Blinded and unblinded hypohydration similarly impair cycling time trial performance in the heat in trained cyclists

Knowledge of hydration status may contribute to hypohydration-induced exercise performance decrements, therefore, this study compared blinded and unblinded hypohydration on cycling performance. Fourteen trained, non-heat acclimated cyclists (age 25 ± 5 y; V̇O2peak 63.3 ± 4.7 mL∙kg-1∙min-1; cycling experience 6 ± 3 y) were pair-matched to blinded (B) or unblinded (UB) groups. After familiarisation, subjects completed euhydrated (B-EUH; UB-EUH) and hypohydrated (B-HYP; UB-HYP) trials in the heat (31˚C); 120 min cycling preload (50% Wpeak) and a time trial (~15 min). During the preload of all trials, 0.2 mL water∙kg body mass-1 was ingested every 10 min, with additional water provided during EUH trials to match sweat losses. To blind the B group, a nasogastric tube was inserted in both trials and used to provide water in B-EUH. The preload induced similar ( P=0.895) changes in body mass between groups (B-EUH -0.6 ± 0.5%; B-HYP -3.0 ± 0.5%; UB-EUH -0.5 ± 0.3%; UB-HYP -3.0 ± 0.3%). All variables responded similarly between B and UB groups ( P≥0.558), except thirst ( P=0.004). Changes typical of hypohydration (increased heart rate, RPE, gastrointestinal temperature, serum osmolality and thirst, decreased plasma volume; P≤0.017) were apparent in HYP by 120 min. Time trial performance was similar between groups ( P=0.710) and slower ( P≤0.013) with HYP for B (B-EUH 903 ± 89 s; B-HYP 1008 ± 121 s; -11.4%) and UB (UB-EUH 874 ± 108 s; UB-HYP 967 ± 170 s; -10.1%). Hypohydration of ~3% body mass impairs time trial performance in the heat, regardless of knowledge of hydration status

Chronic ingestion of a low dose of caffeine induces tolerance to the performance benefits of caffeine

This study examined effects of 4 weeks of caffeine supplementation on endurance performance. Eighteen low-habitual caffeine consumers ( 0.05). Before supplementation, all participants completed one V̇O2peak test, one practice trial and 2 experimental trials (acute 3 mg · kg−1 caffeine [precaf] and placebo [testpla]). During the supplementation period a second V̇O2peak test was completed on day 21 before a final, acute 3 mg · kg−1 caffeine trial (postcaf) on day 29. Trials consisted of 60 min cycle exercise at 60% V̇O2peak followed by a 30 min performance task. All participants produced more external work during the precaf trial than testpla, with increases in the caffeine (383.3 ± 75 kJ vs. 344.9 ± 80.3 kJ; Cohen’s d effect size [ES] = 0.49; P = 0.001) and placebo (354.5 ± 55.2 kJ vs. 333.1 ± 56.4 kJ; ES = 0.38; P = 0.004) supplementation group, respectively. This performance benefit was no longer apparent after 4 weeks of caffeine supplementation (precaf: 383.3 ± 75.0 kJ vs. postcaf: 358.0 ± 89.8 kJ; ES = 0.31; P = 0.025), but was retained in the placebo group (precaf: 354.5 ± 55.2 kJ vs. postcaf: 351.8 ± 49.4 kJ; ES = 0.05; P > 0.05). Circulating caffeine, hormonal concentrations and substrate oxidation did not differ between groups (all P > 0.05). Chronic ingestion of a low dose of caffeine develops tolerance in low-caffeine consumers. Therefore, individuals with low-habitual intakes should refrain from chronic caffeine supplementation to maximise performance benefits from acute caffeine ingestion

24 h severe energy restriction impairs post-prandial glycaemic control in young, lean males

Intermittent energy restriction (IER) involves short periods of severe energy restriction interspersed with periods of adequate energy intake, and can induce weight loss. Insulin sensitivity is impaired by short-term, complete energy restriction, but the effects of IER are not well known. In randomised order, 14 lean men (age: 25 (SD 4) y; BMI: 24 (SD 2) kg·m-2; body fat: 17 (4) %) consumed 24 h diets providing 100% (10441 (SD 812) kJ; EB) or 25% (2622 (SD 204) kJ; ER) of estimated energy requirements, followed by an oral glucose tolerance test (OGTT; 75g glucose drink) overnight fasted. Plasma/ serum glucose, insulin, non-esterified fatty acids (NEFA), glucagon-like peptide-1 (GLP-1), glucose-dependant insulinotropic peptide (GIP) and fibroblast growth factor-21 (FGF21) were assessed before and after (0 h) each 24 h dietary intervention, and throughout the 2 h OGTT. Homeostatic model assessment of insulin resistance (HOMA2-IR) assessed the fasted response and incremental (iAUC) or total (tAUC) area under the curve were calculated during the OGTT. At 0 h, HOMA2-IR was 23% lower after ER compared to EB (P<0.05). During the OGTT, serum glucose iAUC (P<0.001) serum insulin iAUC (P<0.05) and plasma NEFA tAUC (P<0.01) were greater during ER, but GLP-1 (P=0.161), GIP (P=0.473) and FGF21 (P=0.497) tAUC were similar between trials. These results demonstrate that severe energy restriction acutely impairs postprandial glycaemic control in lean men, despite reducing HOMA2-IR. Chronic intervention studies are required to elucidate the long-term effects of IER on indices of insulin sensitivity, particularly in the absence of weight loss