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Analysis of multilevel short-term balance studies
UNU Food and Nutrition Program,
Charles Street Station, P.O. Box 500, Boston, MA 02114-
Descriptors: nitrogen balance, protein requirements, methodology
Estimates of human protein requirements are almost wholly dependent on nitrogen balance measurements. This was also true for the determination of amino acid requirements until the recent development of stable isotope methodology. Because there are a number of inherent sources of error in nitrogen balance measurements and a number of requirements that are not always observed, the methodology has been much criticized. Nitrogen balance data can be reliable and consistent or of doubtful validity and consistency depending on how they are obtained. The following precautions are essential:
1. Caloric intake must match caloric requirement
Nitrogen balance is very sensitive to changes in dietary energy intake above and below that required for zero energy balance. Therefore, estimates of normal protein needs from nitrogen balance are valid only when energy intakes closely approximate those when the individual is consuming his usual diet while maintaining his usual level of activity. Many past studies are invalid because energy levels were too high or too low.
2. An appropriate stabilization period must precede each measurement
Adaptation to a new protein level is relatively rapid and depends on the magnitude of the change. For subjects who are protein replete, a five-day adaptation period has been suggested as sufficient for determining incremental changes in N-balance. For randomized, multilevel studies in adults an initial one-day nitrogen-free period helps to ensure the adequacy of the subsequent adaptation period.
3. Periods on the same diets must be long enough to establish a stable response to the dietary change
Experience has shown that consistent results are obtained using two five-day or three three-day periods per level of intake.
4. Degree of depletion of the subject must be taken into account
Chronically depleted subjects utilize protein more efficiently than those who are normally replete and hence give an overestimate of normal nitrogen retention. They are inappropriate for determining normal protein requirements.
5. No infections, even of seemingly mild degree, can be present
Infections, no matter how mild, increase catabolic nitrogen losses and divert protein for the synthesis of immune proteins (Scrimshaw et al, 1991). It is also virtually impossible to maintain the specific protein intake because anorexia is an early characteristic of acute infections, even when they are subclinical.
6. Variations in daily physical activity must be minimal
Because physical activity is such a large component of energy requirements, it is impossible to maintain an appropriate energy intake if there are large daily variations in activity.
7. Fluid intake must be controlled
A large fluid intake increases urinary nitrogen retention so that variability in fluid intake causes variability in nitrogen balance results.
8. Correction must be made for integumental and miscellaneous losses
Nitrogen balance studies should be conducted under conditions of minimal sweat loss or, if this is not possible, sweat losses should be measured and taken into consideration. Studies of the same individuals showed higher integumental N losses and correspondingly lower urinary N losses in summer than in winter (Huang et al, 1972).
9. Subjects should not be anxious or otherwise disturbed
Psychological stressors cause metabolic responses that are qualitatively similar to those observed with infections.
10. Protein intake be precisely monitored
Misleading results have been obtained in studies where, unknown to the investigators, the subjects consumed additional food or failed to consume some of the food prescribed.
11. Urine collections must be precisely timed and complete
Incomplete urine collections or varying duration of daily collections can introduce large errors into N balance studies. Twenty-four hour urinary creatinine excretion should be monitored as one indicator of the quality of the urine collections.
12. Faecal collections must be complete and very well homogenized before a sample is taken for analysis
A common and serious error is taking a faecal aliquot for analysis without adequate homogenization of the total sample.
13. Determinations of nitrogen in food, urine and faecal collections must be accurate
Nitrogen should be directly determined by analysis and not estimated from food composition data. Inaccurate analyses of the N content of samples is a common source of error in balance studies.
14. The experimental design must be appropriate for the intended purpose
Correspondence to: NS Scrimshaw.
To determine the amount of protein in a diet required for nitrogen balance in adults, a standardized protocol has been successfully applied in multicenter studies sponsored by the United Nations University, the results of which were the basis for the 1985 revised recommendations for adult protein requiremens (Rand et al, 1984). This protocol was devised by an expert working group (UNU/WHP, 1979) and can be summarized as follows.
Taking into consideration all of the above requirements, protein should be given at a minimum of four levels, the highest near the anticipated mean requirement. The 1979 working group recommended levels of 0.03, 0.04, ().05 and 0.06 g/kg for adults and appropriately higher levels for children, offered in a Latin-square protocol. They proposed:
For adults, the difficulties in obtaining accurate estimates of faecal output, and in consideration of adaptation to reduced nitrogen intakes, make desirable, for each test protein, one day on a nitrogen-free diet, five days for adaptation to the test level, and then five days for the urine collection. The collection period should then be followed by a break period of three or more days. Faecal collections are best pooled for the last eight days at each test protein level.'
In measuring the nitrogen-balance
response of children, each protein level should be fed for seven days, and
estimated nitrogen balance should be calculated on the basis of the last three days, the
protein-free day should be omitted, and a three-day break period interposed between the
randomized test diets. Children should receive, during these three days, the same diet
that was given for the three days prior to the beginning of the study.'
Conclusions regarding protein needs derived from short-term balance studies should be confirmed by feeding at the proposed safe level for three to six months. It is not practical to conduct the number of long-term studies that would be required to include the many variations in age, sex, physiological state, and genetic and environmental factors. However, several excellent studies established the adequacy of the level of 0.8 g/kg of protein predicted from the mean and confidence limits of the short-term studies (Rand et al, 1984).
Linearity and significance of the nitrogen balance response to increasing levels of nitrogen intake should be verified for each individual's response. For subjects not meeting at least one of these criteria their values should be eliminated from the pooled data and reported separately, with every effort made to repeat the study with such subjects to determine whether the anomalous results were due to experimental error or to some metabolic difference. A regression line, the mean, and 97.5% confidence intervals for the zero-balance intercept should be calculated using the pooled data.
Huang PC, Chong HE & Rand W (1972): Obligatory urinary and faecal nitrogen losses in young Chinese men. J. Nutr. 102, 16051613.
Rand WM, Uauy R & Scrimshaw NS (1984): Protein energy requirement studies in developing countries: results of international research. Fd Nutr. Bull. Suppl. 10.
Scrimshaw NS, Bistrian BR, Brunser O. Elia M, Jackson AA, Jian Z-M, Kinney JM, Rosenberg IH & Wolfe RR (1991): Effect of disease on desirable protein/energy ratios. In Protein-energy interaction, eds NS Scrimshaw & B Schürch pp 385-398. Lausanne: IDECG.
UNU/WHP (1979): Protein energy requirements under conditions prevailing in developing countries: current knowledge and research needs. Fd Nutr. Bull. Suppl. 1.
European Journal of Clinical
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