Protein metabolism in insulin-treated gestational diabetes.

OBJECTIVE: To test the hypothesis that protein metabolism is not totally normalized in insulin treated gestational diabetes mellitus (GDM) patients compared with normal, pregnant control subjects. RESEARCH DESIGN AND METHODS: Protein metabolism in eight Hispanic women with insulin-treated GDM and eight healthy Hispanic control women was studied in late gestation and at 6 weeks postpartum. Nitrogen flux was assessed from the disposal rate of [15N]-labeled urea over 12 h after a dose of [15N]-labeled leucine. Plasma amino acid concentrations were determined in fasting and 2-h postprandial samples using an amino acid analyzer. RESULTS: Protein turnover was normalized in insulin-treated GDM; however, fasting and postprandial plasma amino acids were elevated antepartum and postpartum. Nitrogen flux was significantly lower during pregnancy (P = 0.04-0.001) and did not differ between groups. Fasting and postprandial plasma amino acids were elevated in GDM antepartum and postpartum, despite satisfactory glycemic control. Fasting levels of taurine, hydroxyproline, glutamic acid, glutamine, cystine, tyrosine, phenylalanine, tryptophan, and histidine were higher in GDM antepartum and postpartum (P < 0.05). Postprandial concentrations of taurine, hydroxyproline, valine, cystine, isoleucine, leucine, tyrosine, phenylalanine, tryptophan, ornithine, lysine, histidine, and arginine were higher in GDM antepartum and postpartum (P < 0.05). With few exceptions, plasma amino acid concentrations were lower antepartum than postpartum (P < 0.05). CONCLUSIONS: Protein turnover was normalized in insulin-treated women with GDM; however, fasting and postprandial plasma concentrations of amino acids were elevated in the antepartum and postpartum periods, despite satisfactory maternal glycemic control.

Qualitative analysis of human milk produced by women consuming a maize-predominant diet typical of rural Mexico.

The milk composition of women on a typical rural Mexican diet was compared with that secreted by American women, consuming a diet typical of affluent countries. Milk concentrations of free fatty acids, cholesterol, total amino acids, and selected key minerals were analyzed at 4 or 6 months postpartum. The total milk fat concentration was lower in the Otomi (22.7 +/- 6.7 mg/g milk) than in the American women (31.3 +/- 5.4 mg/g milk, p = 0.001). Although the absolute concentration did not differ, cholesterol, expressed in terms of total lipid, was higher in the Otomi milk (3.9 +/- 1.1 vs. 3.1 +/- 0.7 mg/g fat, p = 0.005). Saturated medium-chain (C10:0-C14:0) and unsaturated intermediate-chain fatty acids (C16:1 and C18:2) were higher in the Otomi than in the American milk (p < 0.03). The concentrations of C16:0, C18:0, and C18:1 were significantly lower in Otomi than in American milk. The milk concentrations of protein and nonprotein nitrogen were comparable between the two groups. The concentrations of serine, proline, cystine, methionine, and tryptophan were higher in the Otomi than in the American milk (p < 0.05-0.001). The concentrations of valine and isoleucine were significantly lower in the Otomi milk (p = 0.05). Expressed per gram of milk protein, the cystine, methionine, lysine, and tryptophan concentrations were higher, and the glutamine/glutamate, valine, isoleucine, and arginine levels were lower in the Otomi milk. The concentrations of phosphorus and copper were lower in the Otomi than in the American milk at 4 months postpartum (p = 0.05). These differences in milk fatty acid and amino acid patterns and mineral content are unlikely to affect infant growth, but may have other biological consequences yet to be ascertained.

Nutrient accretion in preterm infants fed formula with different protein:energy ratios.

BACKGROUND: Although standard formulas for preterm infants promote intrauterine rates of weight gain, fat deposition in preterm infants fed these formulas has been reported to be considerably higher than that in the fetus. We hypothesized that a preterm infant formula with a higher protein:energy (P:E) ratio would promote accretion rates of fat, fat-free mass, and minerals closer to those of the fetus. METHODS: As part of a larger study to determine whether accretion rates of fat and fat-free mass closer to those of the fetus can be achieved with a higher P:E ratio, we present a descriptive analysis of 72-h nutrient balance studies performed on a subset (n = 15/30) of the infants randomly assigned to be fed formula with a P:E ratio of either 3.2 g/100 kcal or 2.6 g/100 kcal. RESULTS: Despite the higher intake and net absorption of nitrogen by infants fed the higher P:E formula, there was no statistically significant difference in net nitrogen retention between groups. There also were no statistically significant differences between groups in digestible energy, metabolizable energy, energy expenditure, or energy storage. Thus, partitioning of stored energy as protein and fat did not differ between groups. The retention of calcium, phosphorus, sodium, potassium, copper, and zinc also did not differ between groups, and nitrogen intake did not affect mineral retention. CONCLUSIONS: In this study, formula for preterm infants with a P:E ratio of 3.2 g/100 kcal vs. 2.6 g/100 kcal provided no apparent benefit in terms of the proportion of fat to lean tissue accretion as determined from nutrient balance data.

Lactational performance of adolescent mothers shows preliminary differences from that of adult women.

PURPOSE: The purposes of this study were to characterize milk production, milk composition, and the lactational behavior of adolescent mothers, and to compare their lactational performance with that of adult females. METHODS: Twenty-two lactating mothers, 11 adolescents and 11 adults, were studied at 6-week intervals between 6 and 24 weeks postpartum. Milk production was determined by the test-weighing procedure. Milk nutrient composition was determined by standard chemical analyses. Frequency and duration of nursing and the use of supplemental formula and complementary foods were recorded. RESULTS: The amount of milk adolescents produced at 6, 12, 18, and 24 weeks postpartum ranged from 37-54% less (P < .05) than that of the adults and resulted in a 45% weaning rate at 18 weeks postpartum in the younger group. Milk nutrient concentrations were not significantly different between groups, with the exception of significantly higher sodium concentrations during early lactation in the adolescents' milk. Lactational behavior differed significantly between the adolescent and adult groups; however, with the exception of the lower frequency of daytime nursing and the tendency toward the early introduction of supplemental formula in the adolescent group, these behavioral differences were the result of the racial and ethnic differences between the two groups. The differences in lactational behavior did not contribute to the differences in milk production between the adolescents and adult mothers. CONCLUSIONS: This preliminary study suggests that milk production was reduced in adolescent mothers compared with adult females. Although behavioral strategies that increase the frequency of daytime nursing and reduce the frequency of supplemental feedings may enhance the milk production of adolescent mothers, other biological factors may account for their poorer lactational performance.

PIP: In a comparative study of the lactational performance of 11 adolescent and 11 adult breast-feeding mothers from the US, adolescents were found to produce significantly less milk and lactate for a significantly shorter period of time than their adult counterparts. All subjects were assessed at 6-24 weeks postpartum. The adolescents produced 37% and 54% less milk at 6 and 24 weeks postpartum, respectively, than adult women. These differences in milk production were significant even when adjusted for differences in the frequency and duration of breast feeding episodes and use of supplementary feeds. The amount of dietary energy the infants of adolescents received from human milk alone was clearly inadequate, at every time point, to support normal growth rates. In both groups, the average frequency of nursing episodes during the first 12 weeks postpartum was 7 or more per 24 hours (consistent with current recommendations for adequate lactation); adolescents, however, spent significantly less time nursing and provided greater quantities of supplementary feeds. While all adult women breast-fed throughout the study period, 20% of adolescents had stopped breast feeding by 12 weeks, 50% weaned by 18 weeks, and 64% had discontinued breast feeding by 24 weeks. Unexpectedly, the energy, lactose, fat, total nitrogen, protein nitrogen, nonprotein nitrogen, sodium, potassium, calcium, and phosphorous concentrations showed little difference between the two age groups. The absence of data from the first 6 weeks of life makes it impossible to rule out a role for early formula supplementation in the decreased milk production of adolescents. It is believed,however, that adolescents may be biologically incapable of producing a full complement of milk because of their developmental immaturity.

Marginal dietary protein restriction reduced nonprotein nitrogen, but not protein nitrogen, components of human milk.

OBJECTIVE: This study was designed to determine whether marginal dietary protein intakes of lactating women alter their milk production and composition. The study followed the observation that marginal dietary protein intakes lead to significantly negative nitrogen balances in these women. METHODS: Twenty-four healthy, mature lactating women were divided evenly into three groups on the basis of postpartum times; 1, 5, or 12 months. Each woman was given a controlled protein diet of either 1.5 (HP) or 1.0 (MP) g/kg body weight/day for 7 to 10 days. Milk production was measured for 72 hours by test weighing the infant before and after feeding. Milk samples from alternate breasts were collected for 24 hours, pooled, and analyzed for their protein nitrogen, nonprotein nitrogen, free and protein-bound amino acid, and lactoferrin concentrations. RESULTS: Reduced dietary protein intakes were associated with a decrease (p < 0.05) in the nonprotein nitrogen and total free amino acid fractions of milk. Although urea was unaffected, the concentrations of milk free leucine, phenylalanine, histidine, and ornithine were lower (p < 0.05) in the MP than in the HP group, while the output of leucine and histidine tended to be lower in the MP group. The amount of milk produced, as well as the amounts of protein nitrogen, protein-bound amino acid, and lactoferrin and their concentrations were unaffected by a reduction of dietary protein. CONCLUSIONS: Maternal milk production and the protein nitrogen, but not the nonprotein nitrogen, fraction of human milk were relatively well-preserved in the presence of a short-term, marginal reduction of dietary protein.

Insulin, cortisol and thyroid hormones modulate maternal protein status and milk production and composition in humans.

The partitioning of dietary and endogenous nutrients during lactation is not well understood. To examine associations between plasma hormone and substrate profiles and indices of either maternal body protein metabolism or lactational performance, we measured plasma insulin, cortisol, prolactin, thyroxine, triiodothyronine, individual amino acid, blood urea nitrogen, and prealbumin concentrations in lactating and nulliparous women in the postabsorptive state. We related these measurements to the subjects' nitrogen balance, urinary 3-methylhistidine excretion, [1-13C]leucine metabolism and milk production. Insulin concentrations showed significant positive relationships with nitrogen balance and prealbumin concentrations; cortisol levels showed a significant negative relationship with nitrogen balance and a significant positive relationship with leucine incorporation into protein. Thyroid hormone concentrations showed significant positive relationships with urinary 3-methylhistidine excretion, leucine incorporation into protein, and milk production. Proline concentrations were associated positively with nitrogen balance and negatively with leucine incorporation into protein, whereas glutamate-glutamine concentrations showed positive associations with leucine oxidation and milk nitrogen concentrations. We propose that insulin and cortisol modulate the channeling of nutrients between anabolic and anti-anabolic aspects of maternal body protein metabolism, whereas thyroid hormones and cortisol modulate nutrient partitioning toward milk production and visceral protein synthesis. We suggest that some nonessential amino acids (proline, glutamate-glutamine) may become limiting during lactation because of their unique contributions to milk protein synthesis.

Dietary protein and nitrogen balance in lactating and nonlactating women.

Adaptive responses of body protein metabolism to dietary protein intakes of 1.0 g.kg body wt-1.d-1 were determined by nitrogen balance and urinary 3-methylhistidine excretion in lactating and nonlactating women. Despite higher energy intakes (p less than 0.04), lactating women had lower nitrogen balances compared with nonlactating postpartum and nulliparous women (p less than 0.001). Nitrogen losses in milk did not account entirely for these differences. Nitrogen balance showed linear (p less than 0.04) and quadratic (p less than 0.03) trends over time postpartum among the lactating women. Urinary 3-methylhistidine excretion also was reduced (p less than 0.05) in lactating compared with nonlactating women. These observations suggest that protein intakes of 1.0 g.kg body wt-1.d-1 in lactating women are associated with adaptive responses that promote the conservation of skeletal muscle protein stores and that currently recommended dietary protein allowances may be insufficient to meet the nutritional needs of well-nourished lactating women.