fuel supply quite dramatically (this graph is at rest). 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 your workouts, making it critical to choose the appropriate fuel for the appropriate response. (more on this in our recommendation section).

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, has a less erratic effect on blood sugar levels (at rest). Diabetics 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. 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 is an excellent source of carbohydrates.

Galactose is a simple sugar that has recently shown up in sports drinks. 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. *The GI of Galactose could not be found on any of the official GI lists, though G-Push (a popular sports drink) does claim that Galactose is absorbed quickly like glucose without a subsequent increase in insulin release. This is not confirmed with clinical studies.

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 fats 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 with a GI of 68±5 (otherwise known as table sugar) is composed of one molecule of glucose and one molecule of fructose. This is the white sugar that comes in many forms, such as powdered or granulated. It is usually made from refining extracts of sugar beets or sugar cane.

Maltodextrin with a GI of 85±15 (otherwise known glucose polymers or complex carbohydrates) 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. New research by 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 large mixture of sugars (maltodextrin and fructose) at a rate of 1.8 grams per minute (108 grams per hour) as opposed to a single source of sugar (maltodextrin) produces a higher oxidation rate, reaching peak levels of approximately 1.5 grams per minute (90 grams per hour). Similar results are found when using the combination of sucrose and glucose at high ingestion rates (2.4 grams per minute or 144 grams per hour) with the peak carbohydrate oxidation rate 1.2 grams per minute (72 grams per hour) This data provides support that if 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)

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 pooling of blood to the exercising muscle). During these times it is best to consume carbohydrates and foods that are easily digested = High GI. The best advise is to stick to what you are used to. Liquid calorie sources are sometimes more easily tolerated and digested.

A recent 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 the your best. Ingestion of the wrong carbohydrates at the wrong time, or ingesting too little carbohydrate can impair performance both in the short term and long term.

Before: (low to moderate GI) 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 some positive effects during exercise including: 1) Minimizing the hypoglycemia that occurs at the start of exercise. 2) Increasing the concentration of fatty acids in the blood. 3) Increases fat oxidation and reducing reliance on carbohydrate 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.

During (mix 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 with two or more simple sugars. Dr. Jeukendrup theorizes that because carbohydrates are absorbed in different areas of the digestive system, mixing sources allows for great absorption rates. Furthermore stay away from any products with only a single source of carbohydrates or fructose as its primary sugar.

After (high GI): Following exercise your primary concern is to replenish lost glycogen stores. The ability to replenish these stores fully 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 carb, 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 (Dextrose) is the highest GI sugar available. Low molecular weight proteins and high levels of Glutamine have been shown to improve glycogen resynthesis more than carbohydrates alone.

Zawadzki,. K.M., B.B. Yaspelkis, and J.L. Ivy (1992). Carbohydrate-protein complex increases the rate of muscle glycogen storage after exercise. J.Appl. Physiol. 72:1854-1859.

Glycemic Index scores: http://diabetes.about.com/library/mendosagi/ngilists.htm