Maintaining adequate carbohydrate (CHO) stores is essential for those engaging in regular exercise. Advances in sports nutrition have provided coaches with a wealth of information regarding the recommended timing and amount of CHO to be ingested before, during, and after exercise. However, debate still occurs among exercise and health professionals as to which type of CHO will provide the greatest performance benefit to athletes at both the recreational and elite levels. Once a sufficient amount of CHO is ingested before, during, and after exercise, does the CHO source really matter? Do certain types of CHOs affect performance and recovery in different ways? Can different types of CHO be recommended for different training goals? For example, will more efficient "fat oxidation" be achieved during an endurance workout after the consumption of specific CHOs? Recent studies on the glycemic index (GI) have helped answer these questions and provide nutritional guidelines for exercise (1,14,20).
CONCEPT OF GI
The concept of the GI was first introduced in the early 1980s as a method of functionally ranking CHO foods based on their actual postprandial blood glucose response compared with a reference food (either glucose or white bread) (10). The GI is an indication of the rate of CHO digestion and absorption and is influenced by factors such as the macronutrient content of the food, cooking techniques, food processing, and the training status of the individual concerned. Foods that are digested and absorbed rapidly, such as corn flakes, have a high GI score (about 84), whereas foods that are digested and absorbed during a much longer period, such as legumes, have a low GI score (about 16). The purpose of this article was to examine recent findings in GI-related research and to analyze their impact on nutritional strategies for exercise performance and recovery. The relatively new concept of the glucose load (GL), which reflects the total exposure to glycemia during a period and is derived by multiplying the amount of available CHO consumed in the diet by its GI value, is also introduced, and the pertinent studies are discussed.
Traditionally, CHOs were categorized as either "simple" or "complex" according to their chemical structure. Simple CHO foods were deemed to be unhealthy and generally not nutrient rich, whereas complex CHO foods were considered to be healthier and more nutritious. These groupings are misleading, however, because many complex CHO foods (e.g., French fries) are often quite high in fat, whereas many simple CHO foods (e.g., fruit and yogurt) are low in fat and are good sources of protein and other vitamins. To avoid confusion and providing misinformation, many professional associations have begun to recommend CHO ingestion according to GI classifications.
PRE-EXERCISE CHO INGESTION
Preparation for exercise performance is multifaceted and incorporates factors such as training schedule, hydration strategy, and timing and amount of CHO ingestion. The type of CHO ingested is also an integral part of preexercise preparation strategies, and this issue has been widely researched in recent times. Despite some mixed results, a general consensus has emerged that indicates that certain benefits may be gained from the preexercise ingestion of low-GI (LGI) food (18,20). This is largely caused by the resultant decrease in postprandial hyperglycemia and hyperinsulinemia. This causes an increase in free fatty acid oxidation and possibly better maintenance of plasma glucose concentrations, leading to a more sustained CHO availability during exercise (18).
Some recent studies have provided a clearer picture of the different effects that preexercise LGI and high-GI (HGI) CHO ingestion has on metabolism and performance (14,18,20). For many recreational athletes and fitness clients, fat loss and weight maintenance are top priorities when engaging in exercise programs. Although it has been shown that fat oxidation is maximized when exercise takes place in a fasted state (19), many GI studies have highlighted that there is a distinctive increase in fat oxidation during both the postprandial period as well as during subsequent exercise bouts when LGI CHO is ingested 2 to 3 hours preexercise as opposed to HGI CHO. Initial findings resulted from studies carried out on single foods, but subsequent studies have indicated that a similar metabolic response was still elicited from LGI mixed meals, which indicates that the GI approach can now be applied to more lifelike athletic situations. In some cases, LGI ingestion also can result in improvements in endurance capacity because increased fat oxidation can lead to a sparing of muscle glycogen, which allows for more sustained CHO availability during the course of the exercise (20).
MALE VERSUS FEMALE RESPONSES
Until recently, sports coaches and fitness professionals questioned whether sex differences existed in the metabolic responses to LGI and HGI CHO ingestion. However, it has been suggested that similar to males, a higher fat oxidation rate also is observed in females after an LGI meal ingested 3 hours preexercise (15). Research findings also indicate that for both males and females, pre-exercise medium-GI CHO ingestion produces similar metabolic responses to those observed after a meal of entirely HGI foods (1).
DURING-EXERCISE CHO INGESTION
As recommended by many exercise and health professionals, athletes of all levels ingest CHO during exercise, mostly in the form of sports drinks or snack bars (Table 1). However, once a standard amount of CHO is ingested during exercise, that is, as used in many research studies, either 2 g CHO/kg body mass (3) or approximately 6% CHO solution, the typical responses seen with LGI pre-exercise ingestion are overridden, and little, if any, benefit exists in relation to substrate utilization and performance (17). It also has been highlighted more recently that fat oxidation during exercise with CHO intake was not different when compared with exercise in the fasted state (7). Performance enhancements that do occur are generally as a result of exogenous CHO rather than from any GI-related benefit (17). Further research is needed in this area to determine the optimum balance of preexercise and during-exercise CHO ingestion that will positively influence glycemic response and enhance exercise performance.
Just as rehydration is vital after exercise to restore fluid balance, food ingestion plays a crucial role in replenishing muscle glycogen stores during recovery. It is important to rebuild glycogen stores as soon as possible after exercise, particularly if the next bout of activity occurs 24 or 36 hours later. However, the exact type of CHO to be ingested postexercise is an ongoing subject of debate among exercise and health professionals because providing clients with accurate nutrition information is critical to the success of training programs. It has been suggested that a postexercise HGI meal can lead to increased muscle glycogen synthesis (4), but more recently, it has been shown that once sufficient CHO, that is, 2 g CHO/kg body mass, is consumed in the hours immediately postexercise, no differences exist in glycogen storage between HGI and LGI CHO ingestion (13). Later in the recovery period, however, an HGI meal will result in greater plasma glucose responses than an isocaloric LGI meal. This then leads to a higher rate of fat oxidation during subsequent exercise. When the exercise recovery period is quite short (e.g., 3-4 hours), the pattern of CHO intake also can have a bearing on the metabolic responses during subsequent exercise. When small portions are ingested for a longer period, CHO oxidation increases during subsequent endurance exercise. It is worth noting, therefore, that a single meal leads to greater fat oxidation during subsequent exercise sessions versus nibbling overtime. This was highlighted in a study where fat oxidation during a subsequent endurance run to exhaustion at 70% V˙O2max increased when the recovery meal was ingested in one single bolus (12). Therefore, increased fat oxidation may occur without reduced caloric intake, once the type of CHO and timing of ingestion are appropriate. As a result, it is important that clients' exercise goals and targets are fully understood because the appropriate nutritional guidelines can then be incorporated into their training programs. For example, to increase an individual's glucose response postexercise, an HGI meal is recommended (14,16). Whereas if an increase in fat oxidation during exercise is the primary goal, most of the current research suggests that a pre-exercise LGI meal would be most beneficial (17,20).
GI and Intermittent Variable-Intensity Exercise
Although knowledge of the GI and its relevance to exercise and fitness has grown since its introduction in 1981, much of the research relating to GI and CHO ingestion has focused on endurance performance. Questions continued to be raised, however, among coaches and trainers in relation to the effectiveness of the GI during short-duration high-intensity exercise. Very little scientific evidence exists on this particular issue, but initial suggestions are that despite an increase in muscle glycogen concentration after HGI ingestion, the GI of a diet before high-intensity sprint performance has little impact on exercise performance (8). This is a significantly underresearched area and one that requires further investigation.
GL and Exercise Performance
The GL is a relatively new concept, and its effect on health and fitness has not been widely investigated to date. It was initially applied in a more clinical setting, where it was identified as a key variable in determining which risk category for diabetes patients would be placed (2). The GL acts as a ranking system for CHO ingested and is based on the GI as well as on serving sizes (see Table 2 for meal examples). It is calculated by multiplying the CHO content of the food by its GI score and dividing by 100. More recently, it has been incorporated into sports nutrition because it may have a role to play in determining the overall glycemic effect of a diet.
Because GL was being determined by the overall glycemic effect of the diet and not by the amount of CHO alone, some practitioners have started asking the question whether GL provides a much better predictor of glycemic responses than CHO amount/percentage or GI alone, as has been recently suggested (11). For example, a watermelon has quite a high GI score, but very little CHO content, hence a low GL and little effect on blood glucose levels compared with a medium-GI food with a higher CHO content (e.g., brown rice). Tables indicating GL values of various foods have been published, but these must be viewed with caution because portion sizes vary from state to state and country to country. It is advisable that coaches and health professionals use the most authoritative and up-to-date information available to calculate the GL data for themselves and their clients.
The concept of the GL largely has gone unrecognized by many health and fitness trainers, perhaps in part, caused by the calculations involved in arriving at the final GL score. The perception of the GL is, however, certainly rising among the population in general. Indeed, a "glycemic load" search in September 2010 on a popular search engine for books returned 200 titles related to "health, mind, and body." Many Web sites claim the GL diet to be the most practical way to maintain a balanced diet, control blood glucose levels, and lose weight. It avoids the constant calorie counting and CHO restriction that is involved in many other commercial diet plans. Its validity and reliability have been tested (2,11), and there is significant evidence to suggest that the GL may have an important role to play in inducing metabolic changes to increase fat oxidation or enhance performance during exercise (5). For example, a preexercise low-GL meal can reduce fluctuations in glycemic responses during the postprandial period as well as during subsequent exercise when compared with a high-GL meal (5). Glucose load research is still at an embryonic stage, but there are recent findings that suggest that it is the amount, rather than the nature, of CHO consumed that is the most important factor influencing subsequent metabolism and endurance run performance (6) (see Tables 3 and 4).
The possibility exists of enhanced endurance performance after LGI ingestion when compared with HGI ingestion because of sustained CHO availability and glycogen sparing. It also is important for health and fitness professionals to note that GI-induced metabolic changes can result in increased fat oxidation during both the postprandial and subsequent exercise periods and that these principles apply equally across the sexes.
Once a large amount of CHO is taken onboard during exercise, research findings have indicated that the GI of the preexercise ingestion becomes irrelevant. A HGI meal postexercise can possibly speed up glycogen repletion, but once sufficient CHO is taken onboard, the GI of food ingested is largely immaterial. Short-term high-intensity exercise has so far been shown to be unaffected by the GI.
Because the GL incorporates both the GI and serving sizes, it may be a better predictor of glycemic responses than GI alone. It may be possible that preexercise meals can be modified according to their GL to increase the body's ability to oxidize fat during the postprandial period and during subsequent exercise. Research on the topic is at an early stage, but indications are that it may prove to be an important consideration in the area of sports nutrition.
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