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Table 3 Protein needs for growth and maintenance at different ages of the human infant

^a From Table 4, up to 12 months; from Fomon et al (1982) from 1 to 5 years.
^b Maintenance assumed to be 90 mg N/kg/d.

Table 4 Daily increment in body protein of male breast-fed infants (pooled dataset, WHO, 1994)

^a At midpoint of interval.
^b Fomon et al (1982).
^c Fomon (1991) for male infants fed milk-based formulas.

Body composition changes rapidly during infancy. The body composition data presented by Fomon et al (1982) were based on measurements at three ages: birth, 6 months and 9-10 y of age, with interpolation used to obtain values for other ages. As they point out, the estimates were considered preliminary 'because of uncertainties about the data and because of the large number of assumptions that have been required' (p. 1172). Thus, there is a need to reevaluate reference values for body composition, utilizing any new data that may be available for the first year of life. For the purposes of this analysis, the major question is whether the estimates of the percentage of weight gain contributed by FFBM are accurate. There is evidence that percentage body fat is lower among breastfed than formula-fed infants during the second six months of life (Dewey et al, 1993); unpublished data (Dewey et: al) indicate that the estimated percentage of weight gain as FFBM is higher from 3 to 12 months in breastfed than in formula-fed infants (the difference is statistically significant at 6-9 months: 88 ± 21% vs 77 ± 9%, respectively, P = 0.001). If the values of percent gain as FFBM for breastfed infants are used in Table 4 from 6 to 12 months, the calculated body protein gain is slightly higher (an increase of 0.01 g/kg/d), but still not as high as the estimates seven by Fomon (1991).

Therefore, if the factorial model were based on growth data. for breastfed infants, the resulting estimates of protein required for growth during the second six months of life would be somewhat less than the values used in the 1985 report.

It should be noted that protein needs for growth during early infancy will depend on birth weight, as infants with lower birth weights generally will gain weight more rapidly and those with higher birth weights will gain weight less rapidly than average. Because of this, protein requirements based on reference growth rates from affluent populations (in which mean birth weight is relatively high) may be an underestimate during early infancy for populations with lower mean birth weights. For example, mean weight gain from birth to 10 weeks of age of 141 exclusively breastfed infants in Honduras, whose mean birth weight was 2.9 kg, was 36.7 g/d (Cohen et al, 1995), compared to 32.0 g/d during the same interval for the pooled dataset shown in Table 4. The difference of 4.7 g/d would represent an additional deposition of about 0.1 g/kg/d body protein, which would require an additional 0.14 g/kg/d protein intake for infants with lower birth weights. This would increase protein needs during early infancy by about 8.5%.

2.2.4. Intra-individual variation in growth. To account for day-to-day variation in the rate of growth, and thus in the need for protein to support that growth, the 1985 report added 50% to the theoretical daily body nitrogen increment. l he reasoning behind this was that there is a very limited storage capacity for amino acids, and thus it is necessary to allow for extra demand on days when growth is particularly rapid. Recent findings by Lampl et al (1992) lend support to the idea that linear growth occurs in spurts, although this hypothesis has been challenged. However, even if linear growth is indeed saltatory, it is by no means clear which nutrients are likely to be most limiting for the deposition of tissue during these brief spurts, and therefore the quantitative impact on daily protein needs has not been estimated. Weight gain is probably also saltatory, but the proportion of lean to fat tissue in each 'spurt', and thus the need for extra protein, has not been estimated. The 50% augmentation used in the 1985 report was arbitrary, and was justified on the basis that the resulting factorial estimate of average requirement was very similar to the estimated intake from breast milk at 4 months. However, as pointed out in section 2.1.2, the assumption that average requirement should be similar to average protein intake of breastfed infants is incorrect. Therefore, the adjustment for day-to-day variability needs to be reconsidered. In Fomon's (1991) estimates, no adjustment for day-to-day variability in growth was utilized.

2.2.5. Efficiency of conversion from dietary protein to body protein. In the 1985 report, it was assumed that the efficiency of conversion from dietary protein to body protein during growth is 70%, the same efficiency assumed for maintenance needs. However, as Waterlow points out (1990), because the reasons for inefficiency are not fully understood, it is difficult to defend this assumption. Considering the potential for salvage of urea nitrogen (Jackson, 1992), it is likely that the efficiency of conversion could be quite a bit higher than 70%. In Fomon's (1991) estimates, he assumed an efficiency of 90%, based on the presumption that efficiency of conversion should be higher from a milk-based diet than from a mixed diet. It could be that the efficiency of utilization of human milk is yet higher than that of cow's milk-based formulas. Serum urea nitrogen and urinary nitrogen levels tend to be lower in breastfed than formula-fed infants (see section 2.3), even though urea content is relatively high in human milk. However, much of this difference may be attributable simply to the lower total protein intake of breastfed infants. Although breastfed infants gain more weight and lean body mass per gram of protein consumed than do formula-fed infants, the slope of the relationship between protein intake and growth does not differ between feeding groups (Heinig et al, 1993); i.e. infants with a lower protein intake will use more of the protein for growth regardless of feeding mode. Taken together, these findings may mean that protein intakes of formula-fed infants are excessive, rather than that breastfed infants are metabolically more 'efficient' at utilizing protein.

More information on the efficiency of conversion of dietary protein to body protein is needed, particularly under conditions of rapid growth. Efficiency of protein utilization (for all needs, not just for growth) can be estimated from balance studies by calculating the slope of the line (by dividing basal needs (the y intercept) by maintenance needs (the x intercept)), as shown in Table 2. In the studies of children fed a predominantly milk based diet, this value was 73-74% (except for the study of very young infants by Fomon et al (1965), as mentioned in section 2.2.2). The appropriate value for infants younger than 6 months of age requires further investigation. Because growth represents a much larger percentage of total protein needs in the early months (see Table 3), and the efficiency of protein utilization for growth may differ from that for maintenance, it is possible that the slope of the line would be considerably different in younger infants. Although differences in diet composition among studies must be taken into account, the data shown in Table 2 are consistent with the hypothesis that efficiency decreases with age. In animal studies, younger animals are apparently more efficient than older animals at utilizing protein, based on the observation that the latter require higher protein intakes to achieve the same rate of growth (Reeds, personal communication).

Waterlow (1992b) recently summarized data on the efficiency of protein deposition from a variety of studies. Efficiency was calculated as N retained/(N intake obligatory losses). Using this definition, values of 80% or less were observed in situations of rapid growth. However, this definition of 'efficiency' yields values that are strongly influenced by the level of protein provided: if this is in excess relative to the anabolic drive (or to other nutrients required for growth), the amount of nitrogen retained as a proportion of intake will be low. When protein is provided closer to the 'requirement' level (i.e. just meeting the anabolic drive), which is likely to be the case for breastfed infants, the calculated 'efficiency' would probably be considerably higher. This does not necessarily imply, however, that the biochemical efficiency of amino acid utilization for growth would differ according to the level of protein intake.

To adjust for efficiency of protein utilization, the factorial model in the 1985 report used a simple additive model (i.e. adding maintenance needs, with an assumed efficiency of 70 %, to needs for growth, also with an assumed efficiency of 70%). However, it is not clear whether an additive approach is appropriate in situations of rapid growth such as during infancy. In such situations, overall efficiency may be greater because the amino acids not needed for maintenance can be readily used for growth. In support of this idea, Chan and Waterlow (1966) found that efficiency of utilization of milk protein was almost 100% in young children recovering from malnutrition.

2.2.6. Inter-individual variability in nitrogen needed for maintenance and growth. To derive safe levels of protein intake for virtually all healthy infants, the average requirement estimates need to be adjusted based on the magnitude of inter-individual variability in the needs for both maintenance and growth. In the 1985 report, the coefficient of variation (CV) in the maintenance requirement was assumed to be 12.5%, the same as the CV found for adults in numerous short-term nitrogen balance studies. As justification for this, it was stated that the range of inter-individual variation in the nitrogen balance studies of children was similar to that of adults, although no actual CV values were presented.

Much more problematic than the maintenance CV is the inter-individual variability in growth. In the 1985 report, data on variability in weight gain over one month intervals from 3.5 to 6.5 months (based on unpublished data of Fomon) were used to estimate a CV of approximately 35% (shown in the footnote to Table 33 of the 1985 report). It was recognized, however, that the CV for growth depends on the interval of measurement: it will be lower over longer periods and higher over shorter periods. The one-month interval CV was 'accepted as a reasonable compromise'. Not mentioned in the report is the fact that the CV for growth will also increase with age during the first year, because the mean growth rate declines disproportionately more than does the standard deviation. This is illustrated in Table 5, which shows CVs for one month and three-month weight increments of breastfed infants from the pooled dataset used for Table 4. With one-month increments, the mean CV increases from 26% at one months to an astonishing 93% at 11-12 months. By contrast, the 3-month CVs range from 24% at 1-4 months to 46% at 9-12 months. Additional analyses with this dataset (data not shown) have indicated that, using the one-month increments and accounting for the effect of age on growth rate, about 70-80% of total variability can be attributed to intra-subject variability (e.g. month to month) and only about 20-30% to inter-subject variability. In other words, there is relatively little 'tracking' in growth velocity by individual infants (in fact, the opposite probably occurs: an individual who grows slowly one month is likely to grow rapidly the next). For this reason, it makes more sense to err on the low side when estimating inter individual variability in growth (i.e. use longer intervals, such as three months). Although the choice of 3-month CVs is admittedly arbitrary, these values were used in calculating the revised estimates of safe levels of protein intake presented in the next section.

Table 5a CVs for one-month weight increments^a of breast-fed infants (pooled data set, WHO, 1994)

^a Calculated from absolute weight gain, not gain per kg body weight.

Table 5b CVs for 3-month weight increments^a of breast-fed infants (pooled data set, WHO, 1994)

^a Calculated from absolute weight gain, not gain per kg body weight.

2.2.7. Revised estimates of protein requirements and safe levels of intake. Although there are many unanswered questions regarding the assumptions required to estimate protein requirements, the information presented in the sections above was used to generate a revised factorial model for infants. Table 6 presents data analogous to those shown in Table 32 of the 1985 report, but using the following assumptions: (a) body protein gain was estimated as shown in Table 4; (b) there was no additional augmentation for day-to-day (intra-individual) variability in growth (instead, this was considered to be covered by the CV for growth when calculating safe levels - see Table 7); (c) the efficiency of conversion of dietary protein to body protein was estimated to be 70% (based on the slopes from the balance studies in Table 2) and (d) the maintenance requirement was estimated to be 90 mg N/kg/d. The estimates in Table 6 are 27-35% lower than the 1985 values.

It could be debated whether data for breastfed infants should be chosen for the growth increment calculation. However, because the differences in protein gain per kg between breastfed and formula-fed infants are evident primarily in the second 6 months of life (see Table 4), when protein needs for growth are a relatively small proportion of total protein needs, the impact of this choice is relatively minor, If estimates of protein gain based on formula-fed infants (Fomon et al, 1982) had been used for Table 6, the values at 6-12 months would only be about 4% higher than those shown.

Table 7 shows the revised estimates for the safe level of protein intake, analogous to Table 33 in the 1985 report. In this table, the CV_tot is calculated in the same way as in the 1985 report (see footnote to Table 7), but using the 3-month CVs for growth from Table 5 instead of a constant 35%.

Continued

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