Minor Obligatory Routes of Nitrogen Loss in Man

Doris Howes Calloway
Department of Nutritional Sciences
University of California
Berkeley, California 94720

The allowance for nitrogen lost from the body due to epithelial renewal and growth of the hair and nails, to insensible perspiration and to sweating - usually taken to be about 1.3 grams per day for an adult - is based largely on estimates derived from long-term positive N balances observed in healthy adults. The assumptions made are that the persons studied were neither gaining nor losing N from the total body pool and that all N not accounted for in the urine and feces was lost from the body surface. Neither of these assumptions has been experimentally verified in the studies most commonly cited (1).

Isaksson and Sjögren (2) have attempted to improve the reliability of this method by increased precision in their N balance techniques and by correcting the N balance for alteration of N pool size. N pool size was adjusted indirectly from data on the exchangeable potassium pool, assuming that 1 gram of N is deposited with 3 mEq of K. Their corrected data provide an estimate of 0.9 gram N/day (0.2 to 1.6 g range) as the body surface loss.

Mitchell (1) cited evidence (v.i.) that the daily 'sweat' N loss was 0.36 g and suggested that the difference between that amount and the observed positive balance (1.3 g N) was N shed as skin cells, hair and nails. This figure was then referred to body surface as being the most probable relevant variable, yielding a value of 0.5 g N/m2. These integumental tissues are mainly keratin and contain about 14% of N, so the daily loss of a man would have to be about 6 g of hair, skin and nails if the Mitchell figure were correct, a value far in excess of weights of tissues actually collected from men. The measured N loss as head hair, nails and whiskers is about 30 mg per day, with extremes of 22 to 62 mg N/day in healthy young men (3-5). Values for women fall within this range. Thus conventional balance techniques do not provide satisfactory estimates of dermal N loss.

A few investigators have actually measured 24-hour dermal N loss by quantitative bath and laundry procedures. These procedures always involve some error due to the necessity of leaving portions of the body uncovered and there is a possibility of bacterial decomposition over relatively long time periods. Benedict (6) reported daily dermal N output of two men in a respiration calorimeter to be 1.03 and 0.06 g. Voit's (7) normal male value was 0.33 g (122 mg as keratin and 211 mg as water-soluble compounds); Freyberg and Grant (8) found about the same amount, 0.25 and 0.52 g, under non-sweating conditions. Taylor (9) collected 0.19 and 0.16 g/day over 28- and 45-day periods from two laboratory technicians, but hair washing was omitted. Kraut and Müller-Wecker (10) recovered N from laundering undergarments daily for two or three weeks and from weekly bath water. The laundry collection was then adjusted according to the amount of body surface covered (e.g. in one man undergarments 0.896 m2 with 0.18 g N, therefore 0.2 g N/m2 surface area). Application of this technique to collections from six men suggested a total dermal loss of 0.23 g N/m2/day, or 0.42 g for a 1.8 m2 adult. Dermal loss of seven women measured in the same way was 0.10 g/m2 or about 0.16 g/day.

About 0.12 g/day was recovered from the 20 men studied by Sirbu et al. (4) under laboratory conditions for 3- or 6-day periods and Calloway et al. (5) report a value of 0.15 g/day by including data obtained from 15 additional subjects who wore long-sleeved, long-legged underwear and socks. More complete collections, made by enclosing all the body except the face in double layers of treated clothing and laundering the bed linens, recovered 45% more N than the former procedure. The corrected total is 0.22 g N/day for the 35 men maintained in a comfortable laboratory, with activity limited to one hour of walking at 3 mph up a 10% incline and fed a constant diet containing 75 g protein (5).

Additional data from the Berkeley group (5) confirmed earlier (4, 11) observations of a relationship between protein intake and dermal N loss. Uncorrected (i.e. 45% low) values were:

Their complete series of 118 observations fits a second-order equation relating dermal loss to blood urea nitrogen, as follows:

DM, mg/day = 127.17 - 2.40 BUN + 0.27 BUN² (BUN as mg/100 ml blood)

Variability was not reduced by adjusting observed dermal output according to calculated body surface area, a conclusion also reached by Darke (12).

Dermal N losses of sedentary adults fully acclimatized to heat may be somewhat higher than losses in more temperate regions but there are few data to prove it. Darke (12) gives an average value of 0.25 g dermal N/day for sedentary African men housed in a hospital ward in Tanganyika. Bost and Borgstrom (13) reported output to be 0.61 and 0.90 g in a 24-hour period for two men working at light occupations in New Orleans, Louisiana in the summer and Eijkman (14) obtained 0.76 and 1.36 g N/day from two medical students in Malaya. Consolazio et al. (15) collected sweat during two sequential 12-hour periods of experimental exposure to heat - 380 in the day and 33° during hours of sleep- from 12 subjects. The total for the day was 3.75±0.71 g N. These latter men were not fully acclimatized. It is probable that nitrogen lost by sweating diminishes with acclimatization (16, 17), although this point is equivocal (18).

There is much more information concerning losses during short periods of time, and these brief collections do not suffer from errors inherent in the longer term ones. Unfortunately these values cannot be extrapolated to obtain daily output. For example, Bost and Borgstrom (13) reported loss over 2½-3½ hour periods during the day to be 0.121 and 0.124 g N/hour for the same subject in whom the average 24-hour value was 0.038 g/hour. Eijkman (14) found 0.074 g N/hour in a 3-hour study of the subject whose hourly output was 0.032 g/hour in a 24-hour collection. Evidently, night-time dermal N loss is extremely low in acclimatized men under normal conditions of tropical life.

Ashworth and Harrower (19) observed that sweating during the work day accounted for almost all of the water lost cutaneously in 24 hours by their six male subjects in Jamaica. Total daily water loss was only 10% greater than that recorded during the daytime period when dermal N loss was measured. Assuming that N content of all perspiration was not greater than that of actively secreted sweat (the evidence indicates that it might actually be much less (20), total daily N loss might have been only 10% more than the 0.49 g collected during the work interval, or about 0.55 g/day.

In Mitchell and Hamilton's (20) widely quoted study (from which the commonly accepted estimate of 0.36 g/day N loss for sedentary comfortable men was derived), the rate of cutaneous water loss was 90 g/hour in a comfortable chamber and 700 g/hour in the heat (38°). Hourly loss of N from the skin was 15 and 150 mg under the two test conditions, indicating that sweat contained 0.17 and 0.24 mg N/g at the lower and higher sweating rates. If the hourly values are simply multiplied by 24 to obtain daily output, as is usually done, dermal N losses of 0.36 and 3.60 g/day are computed. If, however, the rate of dermal water loss was about 30 g/hour during inactive periods outside the chamber, in accordance with numerous observations of resting men (21), hourly loss would have been 5 mg N at the concentration observed. Thus, the daily total would more logically have been about 0.18-0.24 g N under comfortable conditions and 1.28-1.86 g in the heat.

Values computed in this way from the two studies fall within the range of 24-hour measurements of acclimatized subjects. The fact that Ashworth and Harrower's value is lower than that of Mitchell and Hamilton's heat-exposed men may be due to the fact that the latter subjects ate twice as much protein (16 g N/day) as did the Jamaica group (8 g N/day). Differences in habitual protein intake doubtless account for some of the variability in data from the other studies cited as well.

Physical activity also affects sweating rate and dermal N loss. Benedict (6) reported one man's hourly output to be 0.11, 0.16 and 0.22 g N at increasing levels of bicycle work over 4-5 hour periods. Berry's (22) subject lost 0.14 g N/hour in sweat during a 4-hour test involving 142,800 kg M of bicycle work. Calloway et al. (5) found dermal N loss of men to be 0.30-0.36 mg/m2 body surface area/kcal/minute over a work range of 2 to 15 kcal/minute on a bicycle ergometer and a treadmill, and during field soccer practice. The increase due to working over resting values obtained under the same conditions was, in g N per man per hour: 2-4 kcal/minute, 0.05; 4-8 kcal/minute, 0.12; 8-12 kcal/minute, 0.18; and 12-15 kcal/minute, 0.35.

The key question concerning the effect of dermal N loss on dietary protein needs is whether or not there is a compensatory reduction in urinary and fecal N loss of sweating subjects. The data are conflicting on this point. There was no evidence of renal compensation in the climatic chamber studies of Mitchell and Hamilton (20) or Consolazio et al. (17) where heat could be regarded as a stressor. Values from the acclimatized subjects studied by Harrower (19) and others (18, 23) suggest some conservation of urinary nitrogen.

To the extent that blood urea is simply excreted in sweat as a plasma filtrate, it seems logical that urinary urea N would be reduced with heavy sweating. However, sweat contains some non-plasma urea (24) and ammonia that represent sweat gland activity and impose a demand for precursors. Sweat also contains amino acids (3, 25) which are thus removed from the tissue pool. In sum, at least half of the dermal N loss might represent a specific requirement for which allowance should be made. The safest course is to count the entire output as an uncompensated need.

A final point should be raised with regard to the validity of establishing dietary N requirement from studies in which dermal and miscellaneous losses were not measured. Adequacy of the dietary protein should not be accepted as having been proved unless balance was at least +0.5 g N/day to compensate for minor losses. Unavoidable N losses (5) from men include: Toothbrushing, 14±3 mg per occasion; exhaled as ammonia, 50±6 mg per day; fecal matter on wipes, 4±2 mg per day; and handkerchiefs, 1 to 5 mg/day. Variable losses occur from plate wastage (about 0.05% of intended intake with formula diet in our laboratory (5) and up to 200 mg/day with Isaksson's patients (26); blood (32.4±1.7 mg N/g); saliva (0.96±0.14 mg N/g); and semen (37±10 mg N/ejaculate) (5). Women lose 1.5 to 3.4 g in the monthly menstrual flow (27) or about 60 to 120 mg per day averaged over a 28-day interval.

Data are insufficient from which to make judgements concerning dermal N allowances for children. Infants lose 40 to 250 mg N/day from the body surface (28, 29) which is about 3 to 5% as much as the total urinary and fecal output, a value quite similar to that of the adult.

We recommend that uniform amounts be included in the protein allowances to compensate for all of the minor N losses from sedentary acclimatized adults of either sex. Provisionally, 0.5 g per day would be acceptable in temperate regions and 0.8 g per day in hot climates. For physically active groups, allowances should be increased as follows:

References

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