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3. Protein requirements of children and adolescents

3.1. Basis of the 1985 recommendations

In the 1985 report, the factorial approach was used to estimate protein requirements of children and adolescents. For maintenance nitrogen requirements, values were interpolated based on the two 'anchor' points of 120 mg N/kg/d at 1 year and 100 mg N/kg/d at 20 years of age. The coefficient of variation was assumed to be 12.5%, similar to that observed in adult balance studies. For growth, the expected nitrogen accretion was based on data from Fomon et al (1982) up to age 10; after age 10 it is unclear what reference values were used. The same assumptions as used for infants were included in the calculations: (a) the efficiency of conversion from dietary protein to body protein was assumed to be 70%, (b) the coefficient of variation for growth was taken as 35% and (c) an additional 50% was added to the growth increment to allow for day-today variation in growth rate.

It is worthwhile to reevaluate the 1985 estimates, given that questions could be raised about all the above assumptions. With regard to the efficiency of protein utilization for growth and the CV for growth, revisions in the above assumptions are not likely to have much of an impact on requirement estimates after infancy. This is because protein needs for growth are a small percentage of total protein needs after the first year of life, and the magnitude of changes caused by minor revisions in those assumptions (e.g. using an efficiency of 90% instead of 70%) is thus small. The assumptions that are most likely to influence requirement estimates for children and adolescents are the maintenance nitrogen requirement (and its CV) and the 50% growth increment. The lack of justification for the latter adjustment was discussed in section 2.2.4. In the next section, the maintenance needs of children and adolescents are re-examined, and in the subsequent section, revised estimates are presented.

3.2. Maintenance nitrogen requirements/nitrogen balance studies

In the 1985 report, the maintenance nitrogen requirements for children and adolescents depended heavily on the estimated value of 120 mg N/kg/d at 1 year of age. As explained in section 2.2.2 above, only one study was cited to support this estimate, which was based on infants fed very low amounts of energy. Section 2.2.2 presents an argument for considering a lower value, such as 90 mg N/kg/d. If this were accepted, then the interpolation of values from 1 to 20 years would clearly be very different.

Several short-term balance studies in preschool children were described in Table 30 of the 1985 report (Town et al, 1981a; Intengan et al, 1981; Egaña et al, 1983; Iyengar et al, 1979). In all but one of these (which had a sample size of only 3 with adequate energy intake: Iyengar et al, 1979), the average maintenance requirement (corrected for digestibility) listed in Table 30 was £ 100 mg N/kg/d. One study not cited in the 1985 report measured obligatory nitrogen losses in five preschool children 17 to 31 months of age given a protein-free diet for 7-9 days (Town and Viteri, 1981a). Total obligatory nitrogen losses were 59 mg N/kg/d, assuming 5 mg N/kg/d for integumental nitrogen losses. Assuming that nitrogen intake to meet maintenance needs is 130-145% of inevitable nitrogen losses (FAO/ WHO/UNU, 1985; Young et al, 1989), the maintenance requirement would be 77-86 mg N/kg/d. These studies thus provide support for the idea that maintenance nitrogen requirements at this age may be closer to 90-100 mg N/kg/d than to the previously assumed value of 120 mg N/kg/d. No new balance studies (since the 1985 report) for this age range were located.

Longer-term balance studies (3-6 months) have also been conducted in preschool children (Town and Viteri, 1981b,c; Begum et al, 1970), and were described in the 1985 report (Table 35). In all these studies, the level of protein provided (1.07-1.40 g/kg/d when corrected for digestibility, equivalent to 171-224 mgN/kg/d) was judged to be adequate or more than adequate to support expected growth rates. There is a lack of information regarding long-term nitrogen balance of children with protein intakes closer to the estimated requirement.

One factor that has generally not been considered in the longer-term balance studies is the adequacy of micronutrient intake. This is particularly important when diets relying extensively on plant sources are utilized, given the low bioavailability of some nutrients (e.g. iron and zinc) from such foods. Incorrect estimates of protein requirements may result if growth is limited by nutrients other than protein during balance studies.

There is much less information available for older children and adolescents than for preschool children. The 1985 report cites one study of adolescent males (Prothro et al, 1973) as showing that an intake of 100120 mg N/kg/d from a mixed diet (with considerable animal protein) was necessary to produce consistently positive nitrogen balances. Since the 1985 report, two short-term balance studies have been published (Gattas et al, 1990, 1992) both conducted in Chile and using a mixed, predominantly vegetable diet. In one, the mean intake to achieve balance (allowing 8 mg N/kg/d for unmeasured losses and 10 mg N/kg/d for growth) among males 8-10 y of age was 150 mg N/kg/d (Gattas et al, 1990), not correcting for digestibility. A very similar value was obtained for males 12-14y of age: 147 mg N/kg/d (not correcting for digestibility), allowing 8 mg N/kg/d for unmeasured losses and 19 mg N/kg/d for growth (Gattas et al, 1992). If corrected for digestibility, the estimates would be approximately 125 mg N/ kg/d. In the latter study, it is difficult to evaluate the validity of the allowance for growth (19 mg N/kg/d), as no growth data were presented.

The two studies conducted in Chile estimated 'safe' levels of protein intake assuming a CV of 12.5%. The resulting estimates for intake from a mixed diet (not correcting for digestibility) were 1.2 g protein/kg/d for boys 8-10 y and 1.15 g protein/kg/d for boys 12-14y. However, when calculated using the actual CVs observed in the balance studies (5% and 9%, respectively), the 'safe' levels were 1.03 and 1.09 g/kg/d, respectively.

3.3. Revised estimates of protein requirements for children and adolescents

Based on the information described above, Tables 19 and 20 were developed assuming a maintenance requirement of 100 mg N/kg/d (the same as for adults), and without the 50% augmentation for intra-individual variation in growth used in the 1985 report. Both tables use the same assumptions as the 1985 report for the efficiency of conversion of dietary protein to body protein, the CV for growth, and the CV for maintenance (12%)

Table 19 Revised estimates for Table 33 in the 1985 FAO/WHO/UNU report on Energy and Protein Requirements (children)

^a Assumes 70% efficiency of utilization, as in 1985 report.
^b Using CVs shown in Table 33 of the 1985 report.

Table 20 Revised estimates for Table 34 in the 1985 FAO/WHO/UNU report on Energy and Protein Requirements (adolescents)

^a Assumes 70% efficiency of utilization, as in 1985 report.
^b Using CVs shown in Table 33 of the 1985 report.

Among preschool children, estimated requirements from balance studies using milk-based or mixed diets with considerable animal protein range from 98 to 117 mg N/kg/d (Town et al, 1981a; Intengan et al, 1981; Egana et al, 1983). These are consistent with the values in Table 19, which are about 17-20% lower than the 1985 values for this age range. Among older children and adolescents, estimated requirements from balance studies are 100-120 mg N/kg/d for males 13-17 y in the US (Prothro et al, 1973) and about 125 mg N/kg/d for males 8-10 and 12-14 y in Chile (Gattas et al, 1990, 1992). The estimate from the US study is consistent with Table 20, but the estimate from the latter two studies is higher than estimated needs in Tables 19 and 20. Thus, the few data available suggest that the 1985 values are overestimates for preschool children but may be appropriate for older children and adolescents. It should be noted that most balance studies include only boys; there are almost no data on requirements of girls, which theoretically could be quite different during adolescence.

3.4. Amino acid requirements of children and adolescents

In the 1985 report, the amino acid requirements of preschool children were based on experimental studies in which the amount of each essential amino acid provided was varied one at a time (Pineda et al, 1981; Torun et al, 1981b). The 1991 Expert Consultation on Protein Quality Evaluation (FAO/WHO Expert Consultation, 1991) accepted these values, although it has been pointed out (Millward et al, 1992) that the original data have not been published in full. In addition, the applicability of these estimates (obtained from six previously malnourished children) to other populations is debatable (Millward et al, 1992). Nevertheless, the data for preschoolers are considered the best available to date. One of the co-investigators contributing these data (Town, 1989) has pointed out, however, that the published values should be considered as safe levels rather than minimal requirements.

The amino acid requirements for older children published in the 1985 report were considered inappropriate by the 1991 Consultation (FAO/WHO Expert Consultation, 1991) due to problems in the original balance studies on which they were based. In the 1985 report, amino acid requirements (per kg) were predicted to decrease considerably with age, but the biological plausibility of this 'trend' has since been questioned. Millward et al (1992) suggest that the age-related decline in estimated amino acid needs is merely an artifact of the different dietary designs of the original balance studies. Newer data suggest that amino acid needs of adults may have been underestimated; based on these data, Young and colleagues (Pellett and Young, 1988) proposed a new amino acid pattern to be used for all ages except during infancy. However, the 1991 Consultation considered that more data were required before a new pattern could be adopted. Nonetheless, it was recognized that after the first year of life, amino acid needs per unit body weight should not differ greatly by age because growth is such a small proportion of total needs. Therefore, the 1991 Consultation recommended that until additional information is available, the amino acid pattern for preschool children adopted in the 1985 report should be used to estimate requirements for all age groups except infants.

3.5. Recommendations for revision of the 1985 report

(1) The factorial model used to calculate protein requirements for children and adolescents should not use estimates of maintenance requirements that are interpolated from the 'anchor point' of 120 mg N/kg/d at 12 months, given that the latter value is questionable (see section 2). Rather, a value of 100 mg N/kg/d (at all ages) seems more appropriate, based on data from balance studies of children and adolescents (see section 3.2). When updated information on maintenance needs of adults is available, this should be used to help judge whether the estimate of 100 mg N/kg/d is sufficient for older children and adolescents.

(2) As suggested for the infancy requirements (see section 2.6), the 50% augmentation in the growth component to allow for day-to day variability should be abandoned. If any adjustment is considered necessary to allow for spurts in growth (particularly during adolescence), it should be included in the CV component for growth when calculating safe levels, not added onto the mean protein increment for growth.

(3) The factorial model incorporating the above revisions (Tables 19 and 20) should be considered a good approximation of requirements for preschool children, as it yields estimates that are consistent with short-term balance studies. These estimates are 17-20% lower than the 1985 values. However, limited data on older children and adolescents suggest that the 1985 values may be appropriate for that age range. More information is needed to determine whether the new factorial model for older children and adolescents should be modified.

(4) The recommendations of the 1991 Consultation (FAO/WHO Expert Consultation, 1991) regarding amino acid requirements (see section 3.4) should be adopted until additional data are available, but further attention needs to be paid to the distinction between requirement vs safe level of intake.

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