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TABLE 1. Characteristics of the Men

3. Most of the subjects lost weight during the study. The rate of weight loss diminished as the experiment proceeded. The nature and significance of this remain unclear.

4. There were no demonstrable biochemical or physiological (work performance) changes, except for a marked fall in BUN.

5. The time required to reach a steady state varied from seven to more than 28 days. The reasons for this variability are not clear.

Reference

1. P.V. Sukhatme and S. Margen "Models for Protein Deficiency," Amer J. Clin. Nutr., 31: 1237-1256 (1978).

TABLE 2 Summary of Initial and Final Laboratory Values

a. Reference values in parentheses
b. Values outside reference range

TABLE 3 Diet Composition

TABLE 4. Protein and Energy Intake

a. 57 mg N/kg body weight
b. Mean of four values ± standard deviation.

TABLE 5 Individual Formal Exercise Regimen

TABLE 6 Faecal N (g/24 fur): Average of Four Analyses of Two Samples from Pool (Range of Eight Samples)

TABLE 7 Average N Balance (g/day) in Five to Ten Days and during Last Six Days

4. Investigation of a short-term procedure to evaluate protein quality in adult human subjects - a preliminary report

Nevin S. Scrimshaw, William Rand, Zhi-chien Ho, V.R. Young, and E. Murray

Department of Nutrition and Food Science. Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

It is recognized that the use of experimental animals and biochemical methods of evaluating protein should be verified and calibrated by nitrogen balance studies in human subjects. Because the traditional procedure is costly and time-consuming, it is essential that a shorter and more practical method for evaluating protein quality be developed for worldwide use. This report presents some relevant findings from ongoing experiments in our laboratory.

Experimental Design

Study 1

In 1974 Dr. Myriam Puig of our laboratory conducted an experiment to explore the usefulness of a short-term evaluation of proteins, using the same 12 healthy young men for both a short-term and a standard protocol (1). Her short-term design involved a one-day protein-free diet and six days' adaptation that served as a priming period of low 0.3 9 prot./kg for the ascending design, or high, 0.6 for the descending design, and then ascent or descent in three one-day steps (0.4, 0.5, 0.6. or 0.5, 0.4, 0.3). After approximately a two-week break her subjects completed the standard protocol. Two diets were used, chick-pea and diluted beef, with six subjects on each, three ascending and three descending. The data from this experiment (table 1) suggest that the nitrogen balances for the successive single-day intakes of different levels did not represent a consistent response (see fig. 1). Either the single day was insufficient to reflect the change in intake, or three periods were too few or too close together. Nevertheless, pooling of all data, ascending and descending, gave a zero nitrogen balance intercept not significantly different from the mean of the same subjects studied in the standard manner.

Study 2

Four healthy young men participated (table 2) in the next experiment consisting of five different levels of protein intake for two-day periods each. They received medical and nursing supervision throughout the entire study and maintained relatively constant physical activities from day to day. No clinical changes were noted during the experimental period. Protein was supplied by a soy protein isolate (Supro 710). Caloric intakes were 45 to 48 kcal per kilogram body weight, as estimated to maintain body weight from their intake before the experiment (table 3). Additionally, the subjects received daily vitamin and mineral supplements. and a relatively constant water intake. This diet was provided in three meals a day plus a small snack in the evening.

FIG. 1. Time Course of Urinary Nitrogen (g) for Subject 3 on One-day Ascending
Design. (Circled Data Represent Responses to Intakes of 0.4, 0 5, and 0 6.)

TABLE 1. Urinary Nitrogen (gN)

TABLE 2. Physical Characteristics of the Subjects

FIG. 2. Four Subjects on Two-day Ascending Design.

FIG. 3 Four Subjects on Two-day Descending Design.

TABLE 3. Composition of Formula Diet with Soy Protein Isolate (Supro 710) for a 70 kg Man (in grams)

The duration of the experiment was 30 days with 15 days on ascending protein intake and 15 days on descending protein intake. Each 15-day period began with a 5-day priming period at a constant level on either a low (for ascending) or high (for descending) level of protein intake. Following this, a different level of protein intake was provided every two days as follows:

There was a three-day break on an adequate ad libitum diet between the two dietary phases. Urine and faeces were collected daily for analyses; food colour (blue) and cellulose capsules were used as markers for faecal samples. All samples, including food samples, were analysed by the Kjeldahl method.

Results

Figures 2 and 3 show the time course of the nitrogen balances for all subjects. Table 4 shows the balance data for all subjects.

TABLE 4. Nitrogen Balance (gN)

We find:

1. Balance (nitrogen output) responses can be adequately fitted by a straight line, ignoring the priming phase.

2. The ascending design gives consistently lower requirements. and is usually a steeper response leading to a better defined requirement.

3. There is little agreement between results from the ascending and descending phase of the experiment.

Conclusions

A protocol based on a five-to-seven-day priming phase with two days on graded intakes should be explored as a possible alternative to the standard protocol. The critical additional experiment now required is to repeat the study using the same individuals for both the two-day and standard protocols.

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