Carbohydrates and fat are the main sources of fuel oxidized in muscles during endurance exercise [1
]. In contrast to fat, endogenous stores of carbohydrates are limited. Exercise-induced hypoglycemia contributes to the perception of fatigue and consequently attenuates athletic performance [2
]. However, while the ingestion of adequate amounts of carbohydrates is vital to maintain glucose availability and optimize athletic performance [1
], the carbohydrate quality may also play an important role because various carbohydrate-rich foods affect postprandial glycaemia differently.
The glycemic index (GI) classifies carbohydrate-containing foods based on their postprandial glycemic response [3
]. The GI of a food is defined as the incremental area under the two-hour blood glucose response curve (AUC) following consumption of the tested food usually containing 50 g of CHO and divided by the AUC of a reference food containing the same amount of CHO (either glucose or white bread) and multiplied by 100. In general, foods with a high GI increase blood glucose concentration more rapidly than foods with a low GI. The GI of food is termed low when it is below 55, medium when it is between 55–70, and high when it is above 70 [4
]. Initially, the GI was used to design meals and diets for patients with diabetes, but current applications include weight loss and improvement in athletic performance [7
The effects of a pre-exercise meal’s GI on aerobic capacity and endurance performance have been investigated with equivocal results. Some studies reported improved endurance capacity or performance after ingestion of low-GI (LGI) compared to a high-GI (HGI) meal before exercise [9
], while others have not [7
]. These discrepancies could be caused by tested meal’s carbohydrate content and GI, meal timing, study design, and type of exercise test. For example, in two studies [8
] with similar exercise protocols, cyclists were provided with a pre-exercise meal containing 1 g∙kg−1
CHO and ingested 45 min before with meals, but the obtained results were different. In a study by Kern and colleagues [8
], no differences in performance on the 15-min cycling trial were found, while in the study by Moore and colleagues [9
], performance on the 40-km time trial significantly improved. This discrepancy might have been caused by differences in the GIs between the test meals (moderate vs. high and low vs. high) or by different performance cycling tests used in the Kern at al [8
] and Moore et al. [9
] studies (15 min vs. >90 min), respectively. Nevertheless, one meta-analysis concluded there is no clear benefit of low-GI pre-exercise meal for endurance performance regardless of carbohydrate ingestion during exercise [15
], while another meta-analysis found that endurance performance following an LGI meal is superior to that following an HGI meal [16
The available literature on the effect of diets with high versus low GI fed for 3–5 days on endurance performance or exercise capacity is limited to a few studies with inconclusive results. For example, Chen et al. [17
] showed that the most important factor in improving athletic performance was ingesting a three-day high-carbohydrate diet regardless of diet’s GI. Hamzah et al. [18
] found that consuming a five-day high-carbohydrate diet with either high or low GI had no impact on time to exhaustion or distance covered during a treadmill test at 65% maximal oxygen uptake (
max). However, it is possible that three- or five-day diet might be too short to elicit any metabolic changes and alter athletic performance. The intake of carbohydrates in these studies was relatively high (>70%), potentially limiting changes in substrate oxidation and restricting possible beneficial metabolic adaptations. It is possible that long-term low-GI diets might provide greater metabolic alterations than short-term diets. A plausible physiological rationale for long-term LGI diets is an increased fat oxidation caused by reduced carbohydrate availability during exercise. In addition, increased availability of non-esterified fatty acids could enhance the mitochondrial enzymes activity [19
]. Therefore, training on a LGI diet can be a good strategy for improving endurance adaptations, but more research in this area is necessary. We hypothesized that in actively training athletes a high-carbohydrate diet with low GI compared with moderate GI consumed for three weeks would induce modest improvements in aerobic capacity (maximal oxygen uptake, gas exchange threshold). Our secondary hypotheses were that endurance performance (distance in the 12-min running test and time to exhaustion) and maximal workload in the incremental cycling test (ICT) would differ between low and moderate GI diets for three weeks. To test these hypotheses, we conducted a randomized crossover feeding trial in a group of young actively training endurance runners consuming diets with either low (LGI = 39 ± 1) or moderate (MGI = 69 ± 1) GI for three weeks.
To the best of our knowledge, this is the first study to examine the effect of diets differing in GI over a relatively long time (three weeks). The major findings are that the LGI diet consumed for three weeks by actively training endurance runners may improve distance covered in the 12-min running test and time to exhaustion in ICT. Gas exchange threshold was improved by both experimental diets. Although we observed statistically significant differences in HRmax
within diets, we do not consider these differences clinically important [34
]. The LGI diet caused a slight decrease of body mass. However, our findings have to be interpreted with caution and need to be confirmed in larger trials conducted in various athletes’ populations.
The effect of GI on athletic performance was investigated previously, but the results were inconsistent. Several studies assessed the effect of a single pre-exercise meal with a different GI on endurance performance and found that either the performance was improved after low-GI meal or GI had no influence on performance [9
]. In contrast, other studies showed that a pre-exercise low-GI compared to high-GI meal improved running capacity by 7–23% [13
]. Several studies assessed the effect of a longer, usually 3–5 day, diets with various GI on athletic performance. For example, in a study of Hamzah and colleagues [18
], after five days of either high-GI or low-GI diet, no differences were found in running capacity (time to exhaustion—high GI: 107 min vs. low GI: 110 min) [18
]. Similarly, Jamurtas et al. [7
] did not find a difference in time to exhaustion after the pre-exercise low-GI and high-GI meals. We were not able to find any previous reports that used GI-controlled diet for longer than one week, making an interpretation of our results difficult. Specifically, we found that compared to baseline, the 3-week LGI but not the MGI diet improved time to exhaustion in ICT and distance covered in the 12-min running test. Also, the differences between LGI and MGI diets were not significant. In addition to longer diet time, there were other methodological differences between our and other studies [17
]. We measured performance using distance covered in the 12-min running test and time to exhaustion and maximal workload in the ICT, both of which demand high intensity effort. The ICT used in the present study to assess aerobic capacity, was chosen to eliminate factors inherent to treadmill test such as different running technique and type of shoes that could cause measurement errors [30
]. The previous short-term studies [12
] used lower intensity performance tests, and since at higher workloads fat oxidation is downregulated and CHO utilization increases [40
], our results cannot be compared to the results in these reports.
In this study, distance covered in the 12–min run significantly increased after LGI diet, indicating a possible improvement in performance endurance. It has been reported that a low-GI meal ingested before an endurance exercise resulted in a stable blood glucose concentration during the test [12
]. In contrast, a high-GI meal caused a rapid decrease in blood glucose concentration after 10 to 20 min of exercise, despite the fact that postprandial glucose concentration was higher after high-GI meal [35
]. Lower glucose concentration after high-GI meal was also observed at the end of exercise [12
]. That might have been caused by higher insulinemic response to the high-GI meal than the low-GI meal and a resultant glucose clearance from bloodstream. In our study, the tests were conducted 2–3 h after a meal at all visits and lasted for approximately 12–13 min (running test: 12 min; incremental cycling test: from 12.5 to 13.5 min on average). Since the drop in glucose concentration following high-GI meal from 10 to 20 min after the onset of exercise was previously reported [35
], we surmised that the longer distance covered after the LGI than the MGI diet might have been caused by the differences in blood glucose level, glycogen storage, and usage between the diets and/or better adaptation to more stable glycaemia. However, the interpretation is limited since glucose availability was not measured in the present study.
Although exercise performance is ultimately the most important end-goal for an athlete, monitoring aerobic capacity (GET, O2max) is important for determining the impact of training and nutritional strategies on the athlete’s capabilities. In the present study, baseline values of O2max were slightly lower before the MGI than the LGI diet, but they were similar post-MGI and post-LGI diets. In addition, O2max adjusted for body mass was not different between the diets.
Both diets increased time to GET and WGET
, suggesting an improved aerobic metabolism. It was reported previously that the anaerobic threshold is correlated with maximal fat oxidation [42
] and the point where fat utilization becomes negligible [43
]. Therefore, the occurrence of GET later and at higher exercise intensities may suggest better fat oxidation capacity. However, those assumptions are rather intuitive due to the lack of detailed metabolic data. In previous studies, fat oxidation was generally enhanced after the consumption of LGI meal compared to HGI meal [12
From the endurance athletes’ point of view, achieving and maintaining low body and fat mass is beneficial since it might improve performance [44
]. In the present study, the LGI diet induced a 0.6 kg body mass decrease. Body mass was significantly lower after LGI compared to after MGI diet. In a study by Hamzah et al. [18
], a five-day low-GI diet did not induce changes in body or fat mass. The difference between the studies might be caused by the diet duration (three weeks vs. five days). More research on the effects of a long-term LGI diet in combination with energy restriction on body weight and composition in athletes is warranted. Such studies could be especially important in sports that use a rapid weight reduction before competition such as wrestling [47
]. Although low-GI diets have been shown to promote weight loss and improve lipid profiles in obese and overweight adults [11
], little research has been reported on the influence of the GI on body composition in endurance athletes. Our study adds to the literature by highlighting the possibility that a relatively long (e.g., three weeks) LGI diet might help endurance athletes to accomplish modest changes in body weight and composition.
The limitations include small sample size and self-reported adherence to the diets. Although participants reported high adherence and diet compliance, it is possible that some did not eat all foods or ate foods they did not report. Four participants did not finish the study. However, considering the protocol requirements, 16% attrition seems reasonable. Relatively small sample size did not allow us to assess the effect of gender and age differences on study outcomes. An additional limitation was that we used the 12-min running test for assessing the potential effect of diets on aerobic capacity. However, the test is well known and commonly performed by runners and thus does not require familiarization and allows for evaluation running performance. In addition, we also used the incremental cycling test, which measures aerobic capacity more accurately than the 12-min running test. Finally, we did not measure blood biochemical parameters during and after exercise, making it impossible to identify the mechanism of GI effect on exercise performance and endurance.
The strengths include a relatively homogeneous population of actively-trained endurance runners, a comparatively long three-week dietary intervention, and using reference standard methods to assess differences between the diets in a randomized crossover design.
Future research should focus on confirming our results in a larger and various populations and assessing the difference in aerobic capacity and performance changes between diets with high and low GI. Practical application of our results is that athletes and coaches might consider the GI of food in planning a healthy diet, as the LGI diet might slightly improve performance and help to achieve desirable body composition.