Unlike pregnant adult women, pregnant adolescent girls cannot maintain glycine flux during late pregnancy because of decreased synthesis from serine.

During pregnancy, glycine and serine become more important because they are the primary suppliers of methyl groups for the synthesis of fetal DNA, and more glycine is required for fetal collagen synthesis as pregnancy progresses. In an earlier study, we reported that glycine flux decreased by 39% from the first to the third trimester in pregnant adolescent girls. As serine is a primary precursor for glycine synthesis, the objective of this study was to measure and compare glycine and serine fluxes and inter-conversions in pregnant adolescent girls and adult women in the first and third trimesters. Measurements were made after an overnight fast by continuous intravenous infusions of 2H2-glycine and 15N-serine in eleven adolescent girls (17·4 (se 0·1) years of age) and in ten adult women (25·8 (se 0·5) years of age) for 4 h. Adolescent girls had significantly slower glycine flux and they made less glycine from serine in the third (P<0·05) than in the first trimester. Baby birth length was significantly shorter of adolescent girls (P=0·04) and was significantly associated with third trimester glycine flux. These findings suggest that the pregnant adolescent cannot maintain glycine flux in late pregnancy compared with early pregnancy because of decreased synthesis from serine. It is possible that the inability to maintain glycine synthesis makes her fetus vulnerable to impaired cartilage synthesis, and thus linear growth.

Indian women of childbearing age do not metabolically conserve arginine as do American and Jamaican women.

BACKGROUND: In a previous study in pregnant American women, we reported that arginine flux and nitric oxide synthesis increased in trimester 2. More recently, we reported that Indian women do not increase arginine flux during pregnancy as their American or Jamaican counterparts do. OBJECTIVE: The purpose of this study was to determine whether Indian women of childbearing age are producing less arginine and/or catabolizing more arginine and therefore have less available for anabolic pathways than do Jamaican and American women. METHODS: Thirty healthy women aged 28.3 ± 0.8 y from the United States, India, and Jamaica (n = 10/group) were given 6 h primed, constant intravenous infusions of guanidino-¹5N2-arginine, 5,5-²H2-citrulline, ¹5N2-ornithine, and ring-²H5-phenylalanine, in addition to primed, oral doses of U-¹³C6-arginine in both the fasting and postprandial states. An oral dose of deuterium oxide was also given to determine fat-free mass (FFM). RESULTS: Compared with American women, Indian and Jamaican women had greater ornithine fluxes (µmol · kg fat FFM⁻¹ · h⁻¹) in the fasting and postprandial states (27.3 ± 2.5 vs. 39.6 ± 3.7 and 37.2 ± 2.0, respectively, P = 0.01), indicating greater arginine catabolism. However, Jamaican women had a higher endogenous arginine flux than did Indian and American women in the fasting (66.1 ± 3.1 vs. 54.2 ± 3.1 and 56.1 ± 2.1, respectively, P = 0.01) and postprandial (53.8 ± 2.2 vs. 43.7 ± 4.9 and 42.8 ± 3.1, respectively, P = 0.06) states. As a consequence, Indian women had lower arginine bioavailability (µmol · kg FFM⁻¹ · h⁻¹) in the fasting state (42.0 ± 2.6) than did American (49.9 ± 1.3, P = 0.045) and Jamaican (55.5 ± 3.5, P = 0.004) women, as well as in the postprandial state (40.7 ± 3.5 vs. 51.8 ± 1.2 and 57.5 ± 3.2, respectively, P = 0.001). CONCLUSION: Compared with American and Jamaican women, Indian women of childbearing age have a decreased arginine supply because of increased arginine catabolism without an increase in arginine flux.

Adaptation of in vivo amino acid kinetics facilitates increased amino acid availability for fetal growth in adolescent and adult pregnancies alike.

During pregnancy, adult women with a normal BMI synthesise extra amino acids after an overnight fast by increasing body protein breakdown and decreasing amino acid oxidation. It is not known whether adolescent girls can make these adaptations during pregnancy. The present study aimed to measure and compare the protein, glutamine and alanine kinetics of adult women and adolescent girls at early-, mid- and late-pregnancy. Kinetics were measured in the overnight fasted state using intravenous infusions of 13C-leucine, 15N-glutamine and 15N-alanine in ten adults and twenty adolescents aged 14-17 years in the first and second trimesters (phase 1 study) and infusions of 13C-leucine and 15N2-urea in ten adults and eleven adolescents aged 16-17 years in the first and third trimesters (phase 2 study). In phase 1 study, there were no significant differences between the groups with regard to any of the kinetic parameters measured. In both groups, leucine flux increased (P< 0.05), the percentage of leucine flux oxidised decreased (P< 0.05) and non-oxidative leucine disposal to protein synthesis increased (P< 0.05) from the first to the second trimester. In phase2 study, leucine flux was significantly slower (P< 0.05) in the adult group than in the adolescent group during both trimesters, and whole-body leucine flux and non-oxidative leucine disposal increased significantly in the adolescent group (P< 0.05, respectively) and were higher in the adult group from the first to the third trimester. These results suggest that similar to their adult counterparts after an overnight fast, adolescent girls with a normal BMI provide extra amino acids required for net protein deposition during pregnancy by increasing protein breakdown and decreasing amino acid oxidation.

Arginine flux, but not nitric oxide synthesis, decreases in adolescent girls compared with adult women during pregnancy.

NO has been proposed as a mediator of vascular expansion during pregnancy. Inability to increase NO synthesis and/or production of its precursor, arginine, may contribute to pregnancy-induced hypertension. Adolescents have a higher incidence of gestational hypertension. It is not known whether pregnant adolescents can produce sufficient arginine to meet overall demands. Our objective was to measure and compare the arginine flux and NO synthesis rates of pregnant adolescents and adult women. Arginine, citrulline, and NO kinetics were measured by i.v. infusions of (15)N(2)-argininine and (2)H(2)-citrulline in 8 adolescents and 8 adult women in the fasted state at the end of the first and the beginning of the 3rd trimesters of pregnancy. Arginine flux decreased (P < 0.05) from trimester 1 to 3 in the adolescents but not in the adult women. NO synthesis rate did not change significantly in either group from trimester 1 to 3. In trimester 3, there was a positive association (r = 0.55; P = 0.02) between arginine flux and participants' age, indicating that flux was slower in the younger participants. These findings suggest that after a brief period of food deprivation, the pregnant adolescent cannot maintain arginine production like her adult counterpart in late pregnancy. This inability to maintain arginine production seems to be related to her younger age. It does not, however, affect her ability to synthesize NO in late pregnancy.

Comparing the glucose kinetics of adolescent girls and adult women during pregnancy.

BACKGROUND: Fetal energy demands are met mostly from oxidation of maternally supplied glucose. In pregnant adults this increased glucose requirement is met by an increase in gluconeogenesis. It is not known, however, whether, like their adult counterparts, pregnant adolescent girls can increase gluconeogenesis-hence, glucose production. OBJECTIVE: Our objective was to measure glucose kinetics in 8 pregnant adolescents and 8 adult women. DESIGN: We measured glucose kinetics after an overnight fast by using a primed-constant 6-h U-(13)C-glucose infusion at the end of trimester 1 and early trimester 3. RESULTS: From trimester 1 to trimester 3, whole-body glucose production increased significantly in both groups (P < 0.01). However, whereas the weight-specific rate in adults increased by 18.2%, it increased by only 14.3% in adolescents. In adults, the increase in whole-body glucose production was largely due to a significant increase (P < 0.01) in the rate of gluconeogenesis, but in adolescents there was no change in whole-body gluconeogenesis, and weight-specific gluconeogenesis actually decreased by 11.7%. In both groups, the rate of whole-body glycogenolysis increased significantly (P < 0.05) in trimester 3, and in adolescents, it increased by 95%. CONCLUSIONS: These findings suggest that, in the fasted state in late pregnancy, pregnant adolescents cannot increase weight-specific glucose production by the same magnitude as their adult counterparts. Furthermore, whereas adult women increase glucose production primarily through gluconeogenesis, adolescents do so through glycogenolysis.