This study examined whether the prevention of body mass loss during exercise in the heat by fluid ingestion would attenuate the decline in renal concentrating ability. Seven untrained subjects performed 105 min of intermittent exercise (15 min of exercise alternating with 3 min of rest) on a cycle ergometer at 70% VO2max (32°C and 60% RH). Subjects were tested under four conditions: (1) no fluid ingestion (NF), (2) ad libitum fluid ingestion (AF), (3) fluid ingestion equal to 1% of body mass (1% FL), (4) fluid ingestion equal to 2% of body mass (2% FL). Fluid was ingested immediately before exercise and at 15, 33, 51, 69 and 87 min of exercise in AF, 1% FL and 2% FL. Urine and blood samples were taken before and after exercise. During NF, AF, 1% FL and 2% FL, body mass loss was 2.2±0.1%, 1.1±0.2%, 1.1±0.1%, 0.1±0.1%, respectively. Total sweat loss was approximately 2% of body mass in all trials. Urine flow rate during exercise was 0.3±0.0, 0.4±0.1, 0.4±0.1, 0.5±0.1 ml/min during NF, AF, 1% FL and 2% FL, respectively, and was lower (p<0.05) than the pre-exercise level in NF and AF. Urine to serum osmolality ratio in 2% FL was higher (p<0.05) during exercise than the pre-exercise level. Creatinine clearance decreased (p<0.05) in all trials, and was higher (p<0.05) in 2% FL than in NF. Osmolar clearance was lower (p<0.05) during exercise than the pre-exercise level in NF, AF and 1% FL, and was higher (p<0.05) in 2% FL than in NF. Free water clearance increased (p<0.05) during exercise than the pre-exercise level in NF, AF and 1% FL, and was lower (p<0.05) in 2% FL than in NF. During exercise, 2% FL resulted in a concentrated urine production. Fluid ingestion in amounts equal to body mass loss at approximately 2% reduction in body mass is capable of attenuating the decline in renal concentrating ability during heavy exercise in the heat.
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