Journal Title (Medline/Pubmed accepted abbreviation): Med. Sci. Sports Exerc.
Page numbers: 1949-1957
Summary of background and research design:
Background: Dehydration during exercise can lead to increased body temperature and poor performance. Previous studies have suggested dehydration may affect which type of substrate (i.e. fat or carbohydrate) is burned for energy.
Hypothesis: Dehydration during cycling will lead to a greater elevation in body temperature compared to healthy hydration. Also, when dehydrated, a larger ratio of carbohydrates: fats will be burned for energy. The effects of dehydration will be more pronounced in the second hour of cycling compared to the first.
Subjects: Recreationally active females, age 21.7 ± 0.6 y.
Experimental design: randomized, cross-over
HYD: hydrated- participants consumed water to match losses due to sweat
DEH: dehydrated- participants did not consume fluid during exercise
Protocol: The participants were first evaluated for VO2max (or VO2peak) on a stationary bicycle. They completed two subsequent practice sessions where they cycled 120 min at 65% VO2max either with (HYD) or without (DEH) fluid. Two hours before the cycling session they ingested a standardized meal with 250 mL water. Additionally, they consumed 300 mL of water 90 and 45 min before exercise. A urine sample was collected to determine pre-exercise hydration status and body mass was determined just before cycling. Respiratory gases were measured every 30 min to determine the volume of oxygen consumed and the volume of carbon dioxide produced. These measurements were used to estimate the amount of energy that was produced from carbohydrates or fats. At 60 min, the participants were weighed for estimation of sweat loss. They then completed two similar trials during which data was collected for analysis and reporting. In addition to metrics that were acquired during the practice runs, heart rate was monitored; blood was collected for analysis of hematocrit as well as blood glucose, lactate, free fatty acid, epinephrine, and hemoglobin concentrations; muscle biopsies were collected for analysis of phosphocreatine, creatine, ATP, and lactate concentrations; core temperature was measured; and ratings of perceived exertion were noted. During the HYD trial, the participants were provided with water every 15 min in order to replenish fluid lost to sweat. Trials were separated by 7 days.
Summary of research findings:
- The participants lost 1.2 ± 0.9 L fluid during exercise. Therefore, they consumed this much during the HYD trials.
- During the DEH trials, participants lost 0.9% of their body weight after 1 hr and 2.0% of their initial body weight after 2 hrs of cycling.
- Oxygen consumption increased with time for both trials with no difference between trials. It went from about 1.95 L/min to about 2.05 L/min in 120 min.
- Energy provision shifted from carbohydrates (i.e. glycogen) to fat during exercise. At every time point from 40-120 min, less energy was provided from fat in DEH compared to HYD. Accordingly, there was greater carbohydrate oxidation for DEH compared to HYD.
- Heart rate and core body temperature both increased with time. They were both greater in DEH than HYD for every time point from 30-120 min.
- Ratings of perceived exertion were significantly greater for DEH than HYD from 60-120 min.
- Exercise led to a loss of plasma volume that was accentuated in DEH leading to generally greater concentrations of hemoglobin and percent hematocrit.
- Exercise caused a decrease in blood glucose concentration and an increase in blood lactate, free fatty acid, and epinephrine concentrations with no difference between DEH and HYD.
- Exercise promoted a decline in phosphocreatine with a corresponding increase in creatine in skeletal muscle. There were no differences between groups.
- Exercise promoted an increase in lactate concentration in muscle. Lactate concentrations were greater in DEH than HYD.
- Glycogen content was similar between groups before exercise. The rate of glycogen usage was not statistically different during the first 60 min of exercise but was greater in DEH than HYD during the second hour of exercise.
Key practice applications: While cycling, the athletes that did not drink water (DEH) lost 2% of their body weight in 2 hrs. Also, many physiological stressors associated with exercise were exacerbated, including increased heart rate, core body temperature, ratings of perceived exertion, and lactate concentrations in skeletal muscle. Additionally, those that consumed enough liquid to compensate for sweat losses burned a higher proportion of fat for energy, thereby sparing glycogen stores. In conclusion, it is important to remain hydrated during exercise in order to maximize performance, reduce the rate of glycogen loss, and increase the rate of fat burning.
Key search terms for this article (5-7 terms): hydration, fluid, water, dehydration, substrate oxidation