The effect of glycaemic index of high carbohydrate diets consumed over 5 days on exercise energy metabolism and running capacity in males

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

BACKGROUND

Daily nutrition plays an important role in supporting training adaptions and endurance performance. The objective of this 10-week study was to investigate the consequences of varying carbohydrate consumption and the glycaemic index (GI) together with an endurance training regimen on substrate oxidation, muscle energy storage and endurance performance under free-living conditions. Sixty-five moderately trained healthy men (29 ± 4 years; VO2 peak 55 ± 8 mL min-1 kg-1) were randomized to one of three different nutritional regimes (LOW-GI: 50-60% CHO with ≥ 65% of these CHO with GI < 50 per day, n = 24; HIGH-GI: 50-60% CHO with ≥ 65% CHO with GI > 70 per day, n = 20; LCHF: ≤ 50 g CHO daily, n = 21). Metabolic alterations and performance were assessed at baseline (T0) and after 10 weeks (T10) during a graded exercise treadmill test. Additionally, a 5 km time trial on a 400-m outdoor track was performed and muscle glycogen was measured by magnet resonance spectroscopy.

RESULTS

Total fat oxidation expressed as area under the curve (AUC) during the graded exercise test increased in LCHF (1.3 ± 2.4 g min-1 × km h-1, p < 0.001), remained unchanged in LOW-GI (p > 0.05) and decreased in HIGH-GI (- 1.7 ± 1.5 g min-1 × km h-1, p < 0.001). After the intervention, LOW-GI (- 0.4 ± 0.5 mmol L-1 × km h-1, p < 0.001) and LCHF (- 0.8 ± 0.7 mmol L-1 × km h-1, p < 0.001) showed significantly lower AUC of blood lactate concentrations. Peak running speed increased in LOW-GI (T0: 4.3 ± 0.4 vs. T10: 4.5 ± 0.3 m s-1, p < 0.001) and HIGH-GI (T0: 4.4 ± 0.5 vs. T10: 4.6 ± 0.4 m s-1), while no improvement was observed in LCHF. Yet, time trial performance improved significantly in all groups. Muscle glycogen content increased for participants in HIGH-GI (T0: 97.3 ± 18.5 vs. T10: 144.5 ± 39.8 mmol L wet-tissue-1, p = 0.027) and remained unchanged in the LOW-GI and the LCHF group. At the last examination, muscle glycogen concentration was significantly higher in LOW-GI compared to LCHF (p = 0.014).

CONCLUSION

Changes in fat oxidation were only present in LCHF, however, lower lactate concentrations in LOW-GI resulted in changes indicating an improved substrate metabolism. Compared to a LCHF diet, changes in peak running speed, and muscle glycogen stores were superior in LOW- and HIGH-GI diets. The low GI diet seems to have an influence on substrate metabolism without compromising performance at higher intensities, suggesting that a high-carbohydrate diet with a low GI is a viable alternative to a LCHF or a high GI diet.

TRIAL REGISTRATION

Clinical Trials, NCT05241730. https://clinicaltrials.gov/study/NCT05241730 . Registered 25 January 2021.

Longer-Term Effects of the Glycaemic Index on Substrate Metabolism and Performance in Endurance Athletes

Nutrition has a decisive influence on athletic performance. However, it is not only the nutrient intake during exercise that is important, but the daily diet must also be adapted to the requirements of physical activity in order to optimally promote training adaptations. The goal of prolonged endurance training is to enhance fat oxidation, to maintain aerobic performance at a higher intensity while sparing limited carbohydrate stores. The targeted modification of macronutrient intake is a common method of influencing substrate metabolism, fuel selection, and performance. However, it is not well established whether the glycaemic index of carbohydrates in our daily diet can improve endurance performance by influencing carbohydrate or fat oxidation during training. Therefore, the aim of the following review is to elucidate the possible influence of the glycaemic index on substrate utilization during exercise and to clarify whether the consumption of a long-term high-carbohydrate diet with different glycaemic indices may have an influence on substrate metabolism and endurance performance.

Effect of a High Fat Diet vs. High Carbohydrate Diets With Different Glycemic Indices on Metabolic Parameters in Male Endurance Athletes: A Pilot Trial

Consuming low glycemic carbohydrates leads to an increased muscle fat utilization and preservation of intramuscular glycogen, which is associated with improved flexibility to metabolize either carbohydrates or fats during endurance exercise. The purpose of this trial was to investigate the effect of a 4-week high fat low carbohydrate (HFLC-G: ≥65% high glycemic carbohydrates per day; n = 9) vs. high carbohydrate low glycemic (LGI-G: ≥65% low glycemic carbohydrates daily; n = 10) or high glycemic (HGI-G: ≥65% fat, ≤ 50 g carbohydrates daily; n = 9) diet on fat and carbohydrate metabolism at rest and during exercise in 28 male athletes. Changes in metabolic parameters under resting conditions and during cycle ergometry (submaximal and with incremental workload) from pre- to post-intervention were determined by lactate diagnostics and measurements of the respiratory exchange ratio (RER). Additionally, body composition and perceptual responses to the diets [visual analog scale (VAS)] were measured. A significance level of α = 0.05 was considered. HFLC-G was associated with markedly decreased lactate concentrations during the submaximal (−0.553 ± 0.783 mmol/l, p = 0.067) and incremental cycle test [−5.00 ± 5.71 (mmol/l) × min; p = 0.030] and reduced RER values at rest (−0.058 ± 0.108; p = 0.146) during the submaximal (−0.078 ± 0.046; p = 0.001) and incremental cycle test (−1.64 ± 0.700 RER × minutes; p < 0.001). In the HFLC-G, fat mass (p < 0.001) decreased. In LGI-G lactate, concentrations decreased in the incremental cycle test [−6.56 ± 6.65 (mmol/l) × min; p = 0.012]. In the LGI-G, fat mass (p < 0.01) and VAS values decreased, indicating improved levels of gastrointestinal conditions and perception of effort during training. The main findings in the HGI-G were increased RER (0.047 ± 0.076; p = 0.117) and lactate concentrations (0.170 ± 0.206 mmol/l, p = 0.038) at rest. Although the impact on fat oxidation in the LGI-G was not as pronounced as following the HFLC diet, the adaptations in the LGI-G were consistent with an improved metabolic flexibility and additional benefits regarding exercise performance in male athletes.

Effects of Low Versus Moderate Glycemic Index Diets on Aerobic Capacity in Endurance Runners: Three-Week Randomized Controlled Crossover Trial

The glycemic index (GI) of ingested carbohydrates may influence substrate oxidation during exercise and athletic performance. Therefore, the aim of this study was to assess the effect of low- and moderate-GI three-week diets on aerobic capacity and endurance performance in runners. We conducted a randomized crossover feeding study of matched diets differing only in GI (low vs. moderate) in 21 endurance-trained runners. Each participant consumed both, low- (LGI) and moderate-GI (MGI) high-carbohydrate (~60%) and nutrient-balanced diets for three weeks each. At the beginning and end of each diet, participants had their aerobic capacity and body composition measured and performed a 12-min running test. After LGI, time to exhaustion during incremental cycling test (ICT) and distance covered in the 12-min run were significantly increased. The MGI diet led to an increase in maximal oxygen uptake (V˙O₂max), but no performance benefits were found after the MGI diet. The LGI and MGI diets improved time and workload at gas exchange threshold (GET) during ICT. The results indicate that a three-week high-carbohydrate LGI diet resulted in a small but significant improvement in athletic performance in endurance runners. Observed increase in V˙O₂max on MGI diet did not affect performance.

Influence of low versus moderate glycemic index of diet on substrate oxidation and energy expenditure during incremental exercise in endurance athletes: a randomized counterbalanced cross-over trial

The study was aimed at assessing the influence of 3-week low glycemic index (LGI) versus moderate glycemic index (MGI) diet on substrate oxidation during incremental exercise. 17 runners completed two 3-week trials of either LGI or MGI diet in a randomised counterbalanced manner. Before and after each trial the incremental cycling test was performed. Metabolic alternations were observed only within tested diets and no significant differences in fat and carbohydrate (CHO) oxidation were found between MGI and LGI diets. Following MGI diet CHO oxidation rate increased. The AUC of fat oxidation decreased after both diets. Percent contribution of fat to energy yield declined, whereas contribution of CHO was augmented following MGI diet. This study indicates that the 3-week MGI diet increased the rate of carbohydrate oxidation during incremental cycling test and improved performance in acute intense exercise test, while both high-carbohydrate diets downregulated fat oxidation rate.

Effect of pre-exercise carbohydrate diets with high vs low glycemic index on exercise performance: a meta-analysis

Context

Although pre-exercise consumption of a low-glycemic-index (LGI) carbohydrate meal is generally recommended, the findings regarding subsequent exercise performance are inconsistent.

Objective

This meta-analytic study was conducted to determine whether a pre-exercise LGI carbohydrate meal leads to greater endurance performance than a pre-exercise high-glycemic-index (HGI) meal.

Data sources

The following electronic databases were searched until April 2016: MEDLINE, SPORTDiscus, Web of Science, and Cochrane Central Register of Controlled Trials. The reference lists of selected articles were searched manually.

Study selection

Randomized controlled or crossover trials comparing the effects of LGI and HGI pre-exercise carbohydrate meals on subsequent exercise performance of healthy participants were included.

Data extraction

The Jadad scale was used to assess the methodological quality of the included studies. A fixed-effects model was used to evaluate overall and subgroup estimates.

Results

In total, 15 eligible studies from 727 articles were included in this meta-analysis. All included studies were of low research quality. The synthesized effect size ( d  = 0.42, z  = 3.40, P  = 0.001) indicated that the endurance performance following an LGI meal was superior to that following an HGI meal. Subgroup analyses demonstrated that the treatment effect did not vary across outcome measures (exercise to exhaustion, time trial, and work output) or athletic status (trained or recreational participants).

Conclusions

Weak evidence supports the claim that endurance performance following a pre-exercise LGI meal is superior to that following a pre-exercise HGI meal. Further high-quality research in this area is warranted.

The Effects of Hyperhydrating Supplements Containing Creatine and Glucose on Plasma Lipids and Insulin Sensitivity in Endurance-Trained Athletes

The addition of carbohydrate (CHO) in the form of simple sugars to creatine (Cr) supplements is central. The study aimed to determine whether ingestion of glucose (Glu) simultaneously with Cr and glycerol (Cr/Gly) supplement is detrimental to plasma lipids of endurance-trained individuals and find out whether modification arising can be attenuated by replacing part of the Glu with alpha lipoic acid (Ala). Twenty-two endurance-trained cyclists were randomized to receive Cr/Gly/Glu (11.4 g Cr-H₂O, 1 g Gly/kg BM, and 150 g Glu) or Cr/Gly/Glu/Ala (11.4 g Cr-H₂O, 1 g Gly/kg BM, 100 g Glu, and 1 g Ala) for 7 days. Fasting concentration of TAG increased significantly (P < 0.01) after supplementation with Cr/Gly/Glu (before: 0.9 ± 0.2 mmol/L; after: 1.3 ± 0.4 mmol/L) and Cr/Gly/Glu/Ala (before: 0.8 ± 0.2 mmol/L; after: 1.2 ± 0.5 mmol/L) but changes were not different between the groups. Supplementation significantly (P < 0.05) increased the TAG to HDL-cholesterol ratio but had no effect on fasting concentration of total, HDL-, and LDL-cholesterol and insulin resistance. Thus, addition of Glu to Cr containing supplements enhances plasma TAG concentration and the TAG to HDL-cholesterol ratio and this enhancement cannot be attenuated by partial replacement of Glu with Ala.

Translating the impact of exercise on cognition: methodological issues in animal research

Physical exercise and fitness have been proposed as potential factors that promote healthy cognitive aging. Some of the support for this hypothesis has come from animal research. Animal studies are also used to propose the physiological mechanisms underlying the cognitive performance improvement associated with exercise. In the present review and meta-analysis, we discuss several methodological problems that limit the contribution of animal studies to the understanding of the putative effects of exercise on cognitive aging. We suggest that the most likely measure to equate exercise intensity in rodent and humans may be oxygen consumption (VO2) because observed values are surprisingly similar in young and older rodents and humans. For practical reasons, several animal studies use young rodents kept in social isolation. We show that social isolation is associated with an enhanced impact of exercise on cognitive performance but not on some physiological measures thought to mediate the effect of exercise. Surprisingly, two months or more of exercise intervention appeared to be ineffective to promote cognitive performance compared to shorter durations. We argue that impact of exercise in socially isolated animals is explained by an alleviation of environmental impoverishment as much as an effect of physical exercise. It is possible that the introduction of exercise in rodents is partly mediated by environmental changes. It may explain why larger effects are observed for the shorter durations of exercise while much smaller effects are found after longer periods of exercise.

New perspectives on nutritional interventions to augment lipid utilisation during exercise

The enhancement of fat oxidation during exercise is an aim for both recreational exercising individuals and endurance athletes. Nutritional status may explain a large part of the variation in maximal rates of fat oxidation during exercise. This review reveals novel insights into nutritional manipulation of substrate selection during exercise, explaining putative mechanisms of action and evaluating the current evidence. Lowering the glycaemic index of the pre-exercise meal can enhance lipid utilisation by up to 100 % through reduced insulin concentrations, although its application may be restricted to specific training sessions rather than competition. Chronic effects of dietary glycaemic index are less clear and warrant future study before firm recommendations can be made. A flurry of recent advances has overthrown the conventional view of l-carnitine supplementation, with skeletal muscle uptake possible under certain dietary conditions and providing a strategy to influence energy metabolism in an exercise intensity-dependent manner. Use of non-carbohydrate nutrients to stimulate muscle l-carnitine uptake may prove more beneficial for optimising lipid utilisation, but this requires more research. Studies investigating fish oil supplementation on fat oxidation during exercise are conflicting. In spite of some strong putative mechanisms, the only crossover trial showed no significant effect on lipid use during exercise. Ca may increase NEFA availability although it is not clear whether these effects occur. Ca and caffeine can increase NEFA availability under certain circumstances which could theoretically enhance fat oxidation, yet strong experimental evidence for this effect during exercise is lacking. Co-administration of nutrients to maximise their effectiveness needs further investigation.

Influência do índice glicêmico na glicemia em exercício físico aeróbico

OBJETIVO: Avaliar a influência do índice glicêmico (IG) na resposta glicêmica antes e durante o exercício físico decorrente a diferentes sessões experimentais pré-exercício. MÉTODOS: Doze homens adultos realizaram três sessões experimentais pré-exercício: de alto índice glicêmico (AIG); de baixo índice glicêmico (BIG); e em estado de jejum, sendo nesta última oferecida duas formas diferenciadas de hidratação durante o exercício: água e bebida carboidratada. RESULTADOS: Durante o período de repouso, o tipo de refeição, de AIG ou de BIG, interferiu no comportamento da glicemia, ambas tendendo ao "efeito rebote". Durante o exercício, não foi observada diferença na resposta glicêmica entre as quatro ações testadas; contudo, a intervenção com bebida carboidratada manteve constante a glicemia ao longo dos 60 min do exercício. CONCLUSÃO: O IG é determinante na resposta glicêmica ao longo de uma hora antes do exercício, porém não interfere na resposta glicêmica durante a atividade.