Study shows low and high GI diets increase endurance performance compared to low-carb diets

Study shows low and high GI diets increase endurance performance compared to low-carb diets

A recent oneSports Medicine – OpenThe study conducted a randomized controlled trial to evaluate how varying carbohydrate intake and glycemic index (GI) affect the performance of endurance training programs in men.

background

Carbohydrates are crucial for maintaining performance during long endurance training sessions. Optimal metabolic flexibility paired with full glycogen stores are essential prerequisites for high endurance performance.

Muscle energy requirements increase dramatically during the transition from rest to exercise. At this point, fats or carbohydrates are used to produce adenosine triphosphate (ATP). As training intensity increases, carbohydrates become the most important source of energy, replacing fat.

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

Therefore, high-intensity performance is limited due to reduced glycogen stores and attenuated carbohydrate metabolism. Therefore, the LCHF diet may impair physical performance by reducing exercise capacity, exercise economy, and comfort during higher intensity exercise, as well as increasing the risk of fatigue, poor concentration, and gastrointestinal (GI) effects.

A low-GI, high-carbohydrate diet has the potential to overcome the limitations often associated with the LCHF diet. In fact, recent studies have shown 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 examining the effects of a low GI diet compared to a LCHF diet on performance outcomes and metabolic flexibility.

About the study

The aim of the current 10-week intervention study was to evaluate and compare the effects of the LCHF diet, a high-carbohydrate high-GI diet (HIGH-GI), and a high-carbohydrate 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, it was hypothesized that the low GI and high GI groups would experience similar improvements in TT and PRS. Muscle glycogen stores were expected to decrease on the LCHF diet, while no significant difference was expected in the HIGH GI and LOW GI groups.

Study results

The LOW-GI diet resulted in decreased energy intake, decreased blood lactate concentrations during exercise, higher graded exercise test scores, maintenance of glycogen stores, and improved TT performance.

The LCHF diet also increased fat oxidation in the step test. However, due to insufficient carbohydrate supply, the LCHF diet altered carbohydrate oxidation, muscle glycogen recovery, and training adaptations at higher intensities.

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

Importantly, despite its promising effects on fat oxidation and body composition, the LCHF diet could negatively impact long-term health status. 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 remained unchanged at higher intensities. These two factors, along with facilitating fat utilization, resulted in improved metabolic flexibility.

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

Overall, the study results provide evidence that the low-GI diet may result in beneficial changes in substrate oxidation during longer periods of exercise and improve endurance performance compared to the LCHF or HIGH-GI diet.

Study Restrictions

Dietary monitoring was performed through self-reported 24-hour recalls, which could have introduced recall, reporting, and training bias. However, these calls were made twice a week, which should reduce the chance of random errors. In addition, an additional food frequency questionnaire was used to minimize errors and assess nutritional status before the study.

No differences were found between diets during the TT, which could be due to the fact that running uses more muscle mass than cycling. In addition, no glycogen is deprived from the gastrocnemius and less glycogen is broken down in the leg muscles.

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

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