Effects of endurance exercise on metabolic water production and plasma volume

Abstract

Six endurance-trained and heat-acclimatized adult males ran for 1 h (or until exhaustion) at room temperature (23.8 degrees C) on three occasions. The work loads approximated 37, 56, and 74% of the subjects' aerobic capacities. Venous blood samples were drawn, and urine was collected before and immediately after each exercise bout. Metabolic cost was partitioned by energy substrate, and metabolic water production was quantified from urinary nitrogen, oxygen, and carbon dioxide production. Total body water loss was recorded as the decrease in body weight during the exercise. All subjects completed 1 h of exercise at the two lower exercise intensities but, due to exhaustion, averaged only 35.5 min at the highest work intensity. There were no significant changes in plasma volume after the exercise bouts. Metabolic water production increased with increasing work intensity as did the fraction of total caloric expenditure derived from carbohydrate metabolism. Plasma protein content significantly increased at all levels of exercise intensity. Metabolic water production alone would be of minimal help in plasma volume maintenance and thermoregulation during endurance exercise.

Exercise-Associated Hyponatremia

“…However, the contribution of fuel metabolism or metabolic water production to TBW likely is small. During treadmill running at 74% of maximal oxygen consumption, metabolic water production averages 144 g/h (in contrast, sweat loss during this time was 1200 g/h) (58). There is a possibility that water that is stored with glycogen can be released with glycogen breakdown.…”

Probing the metabolic water contribution to intracellular water using oxygen isotope ratios of PO 4

“…In multicellular eukaryotic organisms, body water includes all water found in various body compartments (e.g., intravascular/intercellular) and bodily fluids (e.g., blood plasma, urine, breath vapor), including water produced by metabolism, and is averaged over the entire organism (25,26). Blood is the largest reservoir of body water in mammals.…”

Characterization of the effects of the vasopressin V2 receptor on sweating, fluid balance, and performance during exercise

“…The capability of water reabsorption within an estimated 1.6 to 4.0 million sweat glands (41) is a teleological appealing possibility, which could maximize water conservation during prolonged periods of thermoregulatory stress. Exercise performed at intensities above 50% of V O 2 maximum can shift up to 92% of all water (and 87% of all sodium losses) away from urine production toward requisite thermoregulatory sweat production (6,29,32). This primary shift in water and sodium excretion routes would be of increasing homeostatic importance during endurance exercise, particularly when performed in hot environments.…”