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FIGURE 10-6 Relationship between individual nitrogen balances, corrected for integumental and nitrogen losses and nitrogen intake in random selection of data. □ = primary data; ▲ = test data; * = obligatory losses data. SOURCE: Rand et al. (2003).

Protein Intake (g/kg/day)

FIGURE 10-6 Relationship between individual nitrogen balances, corrected for integumental and nitrogen losses and nitrogen intake in random selection of data. □ = primary data; ▲ = test data; * = obligatory losses data. SOURCE: Rand et al. (2003).

al., 2003), a separate analysis was conducted to evaluate the extent to which the four factors thought to have the most influence on protein requirements—climate, age, gender, and dietary protein source—were analyzed. As shown in Table 10-13, expected climate in the country of the study had a significant effect (p < 0.47), with differences of the magnitude of about +10 mg N/kg/d (0.06 g protein/kg/d) in tropical climates. The effect of age, as shown in Table 10-13, was a nonsignificant difference of 27 mg N/d (0.17 g protein/d) in the nitrogen requirement between young (19 to 52 years of age) and older (53 years of age and older) individuals per kg of body weight. Although the young individuals had a lower nitrogen requirement than the older individuals, the requirement of young individuals was more variable and more positively skewed than that for the older individuals.

PROTEIN AND AMINO ACIDS 643

In addition, men had a statistically significant higher median nitrogen requirement by about 18 mg N/kg/d (0.11 g protein/kg/d) than did the women studied, although this difference disappears when medians of the primary estimation studies are compared. Ninety-five percent confidence intervals for these estimates are 104 and 114 mg N/kg/d (0.65 and 0.71 g protein/kg/d) for men and 85 and 104 mg N/kg/d (0.53 and 0.65 g protein/kg/d) for women. Finally, the source of protein (90 percent animal, 90 percent vegetable, or mixed) did not significantly affect the median nitrogen requirement, slope, or intercept. It should be noted that almost all of the studies included as 90 percent vegetable were based on complementary proteins. For further discussion on this aspect of the data analysis and for information on vegetarian diets see later sections on "Protein Quality" and on "Vegetarians."

All of the various estimates of protein requirements (Table 10-10) are confounded by variations in energy intake relative to energy balance and expenditure. It has been estimated that an error of about 10 percent in energy intake as estimated from a diet history or a prediction equation (FAO/WHO/UNU, 1985) would cause the nitrogen balance estimate to be affected by about ± 6 mg/kg/d (Pellett and Young, 1992).

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