The text explores the significance of carbohydrates (CHOs) in diet, emphasizing their role as the primary source of energy. It notes that the type of CHO consumed is crucial for influencing exercise performance and mitigating the risk of obesity and associated chronic diseases. The concept of the glycemic index (GI) is introduced, developed in 1981 to categorize CHOs based on their impact on blood glucose (GLU) and insulin (INS) levels. The GI is defined as the incremental area under the blood GLU response curve, expressed as a percentage of the response to a reference food (usually glucose) consumed by the same individual.
The discussion highlights that the glycemic response is influenced by various factors such as pH, cooking, processing, and other food components like fiber, fat, and protein. Despite efforts to compile reliable GI values, challenges in standardizing methods for determining dietary GI in national food databases are acknowledged. The variability in GI values across locations and individuals is emphasized, underlining the importance of local testing for practical application.
The concept of glycemic load (GL) is introduced as a ranking system that considers both the CHO content and GI score of a food. It represents the total exposure to glycemia during a 2-hour period and is calculated by multiplying CHO content (in grams per serving) by GI divided by 100. The text suggests cutoffs for low, medium, and high GL values. While acknowledging the importance of GI, the limitations of solely relying on GI are discussed, emphasizing the need to consider both the amount and GI of CHOs to explain metabolic responses accurately.
The advantages of the GL concept are outlined, as it overcomes the limitations of the GI by incorporating serving sizes. GL is considered a better predictor of glycemic response and insulin demand compared to GI. However, caution is advised in using GL, especially in populations where controlling glycemia and insulinemia is critical for health. The text concludes by noting that a low-GI/high-CHO food and a high-GI/low-CHO food can have the same GL, despite potential differences in their effects on postprandial glycemia, β-cell function, triglyceride concentrations, free fatty acid levels, and satiety. These considerations underscore the complexity of managing postprandial responses to CHO and the importance of a comprehensive approach using both GI and GL.
The text delves into the intricate relationship between glycemic index (GI), glycemic load (GL), and metabolic responses, primarily focusing on their impact on postprandial blood glucose (GLU) and insulin (INS) levels. High-GI foods are noted to induce sharp peaks in GLU levels, leading to elevated INS levels and inhibiting glucagon release. The subsequent metabolic changes involve increased glycogen synthesis, decreased gluconeogenesis and glycogenolysis, heightened lipogenesis, and inhibited lipolysis.
The metabolic responses associated with GI values form the basis for the suggestion that low-GI diets may reduce the risk of metabolic diseases. Such diets are linked to potential improvements in body weight and favorable changes in lipid profiles, while high-GI diets may contribute to obesity and associated chronic diseases, including type 2 diabetes mellitus (T2DM).
The text transitions to the exploration of the long-term metabolic effects of low-GI diets, highlighting inconsistent outcomes in studies assessing the relationship between GI or GL and the risk of developing chronic diseases. Various studies associate both GI and GL with disease risk, while others support the association for either GI or GL alone. Metaanalyses suggest that low-GI and/or low-GL diets are independently associated with a reduced risk of T2DM, heart disease, gallbladder disease, breast cancer, and other diseases, particularly in women. However, caution is advised in generalizing these findings to men.
The association of dietary GI and GL with cardiovascular disease (CVD) risk is discussed, noting variations among different populations. Some studies support associations between GI or GL and increased risk of CVD, while others do not. The discussion extends to the potential links between GI and GL and the risk of cancer, with inconsistent findings across studies for various types of cancer. The text emphasizes the need for further long-term studies to clarify the role of GI in carcinogenesis.
Obesity and metabolic syndrome are identified as common public health concerns with increased risks of various diseases. Low dietary GI and GL are proposed as effective contributors to weight loss and reductions in disease risk. The text suggests that low-GI diets may impact substrate oxidation, fuel partitioning, and satiety, potentially aiding in weight control. Moreover, there is evidence that low-GI and low-GL diets may be more effective in reducing body weight compared to other dietary interventions. The text concludes by highlighting the importance of targeting postprandial hyperglycemia as a risk factor for mortality, not only in patients with chronic diseases but also in healthy individuals.
The text explores the emerging interest in glycemic index (GI) within the realm of sports nutrition, examining how different GIs can elicit diverse metabolic responses during exercise. The relationship between preexercise low GI and enhanced physical performance is a subject of debate, with some studies supporting this connection and others refuting it.
The intensity of glycemic and insulinemic responses in the postprandial phase and during exercise is identified as a significant factor influencing performance. Maintaining euglycemia during exercise and avoiding rebound hypoglycemia are crucial for delaying fatigue and improving performance, particularly in prolonged exercise. The text emphasizes the importance of substrate utilization, where carbohydrates (CHOs) are preserved, and fat becomes the primary energy source during endurance exercise.
Given that GI represents the blood glucose response to CHO-containing foods, manipulating the GI of preexercise meals is proposed as a critical factor in controlling glycemia, insulinemia, and substrate utilization during exercise. High-GI preexercise meals are associated with increased CHO oxidation and reduced fat oxidation, potentially impacting endurance and performance.
The text delves into various studies exploring the effects of different GIs of preexercise meals on physical performance. While some studies report improvements in athletic performance, particularly with low-GI preexercise meals, others do not show any enhancement. Physical performance is assessed through parameters like time trial, time to exhaustion, total work and power output, heart rate, rate of perceived exertion, VO2max, and respiratory exchange ratio.
Studies indicating improvement in performance after consuming low-GI meals highlight enhanced time to exhaustion, increased cycling distance, and lower rates of perceived exertion. These findings suggest that the manipulation of GI in preexercise meals can positively influence athletic performance. However, the text notes the variability in the time of meal ingestion across studies, emphasizing the need for clarity on the appropriate timing of low-GI meal consumption for optimal results.
The text explores the absence of a significant impact of glycemic index (GI) on exercise performance, highlighting studies that have found no conclusive evidence supporting a direct link between the GI of the diet and athletic outcomes.
In a study by Jamurtas et al., the ingestion of different GI meals before cycling until exhaustion did not influence exercise performance or metabolic responses. The study involved untrained healthy males who consumed carbohydrate-rich meals with low or high GI or a placebo 30 minutes before exercise. The results showed no differences in various parameters, including time to exhaustion, respiratory exchange ratio, and metabolic responses during cycling exercise. The authors noted a significant increase in β-endorphin at the end of the exercise across all trials, suggesting that the amount of carbohydrate consumed and exercise intensity might have mitigated the impact of GI on metabolic responses.
Febbraio et al. conducted a study investigating the effects of low and high GI meals consumed 30 minutes before prolonged exercise. Trained males consumed low-GI, high-GI, or a placebo before cycling at 70% peak oxygen uptake for 120 minutes followed by a 30-minute performance cycle. The study found no difference in work output during the performance trial between the three dietary groups. While substrate oxidation showed some variation, there was no significant impact on exercise performance.
Kern et al. explored the effects of preexercise consumption of carbohydrate sources with different GIs on metabolism and cycling performance. Endurance-trained cyclists ingested moderate-GI (raisins) or high-GI (sports gel) sources before a submaximal exercise and a 15-minute performance trial. The study found no differences in work output during the performance trial, with minor differences in metabolic responses between the raisin and sports gel trials.
Little et al. investigated the effects of preexercise carbohydrate-rich meals with different GIs on high-intensity intermittent exercise performance. The study involved male athletes performing 90 minutes of high-intensity intermittent running after ingesting low-GI or high-GI preexercise meals, or exercising in a fasted state. While total distance covered was higher in the GI trials compared to the control trial (fasted), there were no significant differences between low-GI and high-GI trials. Metabolic responses, including fat oxidation and carbohydrate oxidation, did not differ significantly between the two GI trials.
Bennett et al. examined the ingestion of high-GI and low-GI preexercise meals on performance and metabolic responses during extended high-intensity intermittent exercise. The study involved recreational soccer players performing two sessions of 90-minute intermittent high-intensity treadmill running. The results showed that the ingestion of preexercise meals with different GIs did not affect performance, although the low-GI preexercise meal improved metabolic responses during extended high-intensity intermittent exercise.
Brown et al. investigated the effects of low-GI and high-GI meals on the physiological responses to a 3-hour recovery period and subsequent 5-kilometer cycling time trial. While the metabolic profile during the recovery period differed between the two testing meals, there was no significant difference in exercise performance during the 5-kilometer cycling time trial.
The text also briefly touches on the influence of GI on exercise performance when carbohydrate-electrolyte solutions are consumed during a run and explores carbohydrate loading strategies, suggesting that the GI of a high-carbohydrate diet consumed before exercise may not significantly impact exercise performance and substrate oxidation during exercise.
The text delves into the integration of glycemic load (GL) into sports nutrition, exploring its potential impact on the glycemic effect of a diet and its influence on metabolic responses and exercise performance. It notes that GL may affect fat and carbohydrate oxidation or performance during exercise, emphasizing the importance of understanding the role of GL in sports nutrition.
The author points out that very few research attempts have been made to elucidate the metabolic responses of different GLs during the postprandial period or exercise. Only three studies have so far examined the potential effects of different GLs of preexercise meals on metabolic responses, exercise performance, and immune response.
Chen et al. conducted a study investigating the impact of different GLs on metabolic responses and endurance running performance. Three isoenergetic dietary approaches were compared, each consumed 2 hours before a preloaded 1-hour run and a 10-kilometer time trial. The study found that preexercise low-GL meals induced smaller metabolic changes during the postprandial period and exercise than high-GL meals. Higher total carbohydrate oxidation during the postprandial, exercise, and recovery period was observed in the high-GL trial compared to the low-GL trials. The study suggests that the amount of carbohydrate consumed in a preexercise meal may play a significant role in modifying metabolic responses.
In a subsequent study, using the same data, the authors investigated the influence of a GI and/or GL meal on immune responses to prolonged exercise. The study found that the consumption of a high-carbohydrate meal resulted in less perturbation of circulating immune cells and decreased interleukin-6 concentrations immediately after exercise and during the recovery period compared with low-carbohydrate meals.
In another study from the same laboratory, researchers expanded their investigation by incorporating preexercise carbohydrate loading. The results showed that loading with an equally high amount of carbohydrates might produce similar muscle glycogen supercompensation status and exercise metabolic responses during subsequent endurance running, regardless of differences in GI and GL. This study suggested that, in contrast with simple preexercise meals, the amount rather than the nature of the carbohydrates consumed during the 3-day regimen may be the most dominant factor in metabolism and endurance running performance.
The text also touches upon food exchange values in health and exercise, highlighting the food exchange system as a method of meal planning for healthy eating. It explains that food exchange lists, organized based on nutrient content, can be useful for simplifying meal planning, ensuring a consistent and nutritionally balanced diet, and helping individuals monitor their blood glucose levels, particularly those with diabetes. The American Dietetic Association has categorized foods into 11 lists, each containing foods with similar carbohydrate, protein, fat, and caloric values.
The author notes the lack of internationally applicable food exchange lists and emphasizes that these lists have not been utilized in the sports science field. It raises the intriguing question of whether food exchange lists could be beneficial for athletes in menu planning and, consequently, in regulating carbohydrate intake to enhance exercise capacity and performance. The text concludes with a brief explanation of how to calculate food exchanges for starch, meat, and fat based on their respective values.