This study examined effects of 4 weeks of caffeine supplementation on endurance performance. Eighteen low-habitual caffeine consumers (<75 mg · day) were randomly assigned to ingest caffeine (1.5-3.0 mg · kgday; titrated) or placebo for 28 days. Groups were matched for age, body mass, V̇O and W (P > 0.05). Before supplementation, all participants completed one V̇O test, one practice trial and 2 experimental trials (acute 3 mg · kg caffeine [precaf] and placebo [testpla]). During the supplementation period a second V̇O test was completed on day 21 before a final, acute 3 mg · kg caffeine trial (postcaf) on day 29. Trials consisted of 60 min cycle exercise at 60% V̇O 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.

Objectives: Ingested ketogenic agents offer the potential to enhance endurance performance via the provision of an alternative exogenous, metabolically efficient, glycogen-sparing fuel (i.e. ketone bodies). This study aimed to assess the impact of combined carbohydrate and 1,3-butanediol (CHO-BD) supplementation on endurance performance, blood beta-hydroxybutyrate (βHB) concentration and glycolytic activity, in comparison to carbohydrate supplementation alone (CHO). Design: Eleven male runners (age 38 ± 12 years, mass 67.3 ± 6.5 kg, height 174.5 ± 5.0 cm, V O2peak 64.2 ± 5.0 ml•kg-1 •min-1) performed two experimental trials in a randomised crossover design. Methods: Each trial consisted of 60 min of submaximal running, followed by a 5 km running time-trial (TT), and was performed following the ingestion of an energy matched ~650 ml drink (CHO-BD or CHO). Results: There was no difference in TT completion time between the trials (CHO: 1265 ± 93, CHO-BD: 1261 ± 96 s; p=0.723). However, blood βHB concentration in the CHO-BD trial was at least double that of the CHO trial at all time points following supplementation (p<0.05). While blood lactate concentration was lower in the CHO-BD versus CHO trial after 30 min submaximal exercise (CHO-BD: 1.46 ± 0.67 mmol•L-1 , CHO: 1.77 ± 0.46 mmol•L-1 , p=0.040), it was similar at other time points. Blood glucose concentrations were higher post-TT in the CHO-BD trial (CHO-BD: 5.83 ± 1.02 mmol•L-1 , CHO: 5.26 ± 0.95 mmol•L-1 , p=0.015). Conclusions: An energy matched CHO-BD supplementation drink raised βHB concentration and acutely lowered blood lactate concentration, without enhancing 5km TT running performance.

The general scientific consensus is that starting exercise with hypohydration >2% body mass impairs endurance performance/capacity, but most previous studies might be confounded by a lack of subject blinding. This study examined the effect of hypohydration in a single blind manner using combined oral and intragastric rehydration to manipulate hydration status. After familiarization, seven active males (mean ± SD: age 25 ± 2 years, height 1.79 ± 0.07, body mass 78.6 ± 6.2, VO2peak 48 ± 7 mL·kg·min−1) completed two randomized trials at 34°C. Trials involved an intermittent exercise preload (8 × 15 min exercise/5 min rest), followed by a 15‐min all‐out performance test on a cycle ergometer. During the preload, water was ingested orally every 10 min (0.2 mL·kg body mass−1). Additional water was infused into the stomach via a gastric feeding tube to replace sweat loss (EU) or induce hypohydration of ~2.5% body mass (HYP). Blood samples were drawn and thirst sensation rated before, during, and after exercise. Body mass loss during the preload was greater (2.4 ± 0.2% vs. 0.1 ± 0.1%; P < 0.001), while work completed during the performance test was lower (152 ± 24 kJ vs. 165 ± 22 kJ; P < 0.05) during HYP. At the end of the preload, heart rate, RPE, serum osmolality, and thirst were greater and plasma volume lower during HYP (P < 0.05). These results provide novel data demonstrating that exercise performance in the heat is impaired by hypohydration, even when subjects are blinded to the intervention.

Pre-trial body mass (P=0.692), urine osmolality (P=0.838) and serum osmolality (P=0.574) were the same on both trials. FR resulted in a 1.1±0.7% reduction in body mass, compared to -0.1±0.6% in the HYD trial (P=0.002). Urine and serum osmolality were both increased following FR (P<0.05). There was a progressive increase in the total number of driver errors observed during both the HYD and FR trials, but significantly more incidents were recorded throughout the FR trial (HYD 47±44, FR 101±84; ES=0.81; P=0.006) CONCLUSIONS: The results of the present study suggest that mild hypohydration, produced a significant increase in minor driving errors during a prolonged, monotonous drive, compared to that observed while performing the same task in a hydrated condition. The magnitude of decrement reported, was similar to that observed following the ingestion of an alcoholic beverage resulting in a blood alcohol content of approximately 0.08% (the current UK legal driving limit), or while sleep deprived.

The impact of alterations in hydration status on human physiology and performance responses during exercise is one of the oldest research topics in sport and exercise nutrition. This body of work has mainly focussed on the impact of reduced body water stores (i.e. hypohydration) on these outcomes, on the whole demonstrating that hypohydration impairs endurance performance, likely via detrimental effects on a number of physiological functions. However, an important consideration, that has received little attention, is the methods that have traditionally been used to investigate how hypohydration affects exercise outcomes, as those used may confound the results of many studies. There are two main methodological limitations in much of the published literature that perhaps make the results of studies investigating performance outcomes difficult to interpret. First, subjects involved in studies are generally not blinded to the intervention taking place (i.e. they know what their hydration status is), which may introduce expectancy effects. Second, most of the methods used to induce hypohydration are both uncomfortable and unfamiliar to the subjects, meaning that alterations in performance may be caused by this discomfort, rather than hypohydration per se. This review discusses these methodological considerations and provides an overview of the small body of recent work that has attempted to correct some of these methodological issues. On balance, these recent blinded hydration studies suggest hypohydration equivalent to 2-3% body mass decreases endurance cycling performance in the heat, at least when no/little fluid is ingested.