Sweat and Miscellaneous Nitrogen Losses in Human Balance Studies

Abstract

In conventional experiments, nitrogen balance is taken to be the difference between the amount given in the diet and the sum of urinary and fecal nitrogenous wastes. Sustained positive balance is ascribed either to expansion of the body nitrogen pool or to loss from the body surface as sweat, desquamated cells, hair and nails. To refine these estimates, we accumulated information on the 3- or 6-day dermal nitrogen loss recovered in bath water and washings of underwear and socks of sedentary men fed diets containing 0.5 to 96 g nitrogen per day. Dermal nitrogen output varied systematically with nitrogen intake, being 112 ± 42 mg with protein-free diet, 149 ± 51 mg with normal (75 g protein) intake, and 514 ± 161 mg with 600 g protein in the daily diet. This relationship appears to be due to differences in blood urea nitrogen associated with protein intake. All material lost from the body surface was collected in double layers of special clothing over a 9-day period from another three men, two of whom were confined to bed. Data from this more complete collection suggest that the usual values are about 45% lower than the true total when all surfaces are covered. Sweat was collected from men working at rates of 2 to 15 kcal per minute for short periods in the laboratory and while playing soccer in the field. Dermal nitrogen loss increased with increasing work rate and was about 0.3 mg per m² body surface area per kilocalorie per minute in laboratory studies and 0.36 mg in the athletes. Other unavoidable but usually ignored nitrogen losses were found to amount to about 115 mg per day (toothbrushing, toilet tissues, plate wastes, exhaled ammonia). Additional potential routes of loss are blood (32 mg N/100 ml), saliva (0.9 mg per gram) and semen (37 mg per ejaculate). The total error in balances if the dermal and miscellaneous losses are disregarded is about 0.5 g nitrogen per day for sedentary men in a comfortable environment.