Effects of Dehydration during Cycling on Skeletal Muscle Metabolism in Females
ABSTRACT This study investigated the effects of progressive dehydration on the time course of changes to whole body substrate oxidation and skeletal muscle metabolism during 120 min of cycling in hydrated females.
Subjects (n = 9) cycled for 120 min at approximately 65% V˙O2peak on two occasions: with no fluid (DEH) and with fluid (HYD) replacement to match sweat losses. Venous blood samples were taken at rest and every 20 min and muscle biopsies taken at 0, 60, and 120 min of exercise.
DEH subjects lost 0.9% body mass from 0 to 60 min and 1.1% from 60 to 120 min (2.0% total). HR and core temperature (Tc) were significantly greater from 30 to 120 min, plasma volume (Pvol) loss from 40 to 120 min, and RPE from 60 to 120 min in the DEH trial. There were no differences in V˙O2 or sweat loss between trials. RER (HYD, 0.85 ± 0.01, vs. DEH, 0.87 ± 0.01) and total CHO oxidation (175 ± 17 vs. 191 ± 17 g) were higher in the DEH trial. Blood (La) was significantly higher in the DEH trial, with no change in plasma free fatty acid and epinephrine concentrations. Muscle glycogenolysis was 31% greater in the DEH trial (252 ± 49 vs. 330 ± 33 mmol·kg dry muscle), and muscle (La) was also higher at 60 min.
Progressive dehydration significantly increased HR, Tc, RPE, Pvol loss, whole body CHO oxidation, and muscle glycogenolysis, and these changes were already apparent in the first hour of exercise when body mass losses were ≤1%. The increased muscle glycogenolysis with DEH appeared to be due to increased core and muscle temperature, secondary to less efficient movement of heat from the core to the periphery.
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ABSTRACT: Research in "stop-and-go" sports has demonstrated that carbohydrate ingestion improves performance and fatigue, and that dehydration of ∼1.5%-2% body mass (BM) loss results in decreased performance, increased fatigue, and increased core temperature. The purpose of this investigation was to assess the physiological, performance, and fatigue-related effects of maintaining hydration with a carbohydrate-electrolyte solution (CES) versus dehydrating by ∼2% BM (no fluid; NF) during a 70-min ice hockey scrimmage. Skilled male hockey players (n = 14; age, 21.3 ± 0.2 years; BM, 80.1 ± 2.5 kg; height, 182.0 ± 1.2 cm) volunteered for the study. Subjects lost 1.94% ± 0.1% BM in NF, and 0.12% ± 0.1% BM in CES. Core temperature (Tc) throughout the scrimmage (10-50 min) and peak Tc (CES: 38.69 ± 0.10 vs. NF: 38.92 ± 0.11 °C; p < 0.05) were significantly reduced in CES compared with NF. Players in CES had increased mean skating speed and time at high effort between 30-50 min of the scrimmage. They also committed fewer puck turnovers and completed a higher percentage of passes in the last 20 min of play compared with NF. Postscrimmage shuttle skating performance was improved in CES versus NF and fatigue was lower following the CES trial. The results indicated that ingesting a CES to maintain BM throughout a 70-min hockey scrimmage resulted in improved hockey performance and thermoregulation, and decreased fatigue as compared with drinking no fluid and dehydrating by ∼2%.Applied Physiology Nutrition and Metabolism 05/2014; 39(11):1-8. DOI:10.1139/apnm-2014-0091 · 2.23 Impact Factor
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ABSTRACT: Hydration pertains simplistically to body water volume. Functionally, however, hydration is one aspect of fluid regulation that is far more complex, as it involves the homeostatic regulation of total body fluid volume, composition and distribution. Deliberate or pathological alteration of these regulated factors can be disabling or fatal, whereas they are impacted by exercise and by all environmental stressors (e.g. heat, immersion, gravity) both acutely and chronically. For example, dehydration during exercising and environmental heat stress reduces water volume more than electrolyte content, causing hyperosmotic hypohydration. If exercise continues for many hours with access to food and water, composition returns to normal but extracellular volume increases well above baseline (if exercising upright and at low altitude). Repeating bouts of exercise or heat stress does likewise. Dehydration due to physical activity or environmental heat is a routine fluid-regulatory stress. How to gauge such dehydration and - more importantly-what to do about it, are contested heavily within sports medicine and nutrition. Drinking to limit changes in body mass is commonly advocated (to maintain ≤2% reduction), rather than relying on behavioural cues (mainly thirst) because the latter has been deemed too insensitive. This review, as part of the series on moving in extreme environments, critiques the validity, problems and merits of externally versus autonomously controlled fluid-regulatory behaviours, both acutely and chronically. Our contention is that externally advocated hydration policies (especially based on change in body mass with exercise in healthy individuals) have limited merit and are extrapolated and imposed too widely upon society, at the expense of autonomy. More research is warranted to examine whether ad libitum versus avid drinking is beneficial, detrimental or neither in: acute settings; adapting for obligatory dehydration (e.g. elite endurance competition in the heat), and; development of chronic diseases that are associated with an extreme lack of environmental stress.01/2014; 3:18. DOI:10.1186/2046-7648-3-18
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ABSTRACT: The objective of this article is to provide a review of the fundamental aspects of body fluid balance and the physiological consequences of water imbalances, as well as discuss considerations for the optimal composition of a fluid replacement beverage across a broad range of applications. Early pioneering research involving fluid replacement in persons suffering from diarrheal disease and in military, occupational, and athlete populations incurring exercise- and/or heat-induced sweat losses has provided much of the insight regarding basic principles on beverage palatability, voluntary fluid intake, fluid absorption, and fluid retention. We review this work and also discuss more recent advances in the understanding of fluid replacement as it applies to various populations (military, athletes, occupational, men, women, children, and older adults) and situations (pathophysiological factors, spaceflight, bed rest, long plane flights, heat stress, altitude/cold exposure, and recreational exercise). We discuss how beverage carbohydrate and electrolytes impact fluid replacement. We also discuss nutrients and compounds that are often included in fluid-replacement beverages to augment physiological functions unrelated to hydration, such as the provision of energy. The optimal composition of a fluid-replacement beverage depends upon the source of the fluid loss, whether from sweat, urine, respiration, or diarrhea/vomiting. It is also apparent that the optimal fluid-replacement beverage is one that is customized according to specific physiological needs, environmental conditions, desired benefits, and individual characteristics and taste preferences. © 2014 American Physiological Society. Compr Physiol 4:575-620, 2014.04/2014; 4(2):575-620. DOI:10.1002/cphy.c130014