Protein metabolism during moderate exercise in cystic fibrosis patients

Abstract

Recent evidence indicates that protein utilization during exercise is greater than is generally believed. Increases in sweat urea nitrogen suggest that during exercise sweat is a major vehicle for excretion of the toxic NH₃ resulting from protein degradation. Cystic fibrosis is manifested by disturbances in secretions of the respiratory, digestive and sweat glands. To further investigate the importance of the sweating mechanism as a mode of urea nitrogen excretion we studied the response of five cystic fibrosis patients and five healthy controls to acute exercise in the heat. Subjects rode a cycle ergometer at (X± S.D.) 48.7 ± 3.1 % VO₂ peak (approximately 1.113 1 min^‐1) for 60 ± 9.6 min at 37.4±0.6°C and 33.9 ± 11.8% relative humidity. Water was provided freely. Rectal temperature, ECG (CM5), sweat (whole body washdown) and pre‐exercise and post‐exercise urine samples were measured. Both healthy and cystic fibrosis subjects had similar #OPX#PMS.E.#CP peak rectal temperatures (38.4±0.4; 38.3±0.2° C), peak heart rate (170±9; 165±5 beats min^‐1), sweat rates (0.608±0.084; 0.714±0.1641 h^‐1), and pre‐exercise urine urea nitrogen (421±12; 386±45 mmol 1^‐1 ). The cystic fibrosis patients had higher (P<0.05) post‐exercise urine urea nitrogen (521±22 versus 439±32 mmol 1^‐1) and lower (P< 0.05) sweat urea nitrogen (116.8±12.2 versus 144.7±16.8 mg h^‐1) than the controls. If the urea data are converted to the equivalent quantity of degraded protein, due to a higher mean workload in the healthy subjects (75.1 versus 59.8 W), the contribution of protein to total energy expenditure was similar (healthy, 1.31 versus cystic fibrosis, 1.34%). These results suggest that: (a) cystic fibrosis has little effect on sweat urea N excretion during exercise, (b) exercise may cause a greater degree of dehydration in cystic fibrosis, (c) protein catabolism is minimal at 49% VO₂ peak and (d) there may be a threshold for protein catabolism at approximately 55 % VO₂ max.

Key Words:

• Urea nitrogen
• sweat
• urine
• substrate utilization
• heat

Notes

Present address: Department of Pediatrics, Children's Hospital of Pittsburgh, Pittsburgh, Pennsylvania, 15213, USA.

Present address: Pulmonary Physiology Laboratory, Department of Preventive Medicine, University of Wisconsin, Madison, Wisconsin, 53706, USA.