Investigating the causal relationships between excess adiposity and cardiometabolic health in men and women

Causal validation of the relationship between air pollution and lung cancer: A bidirectional Mendelian randomization study and meta-analysis

Recent studies suggest a link between air pollution and lung cancer, but causality remains uncertain due to confounding and reverse causation. Mendelian randomization (MR) reduces such bias and offers a new way to explore this relationship. MR is a method that uses genetic variants as instrumental variables to assess the causal relationship between an exposure and an outcome, effectively controlling for confounding and reverse causation. The inverse-variance weighted method is a commonly used approach in MR analysis, which estimates the overall causal effect by weighting the effect ratios of multiple single nucleotide polymorphisms, assuming all instruments are valid. Based on 2-sample MR, this study incorporated 5 air pollution indices and conducted MR analyses with lung cancer outcome data from 2 different sources. Subsequently, a meta-analysis was performed on the primary inverse-variance weighted results, followed by multiple corrections of the thresholds after the meta-analysis to ensure accuracy. Finally, reverse causality was tested through MR analysis for air pollution indices significantly associated with lung cancer. And the selection criteria for instrumental variables were: P < 5 × 10⁻⁶, F > 10, minor allele frequency > 0.01, clump_kb = 10,000, and clump_r2 = 0.001. Five air pollution indices were analyzed using MR analysis and meta-analysis with lung cancer data from the FinnGen R12 and OpenGWAS databases. Multiple corrections were applied to the significance threshold results after the meta-analysis. The final results showed that only nitrogen dioxide (NO₂) exhibited a significant association, with an OR of 3.426 (95% CI: 1.897-6.186, P = 2.21 × 10⁻⁴). Additionally, the positive air pollution index NO₂ showed no evidence of reverse causality with lung cancer from either data source. This study demonstrates a significant causal association between NO₂ and lung cancer, indicating that NO₂ may be a potential risk factor for lung cancer.

Causal relationship between body mass index and insulin resistance: Linear and nonlinear Mendelian randomization study in a Japanese population

AIMS/INTRODUCTION

Obesity is a known risk factor for several chronic diseases, including type 2 diabetes mellitus, which results from increased insulin resistance and impaired insulin secretion. However, the association between obesity and insulin resistance in Asian populations has not yet been fully elucidated. Therefore, we aimed to investigate the causal relationship between body mass index (BMI) and glycemic traits using Mendelian randomization (MR).

MATERIALS AND METHODS

We performed individual-level MR analyses using genetic risk scores based on BMI-related variants in 3,745 individuals without diabetes mellitus from a Japanese cohort. We examined heterogeneity through subgroup analyses based on potential modifiers and determined the shape of the causal relationship using nonlinear MR analyses to further assess the impact of BMI on the homeostasis model assessment of insulin resistance (HOMA-IR).

RESULTS

MR analyses revealed a significant positive association between BMI and HOMA-IR (β = 0.077; 95% confidence interval, 0.014-0.141; P = 0.016; outcome variable was log-transformed and standardized). Additional analyses revealed heterogeneity among subgroups differentiated by age, sex, lifestyle habits, and cardiometabolic traits. Nonlinear MR analyses suggested a potential J-shaped causal relationship between BMI and HOMA-IR.

CONCLUSIONS

Our findings demonstrated that obesity and low BMI may contribute to increased insulin resistance. Furthermore, the impact of BMI on insulin resistance could vary owing to effect modification. Managing BMI is crucial in individuals at high risk of increased insulin resistance and may have important implications for preventing type 2 diabetes, especially given the low insulin secretory capacity observed in East Asian populations.

Mendelian randomization studies of lifestyle-related risk factors for stroke: a systematic review and meta-analysis

Objective

Stroke risk factors often exert long-term effects, and Mendelian randomization (MR) offers significant advantages over traditional observational studies in evaluating the causal impact of these factors on stroke. This study aims to consolidate and evaluate the relationships between potential causal factors and stroke risk, drawing upon existing MR research.

Methods

A comprehensive search for MR studies related to stroke was conducted up to August 2023 using databases such as PubMed, Web of Science, Embase, and Scopus. This meta-analysis examines the relationships between potential causative factors and stroke risk. Both random-effects and fixed-effects models were utilized to compile the dominance ratios of various causative elements linked to stroke. The reliability of the included studies was assessed according to the Strengthening the Reporting of Observational Studies in Epidemiology incorporating Mendelian Randomization (STROBE-MR) guidelines.

Results

The analysis identified several risk factors for stroke, including obesity, hypertension, low-density lipoprotein cholesterol (LDL-C), chronic kidney disease (CKD), and smoking. Protective factors included high-density lipoprotein cholesterol (HDL-C), estimated glomerular filtration rate (eGFR), and educational attainment. Subgroup analysis revealed that type 2 diabetes mellitus (T2DM), diastolic blood pressure (DBP) are risk factors for ischemic stroke (IS).

Conclusion

This study confirms that variables such as obesity, hypertension, elevated LDL-C levels, CKD, and smoking are significantly linked to the development of stroke. Our findings provide new insights into genetic susceptibility and potential biological pathways involved in stroke development.

Systematic review registration

https://www.crd.york.ac.uk/PROSPERO, identifier CRD42024503049.

Age and sex differences in the impact of common comorbidities on stroke and myocardial infarction: results from the China Patient-Centered Evaluative Assessment of Cardiac Events Million Persons Project

OBJECTIVES

This study aims to evaluate the age- and sex-specific associations of comorbidities with stroke and MI and further calculate the population-attributable fractions (PAFs) of five comorbid diseases for stroke and myocardial infarction (MI) by age and sex.

STUDY DESIGN

This is a prospective cohort study.

METHODS

This study leveraged data from a sub-cohort of the China Patient-Centered Evaluative Assessment of Cardiac Events (PEACE) Million Persons Project. Participants aged 35-75 years without a prevalent stroke and MI were enrolled from January 2016 to December 2020, with follow-up through December 2021. Five common comorbidities were collected at baseline, and the study outcome was hospitalization for stroke and MI identified from the Inpatients Registry.

RESULTS

Of 100,873 participants, the mean age was 54.2 (±10.2) years, 34.2% were ≥60 years old, and 60.8% were women. After a median follow-up of 3.52 years, 4156 participants had stroke/MI. The strengths of the associations between hypertension, diabetes, and obesity with stroke/MI were higher in younger individuals than in seniors, and obesity had a more hazardous impact on stroke/MI in men than in women. The five comorbidities collectively explained a higher population attributable fraction (PAF) for stroke/MI in the young group (51.5[46.9, 55.7] %) than in the senior group (41.3[37.0, 45.4] %), in men (45.6[40.9, 49.9] %) than in women (41.1[36.1, 45.7] %).

CONCLUSIONS

Most of the common comorbidities were significantly associated with stroke and MI. Several age and sex differences in the impacts of comorbidities on stroke/MI were observed, highlighting the importance of age- and sex-specific preventive strategies to reduce premature stroke and MI.

Mendelian randomization studies on coronary artery disease: a systematic review and meta-analysis

BACKGROUND

Coronary artery disease (CAD) remains one of the leading causes of mortality worldwide. We aimed to summarize what is currently known with regard to causal modifiable risk factors associated with CAD in populations of diverse ancestries through conducting a systematic review and meta-analysis of Mendelian randomization (MR) studies on CAD.

METHODS

The databases Embase, Medline, Cochrane Library and Web of Science were searched on the 19th and 20th of December 2022 for MR studies with CAD as a primary outcome; keywords of the search strategy included "coronary artery disease" and "mendelian randomization". Studies were included if they were published in the English language, included only human participants, employed Mendelian randomization as the primary methodology and studied CAD as the outcome of interest. The exclusion criteria resulted in the removal of studies that did not align with the predefined inclusion criteria, as well as studies which were systematic reviews themselves, and used the same exposure and outcome source as another study. An ancestry-specific meta-analysis was subsequently conducted on studies which investigated either body mass index, lipid traits, blood pressure or type 2 diabetes as an exposure variable. Assessment of publication bias and sensitivity analyses was conducted for risk of bias assessment in the included studies.

RESULTS

A total of 1781 studies were identified through the database searches after de-duplication was performed, with 47 studies included in the quantitative synthesis after eligibility screening. Approximately 80% of all included study participants for MR studies on CAD were of European descent irrespective of the exposure of interest, while no study included individuals of African ancestry. We found no evidence of differences in terms of direction of causation between ancestry groups; however, the strength of the respective relationships between each exposure and CAD were different, with this finding most evident when blood pressure was the exposure of interest.

CONCLUSIONS

Findings from this review suggest that patterns regarding the causational relationship between modifiable risk factors and CAD do not differ in terms of direction when compared across diverse ancestry populations. Differences in the observed strengths of the respective relationships however are indicative of the value of increasing representation in non-European populations, as novel genetic pathways or functional SNPs relating to CAD may be uncovered through a more global analysis.

SYSTEMATIC REVIEW REGISTRATION

The protocol for this systematic review was registered to the International Prospective Register of Systematic Reviews (PROSPERO) and is publicly available online (CRD42021272726).

The impact of age-specific childhood body-mass index on adult cardiometabolic traits: a Mendelian randomization study

Objective

To evaluate the causal relationship between childhood body-mass index (BMI) at different ages and adult cardiometabolic traits.

Methods

We retrieved genetic instrument variables (IVs) for exposures (standardized BMI at newborn, infant, toddler and late childhood), cardiometabolic traits and potential confounders or mediators (adult BMI, SHBG, testosterone and age at menarche) from the corresponding genome-wide association analysis. We performed univariate and multivariable Mendelian randomization (MR) to dissect associations between age-specific childhood BMI and adult cardiometabolic outcomes. Odds ratio was used to present the direction of the causal association.

Results

In univariate MR, higher newborn BMI was causally associated with reduced risk for type 2 diabetes in women. Late childhood BMI was associated with increased risk for female diabetes and coronary artery disease (CAD), myocardial infarction (MI), and chronic kidney disease (CKD) in general population. Among these associations, only association between late childhood BMI with MI remained significant after adjusting for adult male BMI and sex hormones, (OR = 1.120, 95% CI 1.023-1.226, p = 0.014). Besides, in multivariable MR, we found evidence for causal association between newborn BMI with reduced risk for CAD (OR = 0.862, 95% CI 0.751-0.989, p = 0.034) and MI (OR = 0.864, 95% CI 0.752-0.991, p = 0.037) in men. No obvious impact of infant or toddler BMI was identified on the above-mentioned diseases. For continuous cardiometabolic traits, in all age epochs except infant, higher BMI was associated with increased level of fasting glucose in women.

Conclusion

BMI at birth and late childhood exerts different impact on adult cardiometabolic diseases, while BMI at infant and toddler ages is not causally associated with these outcomes. The effect of childhood BMI may be influenced by sex disparities.

Sex differences in type 2 diabetes

The prevalence of type 2 diabetes mellitus is increasing in both sexes, but men are usually diagnosed at a younger age and lower body fat mass than women. Worldwide, an estimated 17.7 million more men than women have diabetes mellitus. Women appear to bear a greater risk factor burden at the time of their type 2 diabetes diagnosis, especially obesity. Moreover, psychosocial stress might play a more prominent role in diabetes risk in women. Across their lifespan, women experience greater hormone fluctuations and body changes due to reproductive factors than men. Pregnancies can unmask pre-existing metabolic abnormalities, resulting in the diagnosis of gestational diabetes, which appears to be the most prominent risk factor for progression to type 2 diabetes in women. Additionally, menopause increases women's cardiometabolic risk profile. Due to the progressive rise in obesity, there is a global increase in women with pregestational type 2 diabetes, often with inadequate preconceptual care. There are differences between men and women regarding type 2 diabetes and other cardiovascular risk factors with respect to comorbidities, the manifestation of complications and the initiation of and adherence to therapy. Women with type 2 diabetes show greater relative risk of CVD and mortality than men. Moreover, young women with type 2 diabetes are currently less likely than men to receive the treatment and CVD risk reduction recommended by guidelines. Current medical recommendations do not provide information on sex-specific or gender-sensitive prevention strategies and management. Thus, more research on sex differences, including the underlying mechanisms, is necessary to increase the evidence in the future. Nonetheless, intensified efforts to screen for glucose metabolism disorders and other cardiovascular risk factors, as well as the early establishment of prophylactic measures and aggressive risk management strategies, are still required for both men and women at increased risk of type 2 diabetes. In this narrative review we aim to summarise sex-specific clinical features and differences between women and men with type 2 diabetes into risk factors, screening, diagnosis, complications and treatment.