A recent Sports Medicine – Open study conducted a randomized controlled trial to evaluate how varying carbohydrate intake and the glycemic index (GI) impact performance in endurance training regimens in men.

Study: Effects of a 10-Week Exercise and Nutritional Intervention with Variable Dietary Carbohydrates and Glycaemic Indices on Substrate Metabolism, Glycogen Storage, and Endurance Performance in Men: A Randomized Controlled Trial. Image Credit: Lucky Business / Shutterstock.com

Background

Carbohydrates are crucial in maintaining performance during long sessions of endurance exercise. Optimal metabolic flexibility combined with full glycogen stores are essential prerequisites for high endurance performance.

Energy requirements in the muscles increase dramatically while transitioning from rest to exercise. At this time, fats or carbohydrates are used to produce adenosine triphosphate (ATP). With the growing intensity of exercise, carbohydrates become the key energy source, thus replacing fat.

A long-term diet low in carbohydrates and high in fat (LCHF) increases maximal fat oxidation at rest and during submaximal exercise conditions. Previously, the LCHF diet has been shown to improve the respiratory exchange ratio (RER) but not the time to exhaustion (TTE), even when glycogen stores were replenished shortly before the start of the competition.

Therefore, performance at high intensity is restricted due to reduced glycogen stores and mitigated carbohydrate metabolism. Thus, the LCHF diet may impact physical performance by reducing training capacity, exercise economy, and well-being when exercising at higher intensities, in addition to increasing the risk of fatigue, poor concentration, and gastrointestinal (GI) effects.

A high-carbohydrate diet with low GI has the potential to overcome the limitations often associated with the LCHF diet. In fact, recent studies have reported that this type of diet can lead to improved metabolic flexibility and, as a result, superior performance improvements during an incremental cycling test. However, few long-term studies have been conducted that assess the impact of a low GI diet relative to an LCHF diet on performance outcomes and metabolic flexibility.

About the study

The current 10-week interventional study aimed to assess and compare the effects of the LCHF diet, a carbohydrate-rich high-GI diet (HIGH-GI), and a carbohydrate-rich low-GI diet (LOW-GI) on metabolic parameters, running economy (RE), peak running speed (PRS), and peak oxygen consumption. These parameters were assessed by measuring muscle energy stores, a five-kilometer time trial (TT) performance, and a graded exercise test.

The testable hypothesis was that the LOW-GI diet can influence fat oxidation without restricting carbohydrate oxidation to a similar extent as the LCHF diet. Furthermore, the LOW-GI and HIGH-GI groups were hypothesized to experience similar improvements in TT and PRS. Muscle glycogen stores were expected to decline in the LCHF diet, whereas no significant difference was anticipated in the HIGH-GI and LOW-GI groups.

Study findings

The LOW-GI diet led to reduced energy intake, decreased blood lactate concentrations during exercise, higher values in the graded exercise test, maintenance of glycogen stores, and improved TT performance.

The LCHF diet also enhanced fat oxidation in the incremental test. However, due to insufficient carbohydrate provisions, the LCHF diet altered carbohydrate oxidation, muscle glycogen restoration, and training adaptions at higher intensities.

HIGH-GI improved performance at high intensities and increased muscle glycogen content. However, after 10 weeks, fat oxidation was impaired.

Importantly, the LCHF diet could negatively affect long-term health status despite its promising effects on fat oxidation and body composition. The lower intake of essential macronutrients and high-fat content could contribute to these adverse effects; therefore, this type of diet should be recommended with appropriate caution.

Plasma lactate concentrations were reduced in the LOW-GI group, whereas carbohydrate metabolism was unaltered during higher intensities. These two factors, coupled with the facilitation of fat utilization, resulted in improved metabolic flexibility.

In the HIGH-GI group, muscle glycogen levels increased significantly. However, while responding to different exercise intensities, the changes in metabolism could impair the ability to switch from carbohydrate to fat oxidation.

Overall, the study findings provide evidence that relative to the LCHF or HIGH-GI diet, the LOW-GI diet could lead to beneficial changes in substrate oxidation during extended periods of exercise and improve endurance performance.

Study limitations

Diet monitoring was conducted by self-reported 24-hour recalls, which could have led to recall, reporting, and training biases. However, these calls were completed twice a week, which should reduce the probability of random errors. Furthermore, an additional food frequency questionnaire was used to minimize errors and assess diet status before the study.

No differences across diets were noted during the TT, which could be attributed to running engaging more muscle mass than cycling. Furthermore, the gastrocnemius muscle is not depleted for glycogen, and less glycogen is broken down in the leg muscles.

Future studies are needed to account for different sexes and use different macronutrient intake periodizations to better understand the mechanisms associated with the benefits of these different diets. Metabolomics studies could also elucidate the ongoing adaptions in metabolism.