Lipid adjustment for chemical exposures: accounting for concomitant variables

Perinatal exposure to polychlorinated biphenyls (PCBs) and child neurodevelopment: A comprehensive systematic review of outcomes and methodological approaches

BACKGROUND

Polychlorinated biphenyls (PCBs), extensively used in various products, prompt ongoing concern despite reduced exposure since the 1970s. This systematic review explores prenatal PCB and hydroxylated metabolites (OH-PCBs) exposure's association with child neurodevelopment. Encompassing cognitive, motor development, behavior, attention, ADHD, and ASD risks, it also evaluates diverse methodological approaches in studies.

METHODS

PubMed, Embase, PsycINFO, and Web of Science databases were searched through August 23, 2023, by predefined search strings. Peer-reviewed studies published in English were included. The inclusion criteria were: (i) PCBs/OH-PCBs measured directly in maternal and cord blood, placenta or breast milk collected in the perinatal period; (ii) outcomes of cognitive development, motor development, attention, behavior, attention-deficit/hyperactivity disorder (ADHD), and autism spectrum disorder (ASD) among children≤18 years old. Quality assessment followed the National Heart, Lung, and Blood Institute's tool.

RESULTS

Overall, 87 studies were included in this review. We found evidence for the association between perinatal PCB exposure and adverse cognitive development and attention issues in middle childhood. There appeared to be no or negligible link between perinatal PCB exposure and early childhood motor development or the risk of ADHD/ASD. There was an indication of a sex-specific association with worse cognition and attention scores among boys. Some individual studies suggested a possible association between prenatal exposure to OH-PCBs and neurodevelopmental outcomes. There was significant heterogeneity between the studies in exposure markers, exposure assessment timing, outcome assessment, and statistical analysis.

CONCLUSIONS

Significant methodological, clinical and statistical heterogeneity existed in the included studies. Adverse effects on cognitive development and attention were observed in middle childhood. Little or no apparent link on both motor development and risk of ADHD/ASD was observed in early childhood. Inconclusive evidence prevailed regarding other neurodevelopmental aspects due to limited studies. Future research could further explore sex-specific associations and evaluate associations at lower exposure levels post-PCB ban in the US. It should also consider OH-PCB metabolites, co-pollutants, mixtures, and their potential interactions.

Association between polychlorinated biphenyls exposure and incident type 2 diabetes mellitus: A nested case-control study

BACKGROUND

Previous epidemiological studies indicated that the association between polychlorinated biphenyls (PCB) and type 2 diabetes mellitus (T2DM) was inconclusive.

OBJECTIVE

We investigated the association between PCBs exposure and incident T2DM in a nested case-control study, and further explored the relationship between PCBs and 5-year fasting blood glucose (FBG) changes.

METHODS

Baseline concentrations of seven indicator-PCB (PCB-28, 52, 101, 118, 138, 153, 180) were measured in 1006 pairs of incident T2DM cases and matched controls nested within the Dongfeng-Tongji cohort. Conditional logistic regression models and pre-adjusted residuals method were used to assess the associations between PCBs and incident T2DM. We further computed beta coefficients (βs) of 5-year FBG changes using multivariable generalized linear regression.

RESULTS

Non-dioxin-like PCBs (NDL-PCBs) were significantly associated with higher T2DM incidence after adjustment for all covariates. Significant differences were observed for extreme quartiles comparisons (Q4 vs. Q1) of PCBs except PCB-138, and the incidence of T2DM were 1- to 3-fold higher among those in the highest versus lowest PCBs quartiles. Serum NDL-PCBs were positively associated with changes in FBG (P for overall association ≤0.01). Additionally, triglycerides mediated the associations between PCBs and T2DM incidence.

CONCLUSION

Our findings showed positive associations of NDL-PCBs with incident T2DM and 5-year FBG changes. PCBs increased incident T2DM via lipid metabolic pathways.

Peripubertal serum concentrations of organochlorine pesticides and semen parameters in Russian young men

BACKGROUND

Epidemiologic literature on the relation of organochlorine pesticides (OCPs) with semen quality among adult men has been inconclusive, and no studies have prospectively explored the association between peripubertal serum OCPs and semen parameters in young men.

OBJECTIVE

To evaluate prospective associations of peripubertal serum concentrations of hexachlorobenzene (HCB), β-hexachlorocylohexane (β-HCH), and p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE) with semen parameters among young Russian men.

METHODS

This prospective cohort study included 152 young men who enrolled in the Russian Children's Study (2003-2005) at age 8-9 years and were followed annually until young adulthood. HCB, β-HCH, and p,p'-DDE concentrations were measured at the CDC by mass spectrometry in serum collected at enrollment. Between 18 and 23 years, semen samples (n = 298) were provided for analysis of volume, concentration, and progressive motility; we also calculated total sperm count and total progressive motile count. Linear mixed models were used to examine the longitudinal associations of quartiles of serum HCB, β-HCH and p,p'-DDE with semen parameters, adjusting for total serum lipids, body mass index, smoking, abstinence time and baseline dietary macronutrient intake.

RESULTS

Lipid-adjusted medians (IQR) for serum HCB, βHCH and p,ṕ-DDE, respectively, were 150 ng/g lipid (102-243), 172 ng/g lipid (120-257) and 275 ng/g lipid (190-465). In adjusted models, we observed lower ejaculated volume with higher serum concentrations of HCB and βHCH, along with reduced progressive motility with higher concentrations of βHCH andp,ṕ-DDE. Men in the highest quartile of serum HCB had a mean (95% Confidence Interval, CI) ejaculated volume of 2.25 mL (1.89, 2.60), as compared to those in the lowest quartile with a mean (95% CI) of 2.97 mL (2.46, 3.49) (p = 0.03). Also, men in the highest quartile of serum p,ṕ-DDE had a mean (95% CI) progressive motility of 51.1% (48.6, 53.7), as compared to those in the lowest quartile with a mean (95% CI) of 55.1% (51.7, 58.5) (p = 0.07).

CONCLUSION

In this longitudinal Russian cohort study, peripubertal serum concentrations of selected OCPs were associated with lower ejaculated volume and progressive motility highlighting the importance of the peripubertal window when evaluating chemical exposures in relation to semen quality.

Association between maternal exposure to the pesticide dichlorodiphenyltrichloroethane (DDT) and risk of obesity in middle age

Background:

Obesity is a malnourishment epidemic worldwide. A meta-analysis of prospective human studies across the world demonstrated a consistent positive association between maternal exposure to the pesticide dichlorodiphenyltrichloroethane (DDT) and its metabolite dichlorodiphenyldichloroethylene (DDE) and children with obesity. The present study evaluates the association of maternal exposure to DDT and DDE with the risk of obesity in daughters during their mid-life in a prospective birth cohort with up to 53 years of follow-up.

Methods:

Gravidas’ blood was collected during their 1959–1967 enrollment into the prospective Child Health and Development Studies birth cohort in California. Their daughters aged 44–53 years had their height, weight, and waist circumference measured during a home visit to evaluate associations of daughters’ adiposity and relative risk of overweight and obesity with their mothers’ prenatal serum levels of DDT and DDE quantified by gas chromatograph-tandem mass spectrometer (n=511).

Results:

Maternal o,p’-DDT was positively associated with body mass index (beta=0.59 kg/m² per ln ng/ml (95^th percentile confidence interval, 95CI: 0.17, 1.00)) and waist circumference (beta=1.19 cm per ln ng/ml (95CI: 0.26, 2.13)) in multivariable models. Maternal o,p’-DDT was positively associated with a 26% (95CI: 6–49%) to 31% (95CI: 6–62%) higher risk of overweight and the same magnitude of additional risk for obesity, based on waist circumference and BMI definitions respectively, in multivariable models.

Conclusions:

These data indicate maternal DDT exposure is significantly associated with increased obesity risk among middle-aged women independent of the obesity definition, confounding, and obesity risk factors. Our findings suggest that policies supporting the use of DDT for malaria vector abatement need to consider the obesity risk as a health cost when weighing the benefits of using DDT in malaria vector control.

Association of urinary concentrations of phthalate metabolites with quinolinic acid among women: A potential link to neurological disorders

BACKGROUND

Quinolinic acid (QA), a neuroactive metabolite produced during tryptophan degradation, is implicated in the pathogenesis of several neurological disorders. Phthalates are structurally similar to QA, and exposure to phthalates has demonstrated increased QA production and excretion in rodent studies. We recently showed that very high exposure to dibutyl phthalate was associated with higher concentrations of urinary QA in men. However, no human studies examined the associations between background (low) phthalate exposures and QA.

OBJECTIVES

To examine the associations of urinary concentrations of phthalate metabolites with QA.

METHODS

Female participants (N = 126) who participated in a prospective cohort study at the Massachusetts General Hospital Fertility Center provided 758 urine samples (273 during pregnancy and 485 during non-pregnancy). Concentrations of 11 phthalate metabolites and QA in urine were measured. We used multivariable linear mixed effect models to estimate the percent change in urinary QA concentrations associated with a doubling (100%) of urinary phthalate metabolite concentration, and evaluated whether there was effect modification by pregnancy status.

RESULTS

Women's mean (standard deviation) age was 34.2 (4.0) years with a body mass index of 23.5 (3.7) kg/m2. The women were primarily Caucasian (92%), had at least a college degree (98%), and none were current smokers. Pairwise Spearman correlations between concentrations for phthalate metabolites and QA measured in the same urine samples ranged from 0.36 for MEHP to 0.68 for dibutyl phthalate (DBP) metabolites. In multivariable-adjusted models, the percent change in urinary QA concentrations was significantly higher for each doubling of several urinary phthalate metabolite concentrations. For example, each doubling of DBP metabolites was associated with a 13.7% (95%CI: 10.6, 16.9)% higher QA. Associations between the low molecular weight phthalate metabolites and QA were stronger among samples collected during pregnancy as compared to non-pregnancy samples from the same women.

CONCLUSIONS

Urinary concentrations of several phthalate metabolites were positively associated with QA among women. These findings, along with the known neurotoxicity of QA, warrant the need to examine whether QA concentrations may serve as a pathway for the adverse neurodevelopment outcomes found in children's health studies.

The challenging use and interpretation of blood biomarkers of exposure related to lipophilic endocrine disrupting chemicals in environmental health studies

The use of exposure biomarkers has been growing during the last decades, being considered the 'gold-standard' approach for individual exposure assessment to environmental chemicals. However, lipophilic endocrine disrupting chemicals (LEDC) have specific physicochemical and biological properties implying particular analytical challenges and interpretative caveats. The epidemiological literature is therefore afflicted by methodological inconsistencies and results divergences, in part due to recognised sources of exposure measurement error and misinterpretation of results. The aim of the present review is to identify external and endogenous sources of variability and uncertainty associated with the LEDC blood biomarkers in epidemiological studies. The dynamic nature of blood and an overview of the known mechanisms of transport, storage and partition of LEDCs in the organism are first described. The external sources of variability and uncertainty introduced at pre-analytical and analytical level are subsequently presented. Subsequently, we present some specific cases where the dynamics of lipids and LEDCs may be substantially modified and thus, the interpretation of biomarkers can be particularly challenging. The environmental obesogens as source of biomarkers variability is also discussed in the light of the most recent findings. Finally, different modelling approaches (statistical and pharmacokinetic models) proposed to improve the use and interpretation of biomarkers are appraised.

Methodological issues in a prospective study on plasma concentrations of persistent organic pollutants and pancreatic cancer risk within the EPIC cohort

BACKGROUND

The use of biomarkers of environmental exposure to explore new risk factors for pancreatic cancer presents clinical, logistic, and methodological challenges that are also relevant in research on other complex diseases.

OBJECTIVES

First, to summarize the main design features of a prospective case-control study -nested within the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort- on plasma concentrations of persistent organic pollutants (POPs) and pancreatic cancer risk. And second, to assess the main methodological challenges posed by associations among characteristics and habits of study participants, fasting status, time from blood draw to cancer diagnosis, disease progression bias, basis of cancer diagnosis, and plasma concentrations of lipids and POPs. Results from etiologic analyses on POPs and pancreatic cancer risk, and other analyses, will be reported in future articles.

METHODS

Study subjects were 1533 participants (513 cases and 1020 controls matched by study centre, sex, age at blood collection, date and time of blood collection, and fasting status) enrolled between 1992 and 2000. Plasma concentrations of 22 POPs were measured by gas chromatography - triple quadrupole mass spectrometry (GC-MS/MS). To estimate the magnitude of the associations we calculated multivariate-adjusted odds ratios by unconditional logistic regression, and adjusted geometric means by General Linear Regression Models.

RESULTS

There were differences among countries in subjects' characteristics (as age, gender, smoking, lipid and POP concentrations), and in study characteristics (as time from blood collection to index date, year of last follow-up, length of follow-up, basis of cancer diagnosis, and fasting status). Adjusting for centre and time of blood collection, no factors were significantly associated with fasting status. Plasma concentrations of lipids were related to age, body mass index, fasting, country, and smoking. We detected and quantified 16 of the 22 POPs in more than 90% of individuals. All 22 POPs were detected in some participants, and the smallest number of POPs detected in one person was 15 (median, 19) with few differences by country. The highest concentrations were found for p,p'-DDE, PCBs 153 and 180 (median concentration: 3371, 1023, and 810 pg/mL, respectively). We assessed the possible occurrence of disease progression bias (DPB) in eight situations defined by lipid and POP measurements, on one hand, and by four factors: interval from blood draw to index date, tumour subsite, tumour stage, and grade of differentiation, on the other. In seven of the eight situations results supported the absence of DPB.

CONCLUSIONS

The coexistence of differences across study centres in some design features and participant characteristics is of relevance to other multicentre studies. Relationships among subjects' characteristics and among such characteristics and design features may play important roles in the forthcoming analyses on the association between plasma concentrations of POPs and pancreatic cancer risk.

The challenging use and interpretation of circulating biomarkers of exposure to persistent organic pollutants in environmental health: Comparison of lipid adjustment approaches in a case study related to endometriosis

The gold-standard matrix for measuring the internal levels of persistent organic pollutants (POPs) is the adipose tissue, however in epidemiological studies the use of serum is preferred due to the low cost and higher accessibility. The interpretation of serum biomarkers is tightly related to the understanding of the underlying causal structure relating the POPs, serum lipids and the disease. Considering the extended benefits of using serum biomarkers we aimed to further examine if through statistical modelling we would be able to improve the use and interpretation of serum biomarkers in the study of endometriosis. Hence, we have conducted a systematic comparison of statistical approaches commonly used to lipid-adjust the circulating biomarkers of POPs based on existing methods, using data from a pilot case-control study focused on severe deep infiltrating endometriosis. The odds ratios (ORs) obtained from unconditional regression for those models with serum biomarkers were further compared to those obtained from adipose tissue. The results of this exploratory study did not support the use of blood biomarkers as proxy estimates of POPs in adipose tissue to implement in risk models for endometriosis with the available statistical approaches to correct for lipids. The current statistical approaches commonly used to lipid-adjust circulating POPs, do not fully represent the underlying biological complexity between POPs, lipids and disease (especially those directly or indirectly affecting or affected by lipid metabolism). Hence, further investigations are warranted to improve the use and interpretation of blood biomarkers under complex scenarios of lipid dynamics.

Lipid and Creatinine Adjustment to Evaluate Health Effects of Environmental Exposures

PURPOSE OF REVIEW

Urine- and serum-based biomarkers are useful for assessing individuals' exposure to environmental factors. However, variations in urinary creatinine (a measure of dilution) or serum lipid levels, if not adequately corrected for, can directly impact biomarker concentrations and bias exposure-disease association measures.

RECENT FINDINGS

Recent methodological literature has considered the complex relationships between creatinine or serum lipid levels, exposure biomarkers, outcomes, and other potentially relevant factors using directed acyclic graphs and simulation studies. The optimal measures of urinary dilution and serum lipids have also been investigated. Existing evidence supports the use of covariate-adjusted standardization plus creatinine adjustment for urinary biomarkers and standardization plus serum lipid adjustment for lipophilic, serum-based biomarkers. It is unclear which urinary dilution measure is best, but all serum lipid measures performed similarly. Future research should assess methods for pooled biomarkers and for studying diseases and exposures that affect creatinine or serum lipids directly.

Association between Exposure to p,p'-DDT and Its Metabolite p,p'-DDE with Obesity: Integrated Systematic Review and Meta-Analysis

BACKGROUND

The prevalence of obesity is increasing in all countries, becoming a substantial public health concern worldwide. Increasing evidence has associated obesity with persistent pollutants such as the pesticide DDT and its metabolite p,p'-DDE.

OBJECTIVES

Our objective was to systematically review the literature on the association between exposure to the pesticide DDT and its metabolites and obesity to develop hazard identification conclusions.

METHODS

We applied a systematic review-based strategy to identify and integrate evidence from epidemiological, in vivo, and in vitro studies. The evidence from prospective epidemiological studies was quantitatively synthesized by meta-analysis. We rated the body of evidence and integrated the streams of evidence to systematically develop hazard identification conclusions.

RESULTS

We identified seven epidemiological studies reporting prospective associations between exposure to p,p'-DDE and adiposity assessed by body mass index (BMI) z-score. The results from the meta-analysis revealed positive associations between exposure to p,p'-DDE and BMI z-score (β=0.13 BMI z-score (95% CI: 0.01, 0.25) per log increase of p,p'-DDE). Two studies constituted the primary in vivo evidence. Both studies reported positive associations between exposure to p,p'-DDT and increased adiposity in rodents. We identified 19 in vivo studies and 7 in vitro studies that supported the biological plausibility of the obesogenic effects of p,p'-DDT and p,p'-DDE.

CONCLUSIONS

We classified p,p'-DDT and p,p'-DDE as "presumed" to be obesogenic for humans, based on a moderate level of primary human evidence, a moderate level of primary in vivo evidence, and a moderate level of supporting evidence from in vivo and in vitro studies. https://doi.org/10.1289/EHP527.

[Impact on human health of endocrine disruptors present in environmental water bodies: is there an association with obesity?]

There is growing evidence that endocrine disruptors (ED) may adversely affect humans. Surface and underground water are the main sources for obtaining potable water, however they can be contaminated with ED, which are not completely removed by conventional water and sewage treatment processes. Some health problems are related to the exposure of humans to ED, obesity being one of them. There is currently an increase in the prevalence of obesity worldwide, a fact that is considered a concern in view of its potential impact on the health care system, since obesity is the major risk factor of the leading chronic diseases including diabetes and cardiovascular disease. By means of a review of the literature, this paper sought to gather scientific publications linking exposure to ED with obesity, in order to verify the importance of removal of ED from water bodies, thereby preserving the population's health and aquatic biota. Most of the selected studies suggest an association between ED and obesity in humans.

Environmental Chemicals in Urine and Blood: Improving Methods for Creatinine and Lipid Adjustment

BACKGROUND

Investigators measuring exposure biomarkers in urine typically adjust for creatinine to account for dilution-dependent sample variation in urine concentrations. Similarly, it is standard to adjust for serum lipids when measuring lipophilic chemicals in serum. However, there is controversy regarding the best approach, and existing methods may not effectively correct for measurement error.

OBJECTIVES

We compared adjustment methods, including novel approaches, using simulated case-control data.

METHODS

Using a directed acyclic graph framework, we defined six causal scenarios for epidemiologic studies of environmental chemicals measured in urine or serum. The scenarios include variables known to influence creatinine (e.g., age and hydration) or serum lipid levels (e.g., body mass index and recent fat intake). Over a range of true effect sizes, we analyzed each scenario using seven adjustment approaches and estimated the corresponding bias and confidence interval coverage across 1,000 simulated studies.

RESULTS

For urinary biomarker measurements, our novel method, which incorporates both covariate-adjusted standardization and the inclusion of creatinine as a covariate in the regression model, had low bias and possessed 95% confidence interval coverage of nearly 95% for most simulated scenarios. For serum biomarker measurements, a similar approach involving standardization plus serum lipid level adjustment generally performed well.

CONCLUSIONS

To control measurement error bias caused by variations in serum lipids or by urinary diluteness, we recommend improved methods for standardizing exposure concentrations across individuals.

Prepubertal Serum Concentrations of Organochlorine Pesticides and Age at Sexual Maturity in Russian Boys

BACKGROUND

Few human studies have evaluated the impact of childhood exposure to organochlorine pesticides (OCP) on pubertal development.

OBJECTIVE

We evaluated associations of serum OCP concentrations [hexachlorobenzene (HCB), β-hexachlorocyclohexane (βHCH), and p,p-dichlorodiphenyldichloroethylene (p,p´-DDE)] with age at attainment of sexual maturity among boys.

METHODS

From 2003 through 2005, 350 8- to 9-year-old boys from Chapaevsk, Russia, with measured OCPs were enrolled and followed annually for 8 years. We used multivariable interval-censored models to evaluate associations of OCPs (quartiles) with three physician-assessed measures of sexual maturity: Tanner stage 5 for genitalia growth, Tanner stage 5 for pubic hair growth, or testicular volume (TV) ≥ 20 mL in either testis.

RESULTS

In adjusted models, boys with higher HCB concentrations achieved sexual maturity reflected by TV ≥ 20 mL a mean of 3.1 months (95% CI: -1.7, 7.8), 5.3 months (95% CI: 0.6, 10.1), and 5.0 months (95% CI: 0.2, 9.8) later for quartiles Q2, Q3, and Q4, respectively, compared with Q1 (p trend = 0.04). Tanner stage 5 for genitalia growth was attained a mean of 2.2 months (95% CI: -3.1, 7.5), 5.7 months (95% CI: 0.4, 11.0), and 3.7 months (95% CI: -1.7, 9.1) later for quartiles Q2, Q3, and Q4, respectively, of βHCH compared with Q1 (p trend = 0.09). Tanner stage 5 for pubic hair growth occurred 6-9 months later on average for boys in the highest versus lowest quartile for HCB (p trend < 0.001), βHCH (trend p = 0.01), and p,p´-DDE (p trend = 0.04). No associations were observed between p,p´-DDE and Tanner stage 5 for genitalia growth or TV ≥ 20 mL.

CONCLUSIONS AND RELEVANCE

Higher prepubertal serum HCB and βHCH concentrations were associated with a later age at attainment of sexual maturity. Only the highest quartile of serum p,p´-DDE was associated with later pubic hair maturation.

CITATION

Lam T, Williams PL, Lee MM, Korrick SA, Birnbaum LS, Burns JS, Sergeyev O, Revich B, Altshul LM, Patterson DG Jr, Hauser R. 2015. Prepubertal serum concentrations of organochlorine pesticides and age at sexual maturity in Russian boys. Environ Health Perspect 123:1216-1221; http://dx.doi.org/10.1289/ehp.1409022.

Prepubertal organochlorine pesticide concentrations and age of pubertal onset among Russian boys

BACKGROUND

In animal studies, organochlorine pesticide (OCP) exposure alters pubertal development; however, epidemiological data are limited and inconsistent.

OBJECTIVE

To evaluate the associations of serum OCP concentrations [hexachlorobenzene (HCB), β-hexachlorocyclohexane (β-HCH), and p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE)] with male pubertal onset.

METHODS

In Chapaevsk, Russia, a town environmentally contaminated with OCPs, 350 8-9 year old boys with measured OCPs were enrolled during 2003-2005 and were followed annually for eight years. We evaluated three measures of pubertal onset: testicular volume (TV)>3 mL in either testis, or stage 2 or greater for genitalia (G2+), or pubic hair (P2+). We used multivariable interval-censored models to evaluate associations of OCPs (quartiles) with physician-assessed pubertal onset.

RESULTS

In adjusted models, boys with higher HCB concentrations had later mean ages of TV>3 mL and P2+ (but not G2+). Mean age at attaining TV>3 mL was delayed 3.6 (95% CI: -2.6, 9.7), 7.9 (95% CI: 1.7, 14.0), and 4.7 months (95% CI: -1.4, 10.9) for HCB Q2, Q3, and Q4, respectively, compared to Q1 (trend p: 0.06). Boys with higher HCB concentrations reached P2+ 0.1 months earlier (95% CI: -5.8, 5.6) for Q2, 4.7 months later (95% CI: -1.0, 10.3) for Q3 and 4.6 months later (95% CI: -1.1, 10.3) for Q4 compared to Q1 (trend p: 0.04). There were no associations of serum β-HCH and p,p'-DDE concentrations with age of pubertal onset.

CONCLUSION

Higher prepubertal serum HCB concentrations were associated with later age of gonadarche and pubarche.

Challenges and future directions to evaluating the association between prenatal exposure to endocrine disrupting chemicals and childhood obesity

Obesity is an increasing public health threat worldwide. However, there has been insufficient research addressing the obesogenic potential of prenatal exposure to environmental endocrine disrupting chemicals, largely due to complexities in the design, analysis, and interpretation of such studies. This review describes relevant biological mechanisms, addresses current challenges for investigators, presents potential strategies for overcoming them, and identifies areas where further development is required to improve future research. Special considerations for exposure assessment, outcome heterogeneity, and complex confounding structures are described.

Serum cotinine and whole blood folate concentrations in pregnancy

PURPOSE

Prenatal tobacco smoke exposure may be associated with low maternal folate levels that increase the risk of adverse infant and child health outcomes by reducing folate availability during fetal development.

METHODS

Using data from the Health Outcomes and Measures of the Environment Study, we examined the relationship between secondhand or active tobacco smoke exposure and whole blood folate concentrations in pregnant women from Cincinnati, Ohio (n = 362) at approximately 16-week gestation. We used multivariable linear regression to examine the association between continuous or categorical serum cotinine levels and whole blood folate levels, adjusting for sociodemographic, dietary, and perinatal variables.

RESULTS

After adjustment for potential confounders, an interquartile range increases in serum cotinine concentration (0.012-0.224 ng/mL) was suggestively associated with decreased whole blood folate levels (β, -23 nmol/L; 95% confidence interval (CI), -49, 3; P value = .08). Compared with unexposed women, reductions in mean whole blood folate were observed among active smokers (β, -94, 95% CI, 195, 6 nmol/L; P value = .40); smaller reductions were observed among women with secondhand exposure (β, 26; CI, 84, 32 nmol/L; P value = .07).

CONCLUSIONS

Consistent with prior studies, active smoking was associated with reduced whole blood folate levels among these pregnant women. Secondhand tobacco smoke exposures were associated with small and imprecise reductions in whole blood folate levels.