The aim of this review is to provide an up-to-date summary of the evidence surrounding glycemic index (GI) and endurance performance. Athletes are commonly instructed to consume low-GI (LGI) carbohydrate (CHO) before exercise, but this recommendation appears to be based on the results of only a few studies, whereas others have found that the GI of CHO ingested before exercise has no impact on performance. Only 1 study was designed to directly investigate the impact of the GI of CHO ingested during exercise on endurance performance. Although the results indicate that GI is not as important as consuming CHO itself, more research in this area is clearly needed. Initial research investigating the impact of GI on postexercise recovery indicated consuming high-GI (HGI) CHO increased muscle glycogen resynthesis. However, recent studies indicate an interaction between LGI CHO and fat oxidation, which may play a role in enhancing performance in subsequent exercise. Despite the fact that the relationship between GI and sporting performance has been a topic of research for more than 15 yr, there is no consensus on whether consuming CHO of differing GI improves endurance performance. Until further well-designed research is carried out, athletes are encouraged to follow standard recommendations for CHO consumption and let practical issues and individual experience dictate the use of HGI or LGI meals and supplements before, during, and after exercise.
Carolyn M. Donaldson, Tracy L. Perry and Meredith C. Rose
Willeke Trompers, Tracy L Perry, Meredith C Rose and Nancy J. Rehrer
The purpose of this study was to determine whether glycemic index (GI) is influenced by training state. Participants were tested in a randomized order: twice with a reference solution containing 50 g glucose and once each with 2 commercially available snack bars (Griffin’s Fruitli bar and Peak Fuel’s Summit bar) containing 50 g available carbohydrate. Eleven of the participants (6 men and 5 women, M ± SD age 20.8 ± 2.0 yr) were endurance trained (ET; VO2max 57.5 ± 8.4 ml · kg−1 · min−1), and 9 participants (2 men and 7 women, M ± SD age 22.4 ± 1.8 yr) were sedentary (SE; VO2max 43.7 ± 9.1 ml · kg−1 · min−1). After an overnight fast, participants consumed either the glucose solution or snack bar, with blood samples taken before eating and at 15, 30, 45, 60, 90, and 120 min after eating began. The mean incremental area under the curve (IAUC) of the glucose reference was 31% lower (95% CI 3–52%, p = .03), and the Fruitli bar 38% lower (95% CI 0–61%, p = .05) in ET than in SE participants. There was a trend for the IAUC for the Summit bar to be 35% lower in ET than in SE participants (95% CI –7% to 61% p = .09). There was no significant interaction between training state and test food. The GIs of the Fruitli and Summit bars was not significantly different between ET and SE participants (p = .65 and .54, respectively). ET participants had a lower glycemic response than SE participants; however, training state did not influence GI.
Kirsty A. Fairbairn, Ingrid J.M. Ceelen, C. Murray Skeaff, Claire M. Cameron and Tracy L. Perry
Vitamin D insufficiency is common in athletes and may lower physical performance. Many cross-sectional studies associate vitamin D status with physical performance in athletes; however, there have been few prospective randomized controlled trials with adequate statistical power to test this relationship, and none in the southern hemisphere. Thus, a prospective double-blind, randomized, placebo-controlled intervention trial was conducted involving 57 professional rugby union players in New Zealand. Participants were randomized to receive 50,000 IU of cholecalciferol (equivalent to 3,570 IU/day) or placebo once every two weeks over 11–12 weeks. Serum 25(OH)D concentrations and physical performance were measured at baseline, weeks 5–6, and weeks 11–12. Mean (SD) serum 25(OH)D concentrations for all participants at baseline was 94 (18) nmol/L, with all players above 50 nmol/L. Vitamin D supplementation significantly increased serum 25(OH)D concentrations compared to placebo, with a 32 nmol/L difference between groups at 11–12 weeks (95% CI, 26–38; p < 0.001). Performance in five of the six tests at study completion, including the primary outcome variable of 30-m sprint time, did not differ between the vitamin D supplemented and placebo groups (p > 0.05). Performance on the weighted reverse-grip chin up was significantly higher in players receiving vitamin D compared with placebo, by 5.5 kg (95% CI, 2.0–8.9; p = 0.002). Despite significantly improving vitamin D status in these professional rugby union players, vitamin D supplementation had little impact on physical performance outcomes. Thus, it is unlikely that vitamin D supplementation is an ergogenic aid in this group of athletes.
Meredith C. Peddie, Matthew Reeves, Millie K. Keown, Tracy L. Perry and C. Murray Skeaff
Background: Regular activity breaks positively impact markers of cardiometabolic health when performed in a laboratory. However, identifying compliance to a free-living regular activity breaks intervention is challenging, particularly if intensity is prescribed. Methods: This study had two parts. In Part A, 20 participants performed activity breaks similar to those shown to impart health benefits while wearing an ActiGraph and activPAL accelerometer, and a heart rate monitor. In Part B, the threshold found to identify these activities was used to identify the activity breaks performed by 78 sedentary, university employees wearing an ActiGraph accelerometer for seven days. Results: A cut-point of 1,000 vector magnitude counts per minute accurately identified activity breaks performed in the laboratory. Applying this cut-point to data collected in free living, sedentary participants identified, on average, seven activity breaks were being performed during work-hours. Conclusions: Using a cut-point of 1,000 vector magnitude counts per minute will identify activity breaks of a similar intensity to those found to elicit acute cardiometabolic benefit. Sedentary university employees may benefit from interventions to increase the number of activity breaks performed across their entire day.