This study examined the effects of the glycemic index (GI) of post-exercise carbohydrate (CHO) intake on endurance capacity the following day. Nine active males participated in 2 trials. On day 1, subjects ran for 90 min at 70% VO2max (R1). Thereafter, they were supplied with either a high GI (HGI) or low GI (LGI) CHO diet which provided 8 g CHO/kg body mass (BM). On day 2, after an overnight fast, subjects ran to exhaustion at 70% VO2max (R2). Time to exhaustion during R2 was longer in the LGI trial (108.9 ± 7.4 min) than in the HGI trial (96.9 ± 4.8 min) (P < 0.05). Fat oxidation rates and free fatty acid concentrations were higher in the LGI trial than the HGI trial (P < 0.05). The results suggest that the increased endurance capacity was largely a consequence of the increased fat oxidation following the LGI recovery diet.
Emma Stevenson, Clyde Williams, Gareth McComb, and Christopher Oram
Ching-Lin Wu and Clyde Williams
This study investigated the effects of ingesting a low (LGI) or high (HGI) glyce-mic index carbohydrate (CHO) meal 3 h prior to exercise on endurance running capacity. Eight male recreational runners undertook two trials (LGI or HGI) which were randomized and separated by 7 d. After an overnight fast (12 h) the subjects ingested either a LGI or HGI meal 3 h prior to running at 70% VO2max until exhaustion. The meals contained 2 g/kg body mass CHO and were isocaloric and iso-macronutrient with calculated GI values 77 and 37 for the HGI and LGI respectively. The run times for the LGI and HGI trials were 108.8 ± 4.1 min and 101.4 ± 5.2 min respectively (P = 0.038). Fat oxidation rates were higher during exercise after the LGI meal than after the HGI meal (P < 0.05). In summary, ingestion of a LGI meal 3 h before exercise resulted in a greater endurance capacity than after the ingestion of a HGI meal.
Neil M. Johannsen and Rick L. Sharp
The purpose of this study was to investigate differences in substrate oxidation between dextrose (DEX) and unmodified (UAMS) and acid/alcohol-modified (MAMS) cornstarches. Seven endurance-trained men (VO2peak = 59.1 ± 5.4 mL·kg−1·min−1) participated in 2 h of exercise (66.4% ± 3.3% VO2peak) 30 min after ingesting 1 g/kg body weight of the experimental carbohydrate or placebo (PLA). Plasma glucose and insulin were elevated after DEX (P < 0.05) compared with UAMS, MAMS, and PLA. Although MAMS and DEX raised carbohydrate oxidation rate through 90 min of exercise, only MAMS persisted throughout 120 min (P < 0.05 compared with all trials). Exogenous-carbohydrate oxidation rate was higher in DEX than in MAMS and UAMS until 90 min of exercise. Acid/alcohol modification resulted in augmented carbohydrate oxidation with a small, sustained increase in exogenous-carbohydrate oxidation rate. MAMS appears to be metabolizable and available for oxidation during exercise.
Emma Stevenson, Clyde Williams, Maria Nute, Peter Swaile, and Monica Tsui
The present study investigated the effect of the glycemic index of an evening meal on responses to a standard high glycemic index (HGI) breakfast the following morning. The metabolic responses to exercise 3 h after breakfast were also investigated. Seven active males completed 2 trials. In each trial, participants were provided with an evening meal on day 1, which was composed of either HGI or LGI (high or low glycemic index) carbohydrates. On day 2, participants were provided with a standard HGI breakfast and then performed a 60 min run at 65% VO2max 3 h later. Plasma glucose and serum insulin concentrations following breakfast were higher in the HGI trial compared to the LGI trial (P < 0.05). During exercise, there were no differences in substrate utilization. The results suggest that consuming a single LGI evening meal can improve glucose tolerance at breakfast but the metabolic responses to subsequent exercise were not affected.
Emma Stevenson, Clyde Williams, and Helen Biscoe
This study investigated the metabolic responses to high glycemic index (HGI) or low glycemic index (LGI) meals consumed during recovery from prolonged exercise. Eight male, trained athletes undertook 2 trials. Following an overnight fast, subjects completed a 90-min run at 70% VO2max. Meals were provided 30 min and 2 h following cessation of exercise. The plasma glucose responses to both meals were greater in the HGI trial compared to the LGI trial (P < 0.05). Following breakfast, there were no differences in the serum insulin concentrations between the trials; however, following lunch, concentrations were higher in the HGI trial compared to the LGI trial (P < 0.05). This suggests that the glycemic index of the carbohydrates consumed during the immediate post-exercise period might not be important as long as sufficient carbohydrate is consumed. The high insulin concentrations following a HGI meal later in the recovery period could facilitate further muscle glycogen resynthesis.
Tzai-Li Li, Ching-Ling Wu, Michael Gleeson, and Clyde Williams
The purpose of this study was to determine the effect of pre-exercise high carbohydrate meals with high glycemic index (HGI) or low glycemic index (LGI) on blood leukocyte redistribution during subsequent endurance exercise. Eight male subjects performed a 90-min run on a treadmill at 70% VO2max 3 h after ingesting an isocaloric HGI or LGI meal with GI values of 77 and 37, respectively. Blood counts of leukocytes, and neutrophils and the neutrophil/lymphocyte ratio were significantly lower in LGI than HGI at 90 min of exercise (P < 0.05). The plasma glucose concentrations were significantly higher in LGI than HGI between 15 min and 45 min of exercise. There were, however, no differences in plasma cortisol, growth hormone, and interleukin-6 concentrations between trials. Thus, the GI of a pre-exercise meal influences leukocyte trafficking and plasma glucose but has limited effects on circulating stress hormone and cytokine responses to exercise.
Diana E. Thomas, John R. Brotherhood, and Janette Brand Miller
It was hypothesized that slowly digested carbohydrates, that is, low glycemic index (GI) foods, eaten before prolonged strenuous exercise would increase the blood glucose concentration toward the end of exercise. Six trained cyclists pedaled on a cycle ergometer at 65-70%
Laura J.S. Moore, Adrian W. Midgley, Gemma Thomas, Shane Thurlow, and Lars R. McNaughton
The aim of this work was to determine whether the consumption of pre-exercise high– or low–glycemic index (GI) meals has a beneficial effect on time trial performance.
Eight male cyclists were provided with either a high-GI or low-GI meal, providing 1 g·kg−1 body mass of carbohydrate, 45 min before performing a 40-km time trial on a Velotron cyclePro.
Time trial performance was significantly improved in the low-GI trial (92.5 ± 5.2 min) compared with the high-GI trial (95.6 ± 6.0 min) (P = .009). Blood glucose concentrations at the point of exhaustion were significantly higher in the low-GI trial (5.2± 0.6 mmol·L−1) compared with the high-GI trial (4.7 ± 0.7 mmol·L−1) (P = .001). There was no significant difference in estimated carbohydrate oxidation data between the low-GI (2.51 ± 1.74 g·min−1) and high-GI (2.18 ± 1.53 g·min−1) meals (P = .195). No significant difference in estimated fat oxidation was observed between the low-GI (0.15 ± 0.15 g·min−1) and high-GI (0.29 ± 0.18 g·min−1) diets (P = .83).
The improvement in time trial performance for the low-GI trial may be associated with an increased availability of glucose to the working muscles, contributing additional carbohydrate for oxidation and possibly sparing limited muscle and liver glycogen stores.
Stephanie E. Bonn, Lars Alfredsson, Saedis Saevarsdottir, and Maria E.C. Schelin
Effective interventions are needed to increase physical activity in the general population. To target interventions, we need knowledge of insufficiently active groups in society. This study aims to identify demographic and health-related correlates of leisure-time physical inactivity in a general Scandinavian population.
Study participants comprised 5734 control subjects, age 18 to 70 years, from 2 ongoing Swedish case-control studies. Participants self-reported their leisure-time physical activity level. The odds of being physically inactive were calculated using logistic regression.
A total of 42% of participants were classified as physically inactive during leisure time. A lower prevalence of inactivity was associated with middle age, higher education, having previous experience of sports participation, following a low glycemic index/Mediterranean diet and having a light physical workload. A high prevalence of inactivity was associated with greater age, high body mass index, smoking, never drinking alcohol, having children, having a weak social network or lower levels of emotional support, and a low vegetable intake.
Several factors were associated with leisure-time physical inactivity. Directing interventions to target groups defined by specific factors associated with physical inactivity could be an efficient way to increase activity and improve health in the general population.
Stephen R. Stannard, Martin W. Thompson, and Janette C. Brand Miller
Consumption of low glycemic index (GI) foods before submaximal endurance exercise may be beneficial to performance. To test whether this may also be true for high intensity exercise. 10 trained cyclists began an incremental exercise test to exhaustion 65 min after consuming equal carbohydrate portions of glucose (HGI), pasta (LGI), and a noncarbohydrate control (PL). Time to fatigue did not differ significantly (p = 0.05) between treatments. Plasma glucose concentration was significantly lower after LGI vs. HGI from 15 to 45 min of rest postprandial. During exercise, plasma glucose concentration was significantly lower after HGI vs. LGI from 200 W until exhaustion. Plasma lactate concentration following HGI was significantly higher than PL from 30 min of rest postprandial through to the end of the 200-W workload. Plasma lactate concentration following LGI was significantly lower than after HGI from 45 min of rest postprandial through to the end of the 100-W workload. At higher exercise intensities, there was no significant difference in plasma lactate levels between treatments. These findings suggest that a high GI carbohydrate meal (1 g/kg body wt) 65 min prior to exercise decreases plasma glucose and increases plasma lactate levels compared to a low GI meal, but not enough to be detrimental to incremental exercise performance.