A NOTE ON THE FACTORIAL METHOD FOR CALCULATING PROTEIN
REQUIREMENTS FOR MAINTENANCE IN HUMAN SUBJECTS

By
M.SWAMINATHAN
(Central Food Technological Research Institute, Mysore-2A, India)

ABSTRACT

The FAO/WHO Expert Group on Protein Requirements used values of 2 mgN per basal calorie in urine and 20 mgN per kg body weight each through skin and intestines respectively for endogenous N losses in calculating the protein requirements for maintenance. When the losses through intestines and skin are recalculated per basal calorie, a low value of 2.8 mgN for infants and preschool children and high value of 3.6 mgN for adults for the total endogenous N losses per basal calorie are obtained. The Food and Nutrition Board, National Research Council, U.S.A. used a value of 3.2 mgN per basal calorie while the Expert Panel, U.K., used a value of 2.65 mgN per basal calorie for the total endogenous N losses in all age groups. The value of 2.0 mgN per basal calorie used by the above groups for the endogenous N loss in urine should have included also endogenous N loss in sweat, as a part of the waste nitrogenous products are also excreted in sweat. On the basis of available data a revised set of values for endogenous N losses have been suggested for use in the calculation of protein requirements for maintenance in human subjects of all age groups: (i) 2.0 mg per basal calorie in urine, (ii) 0.6 mg per basal calorie in feces and (iii) 0.1 mg per basal calorie for integumental losses, adding to a total endogenous N loss of 2.7 mg per basal calorie for all age groups.

INTRODUCTION:

A factorial method for calculating the protein requirements based on endogenous losses of nitrogen through kidneys, intestines and skin and nitrogen required for growth was suggested by the FAO/ WHO Expert Group on protein requirements (1). The method with slight modifications was used by the Food and Nutrition Board, National Research Council, U.S.A. (2), Expert Panel, U.K. (3) and I.C.M.R. Nutrition Expert Group, India (4) for calculating human protein requirements. In the present paper, a critical assessment of the factorial method is presented.

Obligatory endogenous N losses:

The obligatory endogenous N losses occur through the kidney, skin and intestinal tract. The available data have been briefly discussed below:-

N Loss in urine and sweat:

When a protein free diet is fed to animals or humans, loss of body protein occurs. The amino acids liberated by the break-down of tissue proteins are deaminated in various tissues and ammonia liberated is converted into urea in the liver. Urea and other nitrogenous com- pounds such as uric acid and creatinine are passed on to the general circulation and excreted mainly by the kidney. A small part is excreted however, in the sweat, the loss in sweat may vary depending on the degree of sweating. In addition small quantities of free amino acids are excreted both in urine and sweat. There is evidence indicating that in a wide range of mammals the 'endogenous' loss of nitrogen due to catabolism of tissue proteins is about 2 mg/basal calorie (5), It has not so far been realised, however, that the endogenous loss of 2 mg N per basal calorie includes both the losses in urine as well as in sweat. This may explain the lower values for urinary endogenous nitro- gen reported by several workers in adults, as the losses in sweat will be appreciable in warm climates (6-7). The data for endogenous loss of nitrogen in infants and children are meagre (8). The available data indicate that endogenous N losses in urine in infants and children are lower than those reported in adults (8). The energy needs of fasting animals and humans are met by the oxidation of carbohydrates, fats and proteins present in the body. The fasting animal or human may derive about 10 to 12% of the energy requirements from the catabolism of tissue proteins (9). When a protein free diet is administered, the catabolism of tissue proteins decreases to about half the above level (10). If we assume that catabolism of tissue proteins contributes about 5% of the resting energy metabolism on a protein free diet, the endogenous N loss in urine and sweat, will work out to 40 mg per gram of tissue protein oxidised or 2mgN per basal calorie. The figure of 2 mgN per basal calorie for all age groups adopted by the FAO/WHO Expert Group (1), the Food and Nutrition Board, NRC, U.S.A. (2) and the Expert Group, U.K. (3) appears to be reasonable. The I.C.M.R. Nutrition Expert Group, India (4), however, adopted a lower value of 1.5 mgN/ basal calorie.

Loss of N in feces

The endogenous N loss in feces in adults has been reported to range from 10-20 mg N per kg body weight (6,10). These will work out to 0.4 to 0.8 mg N per basal calorie. The losses of fecal N in infants and children have been found to be about 20 mg and 25 mg per kg body weight respectively (8). These will work out to 0.4 to 0.5 mg N per basal calorie. The FAO/WHO Expert Group (1) adopted a value of 20 mg N/kg body weight for all age groups. This value will work out to 0.4 mg N for infants and pre-school children and 0.8 mg N per basal calorie for adults. More recently, the Food and Nutrition Board, U.S.A. (2) and the Expert Panel, U.K. (3) have suggested figures of 0.4 mg and 0.57 mg N per basal calorie respectively for endogenous fecal loss in all age groups. It is suggested that a value of 0.6 mg N per basal calorie be adopted for all age groups for meeting the endogenous loss of N in feces.

Cutaneous and other losses:

The excretion of endogenous N through the skin occurs in sweat. This is derived from urea and other nitrogenous compounds circulating in blood and derived from the oxidation of tissue proteins in animals and humans receiving a protein free diet. As pointed out earlier, the N loss in sweat will form part of the 2 mg N per basal calorie derived from the catabolism of tissue proteins. Hence, there is no need for making a separate allowance for endogenous N loss in sweat.

The N loss due to loss of integumental cells, hair and nails has been estimated to be 0.56 mg N per m² surface area for men and 0.37 g N per m² surface area for women (11). More recently, Sirbu et al (12) have reported a very low figure of 25 mg N/day as integumental losses in adults. The Food and Nutrition Board, N.R.C., U.S.A. (2) used a figure of 0.8 mg N per basal calorie to include sweat and integumental losses, while the Expert Panel, U.K. (3) used a figure of 0.08 mg N per basal calorie for integumental loss. It is suggested an allowance of 0.1 mg N per basal calorie will be adequate to meet the integumental loss of proteins from the body.

DISCUSSION

It is evident from the data given in Table I that the factorial methods suggested by the FAO/WHO Expert Group (1) and ICMR Expert Group, India (4) give low values for the nitrogen requirements for maintenance of infants and preschool children and high values for adults when the N losses are calculated per basal calorie. In order to overcome these discrepancies it is essential to express the endogenous N losses in terms of basal calories, as has been done by Food and Nutrition Board, N.R.C., U.S.A. (2) and Expert Panel, U.K. (3). Further it is important to note that the nitrogenous products derived from the catabolism of tissue protein in the body on a protein free diet are excreted both in urine as well as in sweat, the loss in sweat being appreciable in warm climates. Hence the value of 2.0 mg N per basal calorie in urine adopted by several expert groups (1-3) should include the endogenous losses in urine as well as in sweat. The lower values reported in the literature for the endogenous N loss in urine is due to the fact that a part of the endogenous N loss occurs in sweat and N losses in sweat have not been estimated in the above studies (6-8). On the basis of data presented in this paper, a revised set of values for endogenous N losses for calculating protein requirements for maintenance for all age groups, have been suggested:- (1) 2.0 mg N per basal calorie in urine and sweat (ii) 0.6 mg N per basal calorie in feces and (iii) 0.1 mg N per basal calorie for integumental losses, adding to a total endogenous loss of 2.7 mg N per basal calorie.

Table I : Estimates of endogenous losses of nitrogen in human subjects

REFERENCES

1. FAO/WHO Expert Group on Protein Requirements, FAO Meeting Report Series, No.37 (1965).

2. Food & Nutrition Board, Recommended Dietary Allowances, 7th Ed. Publ No. 1694, National Academy of Sciences, Washington D.C., U.S.A. (1968).

3. Expert Panel, Dept. Health & Social Security, Recommended Intakes of Nutrients for the United Kingdom, Rep. No. 120; Her Majesty's Stationery Office, London (1969).

4. ICMR Nutrition Expert Group, Recommended Daily Allowances of Nutrients and Balanced Diet, Indian Council of Medical Research, New Delhi (1968).

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11. Mitchell, H.H. and Edman, M., Amer. J. Clin. Nutr., 10, 163 (1962).

12. Sirbu, E.R., Margon, S. and Calloway, D.H., Amer. J. Clin. Nutr., 20, 1158 (1967).