A summary analysis of the nitrogen-balance data

Following the standard UNU protocol, six investigators conducted a total of 11 studies on adults. This presentation summarizes these studies.

Skin Losses

One investigator, P.C. Huang, measured skin losses of nitrogen in subjects consuming two separate diets. Analysis of his data (see table 1 ) shows that the two diets differed significantly in terms of the amount of nitrogen lost through the skin. Additionally, although skin losses did differ with different levels of intake, these differences were

TABLE 1. Skin Losses (mg N/kg/body weight)

S.D. = standard deviation.

TABLE 2. Faecal Losses (mg N/kg body weight)

Faecal Losses

For nine of the ten studies for which faecal nitrogen values are available, there were no significant differences between those values for different levels of nitrogen intake (see table 2). The data of L. Fajardo obtained with a vegetable protein diet did indicate differences; this study examined six levels of intake from 72 to 152 mg N/kg. These results suggest that perhaps diets can be well characterized by faecal nitrogen losses. Furthermore, it appears that absorption measurements will vary with the level of intake.

Nitrogen Balance

Table 3 gives estimated means, standard deviations, and 97.5 per cent population requirements for the 11 studies. Note that the pooled coefficient of variation is 18 per cent. Table 4 shows the results of a series of standard protocol experiments conducted at MIT. Table 5 shows all these data arranged by diet.

TABLE 3. Nitrogen Balance (mg N/kg body weight)

Table 4 Summary of MIT Experiments

¹Supro 620 (Ralston-Purina).
²Supro 710 (Ralston-Purina).

TABLE 5. Experiments by Protein Source

¹ Supro 620.
² Supro 710.

TABLE 6. Repeated Subjects

Paired Experiments

Two investigators, R. Uauy and G. Inoue, conducted experiments comparing two different diets using the same individuals. For these experiments, between-subject variability can be separated from withinsubject variability. The results are shown in table 6. These results suggest (a) that efficiency measures (slopes of response curves) are too variable to be of value, and (b) that the errors in determining mean requirement levels are so large (~ 20 mg/N) as to permit discrimination only between very different dietary proteins.