ExtractCystathionase activity was absent from human fetal liver and brain as early as 6 weeks of gestation. Hepatic methionine-activating enzyme (26 ± 3 nmoles/mg protein/hr) and hepatic cystathionine synthase (21 ± 4 nmoles/mg protein/hr) were present (cf. 86 ±16 and 98 ± 19 nmoles/mg protein/hr, respectively, in mature human liver). All three activities were absent from the placenta. Human fetal liver contained higher concentrations of cystathionine (14 ± 2 /xmoles/100 g wet weight) than mature human liver (0) and human fetal brain (4.0 ± 0.6 ,umoles/100 g wet weight). Methionineactivating enzyme of human fetal brain, but not liver, showed a tendency to increase with development (coefficient of correlation was 0.62; 0.01 < P < 0.05).35 S-L-methionine injected into the umbilical vein of six human fetuses was incorporated into free methionine in liver and brain, but not into free cyst(e)ine, homocyst(e)ine, taurine or, except for the smallest fetus, cystathionine. 35S-L-cysteine similarly injected was incorporated into free cysteine in liver and brain to a greater extent than in plasma, whereas it was incorporated into cystine in plasma to a greater extent than in either liver or brain. Incorporation of S-L-cysteine incubated with minced liver from four human fetuses showed more active incorporation of methionine (11,836-15,045 dpm/mg protein) than cysteine (7,044-9,856 dpm/mg protein). SpeculationThese studies suggest that cysteine is an essential amino acid in human fetuses and in infants for some time after birth, especially if they were born prematurely. Introductionknown about the early development of these pathways. Although there is considerable information on controlIn normal adult humans, about 90% of ingested meof the metabolism of sulfur-containing amino acids thionine is converted to cyst(e)ine [21], a nonessential and transsulfuration in mature humans, little is amino acid, via the transsulfuration pathway (Fig. 1).

Cystathionase activity is not measurable in the livers of 24 human fetuses and 3 premature infants, and the concentration of cystathionine in the liver is higher than that of the brain. The placenta does not subserve the trans-sulfuration function. Cystine (or cysteine) thus may be an essential amino acid in the immature human.

Protein analyses of the milk before individual fortification provides a new tool for an individualized feeding system of the preterm infant. The bovine whey protein fortifier attained biochemical and growth results similar to those found in infants fed human milk protein exclusively with the corresponding protein intakes.

ABSTRACT. In a double-blind, randomized study, 28 healthy, growing very low birth wt, appropriate-for-gestational-age infants were fed human milk, preferably mother's own, fortified daily with human milk protein and/or human milk fat. The infants entered the study when they were stable on complete enteral intakes of 170 mL/kg/d (mean age = 19 d). The study lasted for a mean of 4 wk.Samples from all the milks were collected daily, and intakes of protein, fat, carbohydrates, energy, and electrolytes were calculated weekly during the whole study period.Protein intakes ranged from 1.7 to 3.9 g/kg/d, and energy intakes from 100 to 150 kcal/kg/d. Wt and length gain in the nonprotein-enriched groups were 15.6 + 2.7 g/kg/d (mean + SD) and 0.88 + 0.17 cm/wk; the corresponding figures for the protein-enriched groups were 20.2 + 2.1 g/ kg/d and 1.24 & 0.14 crnlwk. There was a strong correlation between protein intake and growth in wt and length up to an intake of about 3 g/kg/d; more protein did not result in increased growth. The same was true for energy intake, with a maximal growth rate a t an intake of about 120 kcal/ kg/d. A protein intake of more than 3 g/kg/d resulted in a growth rate equal to or higher than the estimated intrauterine growth rate. Some infants fed mature banked human milk alone had a poor growth. Sodium intake was low, ranging from 1.5 to 2.6 mmol/kg/d. No correlation was found between sodium intake and growth rates.

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ABSTRACT. The effects of different protein intakes on wt gain, insulin secretion, and plasma concentrations of amino acids have been evaluated in a prospective study involving 30 normal term infants. The infants were studied from 4.0 to 6.0 mo of age. Ten infants were breast-fed (BF), the others were randomly divided into two groups of 10 infants. One group was fed a formula containing 1.3 g protein/100 mL (F 1.3), the other a formula with 1.8 g protein/100 mL (F 1.8). The formulas were isocaloric (72 kcaI/100 mL), and the fat concentrations were 3.5 g/100 mL (F 1.3) and 3.2 g/100 mL (F 1.8). AH infants received the same supplementary foods. The urinary C-peptide excretion in the infants fed the F 1.8-formula was 4.4 ± 2.1 nmol/mmol creatinine or 19.4 ± 12.9 nmol/m 2 , significantly higher than that in the infants fed the F 1.3-formula (2.6 ±1.5 and 7.9 ± 5.1) or the BF infants (1.7 ± 1.4 and 6.3 ± 6.0). Gain in wt was 18.0 ± 4.3,19.9 ± 3.9, 22.8 ± 1.6 g/kg/wk and corresponded to protein intakes of 1.3 ± 0.2, 1.9 ± 0.3, and 2.6 ± 0.2 g/kg/d, in the BF, F 1.3, and F 1.8 groups, respectively. Gain in length was 6.7 ±1.8 (BFgroup), 6.2 ± 2.5 (F 1.3-group), and 7.6 ± 2.2 (F 1.8-group) mm/m/wk. Wt gain correlated with urinary Cpeptide excretion at 6.0 mo (r = 0.51, p < 0.01) and with protein intake (r = 0.43, p < 0.01). Furthermore, protein intake correlated with urinary C-peptide excretion (/• = 0.66, p < 0.001). Caloric intake from carbohydrate and fat correlated both with wt gain (r = 0.34, p < 0.05) and with urinary C-peptide (r = 0.44, p < 0.05). A higher protein intake during a meal resulted in a higher postprandial excretion of urinary C-peptide. The difference between plasma C-peptide (AC-peptide) before and after a meal was highest in the groups of infants fed the F 1.8-formula. The intakes of the insulin-releasing amino acids (arginine, lysine, leucine, phenylalanine, valine, isoleucine, and threonine) were higher in the F 1.8-group than in the F 1.3-and BF-groups. The sums of the fasting plasma concentrations of these amino acids were 64.8 (BF), 83.6 (F 1.3), and 96.3 (F 1.8) /wnol/100 mL. The plasma concentrations of the branched-chain amino acids, valine, leucine, and isoleucine correlated with wt gain and plasma valine-glycine ratio increased with higher protein intake. These results suggest that protein amino acid-induced insulin secretion could be a factor promoting growth in infants on high protein intakes.

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Improvement of the amino acid profile permits a whey predominant starter formula with 1.8 g protein per 100 kcal to meet the needs of normal term infants during the first four months of life.