2.2. Children

The foregoing has dealt with studies performed on adults. Brief consideration will now be given to protein-energy needs of children. First, however, an overview of assessment methods will be presented. During the first year of life, the protein content of the body increases rapidly. The average increase in body protein is about 3.5 g/d during the first 4 months of life and 3.1 g/d during the next 8 months. By 4 years of age, body protein concentration reaches the adult value of 18 to 19% of body weight. As the growth rate drops rapidly after the first year of life, the maintenance requirement represents a gradually increasing and major proportion of the total protein requirement (FAO/WHO/UNU, 1985).

For the first months of life, requirements are based on intake data because of the difficulties in estimating accurate allowances for growth and maturation. Infants breast-fed by healthy, well-nourished mothers can grow at a satisfactory rate for 4 to 6 months. Measurements of human milk consumption (FAO/WHO/UNU, 1985) demonstrated that protein intakes ranged from 2.43 g/kg/d in the first month to 1.51 g/kg/d in the fourth month. A rounded value of 2.00 g/kg/d was selected as an allowance for this age group, but it was recommended that protein requirements should be considered in relation to energy needs. The protein needs of an infant up to 4 months of age will be met if the energy needs are met, provided the food contains protein of quality and quantity equivalent to that of breast milk. This implies that at least 6°10 of the food-energy in the form of high-quality protein is desirable.

A modified factorial procedure was used to estimate needs for older children, and the FAO/WHO/UNU (1985) group also recognized the need for continuous provision of protein in anticipation of extra, unpredictable demands from the daily variations in growth rate, since protein provided on a day of no growth is probably not held in reserve for later growth. There were no data on which to base reliable estimates for the extra protein needed for this purpose, but a value 50°70 higher than the estimated daily nitrogen increment was considered realistic.

Tabulated values for daily protein and energy allowances for the various age and sex groupings are shown in Table 5 and are initially expressed as high-quality, highly digestible proteins such as meat, milk, fish and eggs. These would need to be corrected for amino acid composition and digestibility to be converted to protein as consumed in ordinary dietaries. Final recommended allowances for dietary protein, as provided by a diet typical of an industrialized country, would thus range from somewhat over 2 g/kg/d in the first year of life to approximately 1 g/kg/d for adolescents. The protein/energy ratios for the various age and sex recommendations will be discussed in a subsequent section.

Table 5. Recommended Allowances for Protein and Energy (NAS-NRC 1989) and calculated values for PCal%

1 PCal% = Protein g per 100g x 400/kcal per 100g. Since energy allowances are averages and protein allowances are safe levels (means + 2SD) the direct ratios of allowances (A) are misleading. The ratios (B) are calculated using average protein needs and are, thus, lower.
² Mature milk composition.
³ Second and third trimesters.

Protein-energy interactions in diets for children have been reviewed by ZLOTKIN (1986). Although recommended intakes of nutrients are listed according to population norms for age and size on an individual basis, protein and energy intakes are generally adjusted to meet specific outcome goals. Recommended intakes for protein and energy for the full-term infant are based on the composition and volume intake of mature human milk, defined as milk produced sometime after 30 days post partum. For infants born prior to term, oral nutrient intake recommendations have been established either from:

(a) analysis of body composition
(b) direct clinical studies of various feeding regimens, or
(c) analysis of the nutrient content of mature human milk.

The establishment of an appropriate outcome for the newborn infant, who is parenterally fed, is even more complicated, since there are potential differences in the body's handling of intravenously versus orally ingested nutrients. As reviewed by ZLOTKIN (1986), the literature describes intravenous formulations delivering 1.5 to 4 g/kg/d of amino acids and 50 to 140 kcal/kg/d for energy, depending on clinical circumstances and whether the infusion is via the central or peripheral vein.

For children, the energy intake necessary to minimize nitrogen loss associated with an amino acid free diet has not been established but is likely to be in the range of 70 kcal/kg/d (ZLOTKIN 1986). For infants who are receiving an energy intake that is adequate to maintain good health, daily nitrogen retention can be shown to increase with increasing nitrogen intake. Even at the highest nitrogen intake level (640 mg/kg/d), there was no decrease in the linear increase in nitrogen retention.

This response is in marked contrast to that seen in the adult where nitrogen retention is assumed, by current theory, to plateau at or slightly above the zero balance line, when excess dietary nitrogen is provided. It should be noted, however, that infants receiving the highest nitrogen intakes also had the highest plasma amino acid levels, with these levels reflecting the composition and quantity of the amino acid solutions infused. Thus, achieving even higher nitrogen retention in infants is possible, though probably undesirable, because of the concomitant metabolic consequences (ZLOTKIN, 1986).

When both nitrogen and energy are taken concurrently both interact to affect nitrogen retention. If energy intake is inadequate to meet maintenance requirements, then protein may be used as an energy source. Similarly, if protein intake is inadequate to meet maintenance needs, then increasing energy intakes will spare protein for protein synthesis. This holds true also for infants and it has been demonstrated (ZLOTKIN, 1986) that, for an energy intake just below maintenance requirements (50-60 non-protein kcal/kg/d), increasing nitrogen intake from 400 to 500 mg/kg/d results in a significant increase in nitrogen retention.

At this same low energy intake, however, a further increase in nitrogen intake to 655 mg/kg/d did not result in any further increase in nitrogen retention. The clinical consequences suggest that, when infants are maintained on a hypocaloric intake, nitrogen should be provided at no more than 480 mg/kg/d, since the administration of excess nitrogen will not contribute to nitrogen retention and may lead to increasing plasma urea and amino acid levels.

Milk formula will meet the protein needs of an infant up to about 4 months, if energy needs are met, since the protein and energy content of infant formulas, at least quantitatively, reflects mature human milk (ZLOTKIN, 1986). Therefore, once again, if the energy needs of the infant are met, so too will be the protein needs. It is generally accepted that the value of 1.5 g of crude protein per 100 kcal (Pcal% = 6) approaches the ideal protein/energy ratio for infant foods (FAO/WHO/UNU, 1985).

For the premature infant fed orally, the situation is much more complicated, since it is not universally accepted that mature human milk is the ideal food for the premature infant. The nutrient content of the milk produced by mothers giving birth to premature infants is different from mature human milk (ATKINSON et al., 1978). The estimated protein needs of the premature infant are 3 g/kg/d and energy needs at 120 kcal/kg/d. Thus, the protein/energy ratio for these infants would be 2.5 g of protein per 100 kcal or Pcal% = 10).

Although the actual requirements for growth and development are independent of the route of feeding, the latter has a significant effect on recommended intakes because bioavailability and absorption considerations are of less consequence and activity levels may differ. ZLOTKIN (1986) has calculated that a energy intake of 80 to 100 kcal/kg/d should be adequate to meet requirements of intravenously fed infants. This value is some 20% lower than the recommended oral intake. It has been estimated also that, for the pre-term infant, as long as adequate energy is provided, a nitrogen intake of about 480 mg/kg/d will safely achieve the goal of duplication of the rates of intrauterine nitrogen accretion. This corresponds to about 13% calories from protein.

The relative effect of glucose and lipids on whole-body protein-metabolism kinetics was assessed in seven infants undergoing parenteral feeding by BRESSON et al. (1991). Protein intake was kept constant, and non-protein energy was either provided as glucose alone or as an isoenergetic glucose-lipid mixture. Protein metabolism and energy-substrate utilization were assessed by a primed, constant L-[13C]leucine infusion, combined with indirect calorimetry. There was a significant difference in the pattern of energy-substrate utilization according to regime. Protein turnover, protein breakdown, and amino acid oxidation rates were higher for the glucose than the glucose-lipid treatment, whereas protein-synthesis rates did not significantly differ. The nature of energy substrates delivered to parenterally fed infants may thus affect protein metabolism.

For requirements of children living in developing countries, the studies that we have considered are those from Thailand, Guatemala and Jamaica. Protein-energy relationships have been reported for children in Thailand by TONTISIRIN et al. (1984b). Nine children, aged 9 to 36 months, weighing 8.1 to 11.1 kg and living in a metabolic unit were given normal local weaning diets at three levels of energy intake, varying from 87 to 118 kcal/kg/d. The diets consisted of rice, fish and banana, and the protein intake was fixed at the safe level of 1.7 g/kg/d. Each level of energy was fed for 7 days and vitamin and mineral supplements were given daily. At the lowest level of energy intake, 87 kcal/kg/d, apparent nitrogen retention and weight gain were quite low being 44 mg N/kg and 3.5 g/d, respectively. At the two higher levels of energy intake (100 and 118 kcal/kg), apparent nitrogen retention was greater than 60 mg/kg per day, and weight gain was 20 g per day or more. The results from this study suggested that at the safe level of protein intake, as recommended by FAO/WHO in 1973, the usual Thai weaning food provided adequate protein for the needs of young children, if energy intake was supplied at 100 kcal/kg/d or more. A longer-term study, lasting 4 months for six normal, healthy young male children, aged 8 to 12 months, that had been rehabilitated for 8 weeks or longer following protein-energy malnutrition and had reached normal weights for height, confirmed the earlier observations that 1.7 g/kg protein was adequate for one-year-old children, provided that the energy intake was 100 kcal/kg/d or more. This would represent Pcal%. = 6.8. Habitual Guatemalan diets were investigated for their capacity to allow for catch-up growth in children with mild to moderate malnutrition (TORUN et al., 1984). Children were fed diets mainly based on corn and beans, but with some animal foods such that protein contents averaged from 2.1 to 1.8 g/kg, depending on the stage of the recovery. The energy density of the diet was increased by adding oil to the beans and sugar to the beverages. It was observed that needs of children 2-4 years old could be satisfied with 85-90 kcal/kg (Pcal% = 8.5-9.5) while catch-up growth required 95-105 kcal/kg (Pcal% = 8).

Nitrogen balance and whole-body protein turnover were measured in children aged about one year by JACKSON et al. (1983). Diets provided 1.7 or 0.7 g milk protein/kg/d and were fed at three levels of metabolizable energy, 80, 90 and 100 kcal/kg/d. The Pcal% values of the diets thus ranged from 2.8 to 8.5%. All the children were in positive nitrogen balance at all levels of energy intake on 1.7 g protein/kg/d. Nitrogen equilibrium was maintained on 0.7 g protein/kg/d when the energy intake exceeded 90 kcal/kg/d, but on 80 kcal/kg/d nitrogen balance was negative. Whole-body protein turnover was measured from the enrichment in urinary ammonia, following a continuous infusion of ¹⁵N-glycine.

It was observed that the variation between individuals on the same diet was significantly greater than the variation within individuals at different levels of energy intake. For the group as a whole, protein synthesis on 1.7 g protein/kg/d was 0.74, 0.75 and 0.87 g N/kg/d on 100, 90 and 80 kcal/kg/d, respectively; whereas on 0.7 g protein/kg/d it was 0.37, 0.38 and 0. 40 g N/kg/d. These results show that, over this range of intakes, protein synthesis decreased as dietary protein fell, but tended to remain unchanged or even increase as energy intake fell.

Physical activity is also a factor in energy-protein interrelationships (YOUNG et al., 1983) and may be especially important in children. A study by TORUN et al. (1976) revealed the beneficial effects of continued mild exercise on the utilization of dietary protein in young children aged 2 to 4 years undergoing treatment for protein-energy malnutrition. These children were stimulated to be more physically active through a daily program of games that required a mild increase in energy expenditure. Measurements were made of growth and of body energy and nitrogen balances, and results were compared with those obtained in a similar group of children treated in the traditional manner at INCAP. The more active children were shown to grow better in height and in lean body mass. Thus, the efficiency of utilization of dietary protein-energy for growth was greater in the more active children. It could be concluded that moderate, systemic exercise has a growth-enhancing affect and a favorable impact on the utilization of dietary protein. Similar relationships for other age groups are described elsewhere in this volume.