| Abstract |
Summary
Background: A substantial body of epidemiological evidence suggests that an adverse intrauterine environment, elicited by maternal dietary or placental insufficiency, may “program” susceptibility in the fetus for a low birth weight and a later development of cardiovascular or metabolic diseases such as obesity, insulin resistance and type 2 diabetes. On the other hand, increased birth weight has also been implicated in the development of metabolic disturbances. Maternal metabolic status with its various components, along with the thyroid status during pregnancy could influence the intrauterine environment substantially and represent a potential causative mechanism, however this hypothesis has not been adequately explored. In this regard, the quest for the study of in utero environment has generated an effort to identify biomarkers that could predict the risk of early development of obesity. Adipokines and particularly leptin are among the most promising markers examined. Recent work has highlighted the importance of this hormone during critical windows of development in the pathogenesis of programming related disorders with evidence that both central and peripheral mechanisms are involved.
Aim: The overall aim of the project was to examine the effect of maternal metabolic and thyroid status during pregnancy on the regulation of pre- and postnatal growth (infancy and early childhood), via its possible effects of cord blood leptin (“intrauterine programming”)
Specific aims included:
1. creation of reference ranges for the levels of TSH, free T3 and T4 in pregnant women in Crete,
2. assessment of the relationship between thyroid disorders/serologic thyroid autoimmunity and pregnancy complications/birth outcomes,
3. evaluation of the relationship between early pregnancy metabolic syndrome and risk for gestational diabetes and preterm term,
4. creation of reference ranges for the normal values of cord blood leptin,
5. evaluation of the effects of maternal weight before and during pregnancy on the levels of cord blood leptin,
6. study of the relationship between cord blood leptin and neonatal anthropometric measurements in the literature and performance of a meta analysis, and
7. assessment of the relationship between cord leptin and fetal/infant and childhood growth
Methods:
The present analysis uses data from the “Rhea” birth cohort. The “Rhea” project is a mother-child study which examines prospectively a population-based cohort of pregnant women and their children at the prefecture of Heraklion, Crete, Greece. Pregnant women (Greek and immigrant) who becamepregnant within a 12-month period, starting in February 2007, were contacted and asked to participate in the study. The first contact was made before 15 weeks’ gestation, at the time of the first major ultrasound examination, and participants were invited to provide blood and urine samples and to participate in a face-to-face interview. Women were contacted again at various times during pregnancy, at birth, at 8-10 weeks after delivery and for child follow-up at the 6th and 18th month, and at 4 years of age. Face-to face completed questionnaires together with self-administered questionnaires and medical records were used to obtain information on dietary, environmental and psychosocial exposures during pregnancy and early childhood. The study was approved by the ethical committee of the University hospital of Heraklion, Crete, Greece and all mothers provided written informed consent after complete description of the study.
Analyses of all biological samples (maternal, umbilical cord blood and child serum) were performed in the Laboratory of Experimental Endocrinology, Biochemistry and Immunology of the University Hospital of Heraklion, Crete (Greece).
The conduction of the meta analysis as well as the determination of reference ranges for the levels of maternal thyroid hormones and cord blood leptin for was performed using standard guidelines.
Potential confounders included characteristics that have an established or potential association with the exposure (metabolic status, thyroid status and cord blood leptin) and/or the outcomes of interest (pregnancy complications, birth and growth outcomes).
Univariate associations between dependent and independent variables were studied using Pearson’s Chi-square test for categorical variables and ANOVA tests for continuous ones. In cases of non-normally distributed variables, non-parametric Mann-Whitney or Kruskal-Walis tests were used instead.
Associations of exposure with the outcomes of interest were estimated with multivariable log-binomial or log-Poisson regression models, for binary outcomes or, multivariable linear regression models for continuous outcomes after adjusting for confounders. We reported b coefficients and 95% CIs for all continuous outcomes, and relative risks (RRs) with 95% CIs for categorical outcomes. Effect modification was assessed through inclusion of the interaction term in the models and stratified analyses accordingly. All association testing was conducted assuming a 0.05 significance level and a two-sided alternative hypothesis.
Statistical analysis was performed using the statistical package STATA, version 13 (StataCorp, College Station, TX, USA) and SPSS, version 18 (SPSS INC, Chicago, IL).
Results
Laboratory- and geography- specific reference intervals were calculated for thyroid hormones during early pregnancy in Crete (first trimester: TSH: 0.05-2.53 μIU/mL, free T3:2.36-8.01 pmol/L and free T4: 12.22-19.69 pmol/L and second trimester: TSH: 0.18-2.73 μIU/mL, free T3:2.73-8.12 pmol/L and free T4: 11.19-18.66 pmol/L.The combination of high TSH and thyroid autoimmunity was associated with a 4 fold increased risk for gestational diabetes (RR: 4.3, 95% CI = 2.1, 8.9). High TSH values without the presence of thyroid autoimmunity were associated with 10% increased risk of overall caesarean sections (RR: 1.1, 95% CI = 1.0, 1.2). Thyroid antibodies per se increased the risk of spontaneous preterm birth by 70% (RR: 1.7, 95%CI = 1.1, 2.8). The combination of high TSH and thyroid autoimmunity was associated with a 3-fold increased risk for low birth weight neonates (RR: 3.1, 95% CI = 1.2 – 8.0), while high TSH values without thyroid autoimmunity were also associated with increased risk for low birth weight neonates (RR: 2.6, 95% CI = 1.1, 5.9).
Women with metabolic syndrome were in high risk for gestational diabetes (GDM, RR=3.17, 95 %CI: 1.06, 9.50).and preterm birth (relative risk (RR) = 2.93, 95% CI: 1.53, 5.58), whereas among the components of metabolic syndrome, the most significant risk factor for prematurity was hypertension (RR = 2.32, 95% CI: 1.28, 4.20). The risk for medically indicated preterm deliveries was increased in women with metabolic syndrome (RR = 5.13, 95% CI: 1.97, 13.38). Fetal weight growth restriction was associated with elevated levels of insulin in early pregnancy (RR = 1.14, 95% CI: 1.08, 1.20) and elevated levels of diastolic blood pressure (RR = 1.27, 95% CI: 1.00, 1.61).
Regarding the meta analysis, a PubMed search was performed between 1994 and 2009 and manual search of reference lists of retrieved articles. Forty-four studies met the inclusion criteria. All studies reported a positive correlation between leptin levels and birth weight. The combined correlation coefficient (r) was 0.46 [95%CI 0.43, 0.5]. Statistically significant positive correlations were also found for birth length (r=0.29, 0.23-0.34) and ponderal index (r=0.36, 0.31-0.41). There was no indication of publication bias.
The derived reference intervals of cord blood leptin levels for male neonates were 1.4 – 18.2 ng/mL and for females 2.0 – 25.8 ng/mL.
Each 1 kg/m2 increase in pre-pregnancy BMI was associated with a 10% increased risk for delivering a neonate with high cord blood leptin levels, while maternal pre-pregnancy overweight/obesity increased two fold the risk of giving birth to a neonate with hyperleptinemia (adjusted RR: 2.1 [95% CI: 1.4, 3.2] after adjustment for confounders. Excessive weight gain during pregnancy, i.e. more than recommended, was associated with a 3-fold increased risk for cord blood hyperleptinemia (RR: 3.0, [95% CI: 1.5, 6.3]).
High levels of log leptin were positively associated with birth weight (β-coef: 176.5 [95% CI: 133.0, 220.0]), head circumference (β-coef: 2.5 [95% CI: 0.7, 4.2]) and ponderal index (β-coef: 1.0 [95% CI: 0.6, 1.4]) after adjustment for confounders. In overweight-obese mothers, increased log leptin was associated with increased birth weight β-coef: 219.1g [95% CI: 152.3, 285.9]), while the effect was much lower for mothers with normal weight (β-coef: 150.5 g [95% CI: 93.1, 207.9]).
In a multivariable analysis, higher cord blood leptin was strongly associated with slow weight gain during the first 3 and 6 months of age (RR: 1.23 [95%CI: 1.05, 1.45] and RR: 1.52 [95%CI: 1.09,2.13] for slow weight gain, respectively). Conversely, low leptin levels at birth conferred an 80% higher risk of a rapid weight gain during the first 3 months of life (RR: 1.83 [95%CI: 1.17, 2.85]).
Each SD increase in cord leptin was associated with decreased weight and height in all ages 2 to 4 years. Neonates born with low leptin levels had more than 1 cm increased height in early childhood, while those with higher cord leptin developed lower weight and BMI during this period. Cord blood leptin was not associated with any of the cardiometabolic risk factors measured in early childhood [skinfold thicknesses, blood pressure, lipid levels, CRP and adipose tissue hormones].
Conclusions
The aim of this thesis was the study of the intrauterine programming of neonatal and infant growth and the way this is affected by the maternal metabolic status. Maternal status was evaluated both with pre-pregnancy and gestational metabolic syndrome and obesity, as well as with thyroidal status in the first trimester of pregnancy. Both of these constituents of maternal metabolism proved able to affect the intrauterine milieu, lead to adaptive changes in organs and tissues and ultimately modify the pattern of neonatal growth. Both neonatal and infant growth predispose to the future development of metabolic syndrome and obesity in childhood and adult life. For the study of the in utero environment we used cord blood leptin which proved to be a valid biomarker capable to predict neonatal, infant and childhood growth.
According to our results, metabolic syndrome during the first trimester of pregnancy (3.6% of Rhea population) was associated with preterm birth, while some of its components (insulin and diastolic pressure) correlated with intrauterine growth restriction. Additionally, increased maternal body weight, either pre-pregnancy (33.3% of mothers had an increased BMI) or during gestation (23.4% of mothers gained more than the recommended weight), had an impact on the intrauterine environment, as the latter was reflected on the levels of cord blood leptin. The correlation between maternal undernutrition and restricted fetal growth, low birth weight and ultimately increased risk for metabolic syndrome in adulthood were already known from the late ‘80s. With current data, one can hypothesize a transgenerational obesity cycle, whereby women with high pre-pregnancy BMI give birth to overweight children with a high risk to become obese later in life.
Maternal thyroidal status during the first trimester of pregnancy was associated with an increased risk for the delivery of a low birth weight neonate, as well as with prematurity. The present study is the first study performed in pregnant women in the geographical area of Crete. Although the prevalence of thyroid autoimmunity was comparable to other countries, we observed a two-fold increased frequency of high TSH levels. Moreover, women with high TSH had a twofold higher risk for low birth weight, while the combined presence of high TSH and thyroid autoantibodies tripled this risk. The fundamental role of thyroid hormones in fetal growth and tissue maturation, through the anabolic roles of growth hormone and IGF-1, is well established.In our study, we assessed the value of cord blood leptin as a biomarker of neonatal and infant growth and its capability to predict childhood obesity. Specifically, leptin showed a potential to modulate intrauterine programming of childhood obesity, through its direct influence of neonatal growth (as the latter is reflected in the anthropometric measurements at birth), but also through its effect on the pattern of growth during early infancy and on early childhood BMI.
In conclusion, both the pre- and the postnatal environment constitute critical periods for the determination of long-term metabolic risk. Leptin, with its pleiotropic actions, participates in the mechanisms of appetite, growth and metabolic rate early in life. Intrauterine exposure to an obesity-prone environment, along with a genetic predisposition, is capable of “programming: the metabolic profile of these children. Additional data on the short- and long-term consequences of the intrauterine exposure to leptin are necessary, in order to delineate the early determinants of obesity and develop preventive strategies to effectively prevent obesity and its metabolic consequences in childhood
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Αδιποκίνες ομφαλίου λώρου και η σχέση τους με την ανάπτυξη κατά την ενδομήτρια ζωή και την πρώιμη βρεφική ηλικία
