Genetically low vitamin D concentrations and myopic refractive error: a Mendelian randomization study

Application of mendelian randomization in ocular diseases: a review

Ocular disorders can significantly lower patients' quality of life and impose an economic burden on families and society. However, for the majority of these diseases, their prevalence and mechanisms are yet unknown, making prevention, management, and therapy challenging. Although connections between exposure factors and diseases can be drawn through observational research, it is challenging to rule out the interference of confounding variables and reverse causation. Mendelian Randomization (MR), a method of research that combines genetics and epidemiology, has its advantage to solve this problem and thus has been extensively utilized in the etiological study of ophthalmic diseases. This paper reviews the implementation of MR in the research of ocular diseases and provides approaches for the investigation of related mechanisms as well as the intervention strategies.

Myopia Control: Are We Ready for an Evidence Based Approach?

INTRODUCTION

Myopia and its vision-threatening complications present a significant public health problem. This review aims to provide an updated overview of the multitude of known and emerging interventions to control myopia, including their potential effect, safety, and costs.

METHODS

A systematic literature search of three databases was conducted. Interventions were grouped into four categories: environmental/behavioral (outdoor time, near work), pharmacological (e.g., atropine), optical interventions (spectacles and contact lenses), and novel approaches such as red-light (RLRL) therapies. Review articles and original articles on randomized controlled trials (RCT) were selected.

RESULTS

From the initial 3224 retrieved records, 18 reviews and 41 original articles reporting results from RCTs were included. While there is more evidence supporting the efficacy of low-dose atropine and certain myopia-controlling contact lenses in slowing myopia progression, the evidence about the efficacy of the newer interventions, such as spectacle lenses (e.g., defocus incorporated multiple segments and highly aspheric lenslets) is more limited. Behavioral interventions, i.e., increased outdoor time, seem effective for preventing the onset of myopia if implemented successfully in schools and homes. While environmental interventions and spectacles are regarded as generally safe, pharmacological interventions, contact lenses, and RLRL may be associated with adverse effects. All interventions, except for behavioral change, are tied to moderate to high expenditures.

CONCLUSION

Our review suggests that myopia control interventions are recommended and prescribed on the basis of accessibility and clinical practice patterns, which vary widely around the world. Clinical trials indicate short- to medium-term efficacy in reducing myopia progression for various interventions, but none have demonstrated long-term effectiveness in preventing high myopia and potential complications in adulthood. There is an unmet need for a unified consensus for strategies that balance risk and effectiveness for these methods for personalized myopia management.

Mendelian Randomisation Analysis of Causal Association between Lifestyle, Health Factors, and Keratoconus

Keratoconus (KC), a leading cause of vision impairment, has an unclear aetiology. This study used Mendelian randomization (MR) to explore the causal links between various factors (smoking, asthma, Down syndrome, inflammatory bowel disease, atopic dermatitis, and serum 25-hydroxyvitamin D levels) and KC. A two-sample MR design, grounded in genome-wide association study (GWAS) summary statistics, was adopted using data from FinnGen, UK Biobank, and other GWAS-related articles. The inverse-variance weighted (IVW) method was employed, complemented by the Wald ratio method for factors with only one single-nucleotide polymorphism (SNP). Sensitivity and stability were assessed through Cochrane's Q test, the MR-Egger intercept test, MR-PRESSO outlier test, and the leave-one-out analysis. The IVW results for the ORA (Ocular Response Analyzer) biomechanical parameters indicated significant associations between tobacco smoking (CH: p < 0.001; CRF: p = 0.009) and inflammatory bowel disease (CH: p = 0.032; CRF: p = 0.001) and corneal biomechanics. The Wald ratio method showed tobacco smoking was associated with a lower risk of KC (p = 0.024). Conversely, asthma (p = 0.009), atopic dermatitis (p = 0.012), inflammatory bowel disease (p = 0.017), and serum 25-hydroxyvitamin D levels (p = 0.039) were associated with a higher risk of KC by IVW, and the same applied to Down syndrome (p = 0.004) using the Wald ratio. These results underscore the role of corneal biomechanics as potential mediators in KC risk, warranting further investigation using Corvis ST and Brillouin microscopy. The findings emphasise the importance of timely screening for specific populations in KC prevention and management.

Nutritional intake, environmental factors, and their impact on myopia prevalence in Korean children aged 5-12 years

BACKGROUND

Myopia is a complex condition influenced by numerous factors, including genetic predisposition, environmental factors, and lifestyle choices. Although evidence indicates that certain dietary factors may influence the development of myopia, this relationship is still not completely understood and is a topic of ongoing research.

METHODS

This study analyzed the relationship between dietary habits, environmental factors, and the prevalence of myopia in a sample of 24,345 children aged 5-12 years from the seventh Korea National Health and Nutrition Examination Survey (KNHANES VII). The average daily intake of dietary nutrients associated with the refractive error status of the participants was analyzed using analysis of variance (GLM) and the Scheffe method for post-hoc comparison. Multiple logistic regression analysis was conducted between the participant's refractive error status and daily dietary nutrient intake, while taking into consideration the age, sex, BMI, parental myopia, and near-work hours.

RESULTS

The risk of myopia increased with age, especially notable between ages 11 and 12, and was higher in children with both parents having myopia. Dietary factors played a crucial role; children with myopia had significantly lower intake of fat, omega-3 fatty acids, and retinol but higher intake of other nutrients compared to emmetropic and hyperopic counterparts. High consumption of carbohydrates, protein, phosphorus, iron, potassium, and sodium was associated with increased myopia risk. High sodium intake was particularly associated with a 2.05-fold increased myopia risk.

CONCLUSIONS

This study highlights the significant role of diet and lifestyle choices in the development of myopia in children. Our findings suggest the importance of considering these specific factors in the management and prevention strategies for myopia, underscoring the need for targeted interventions in children's health and vision care.

Predicting the child who will become myopic - can we prevent onset?

Myopia has become a global epidemic with significant public health impacts. Identifying the child at risk of developing myopia, i.e. the pre-myopic child and implementing strategies to prevent the onset of myopia, could significantly reduce the burden of myopia on an individual and society. This paper is a review of publications that have identified ocular characteristics of children at risk of future myopia development including a lower than age normal amount of hyperopia and accelerated axial length elongation. Risk factors associated with increased risk of myopia development such as education exposure and reduced outdoor time, and strategies that could be implemented to prevent myopia onset in children are also explored. The strong causal role of education and outdoor time on myopia development suggests that lifestyle modifications could be implemented as preventative measures to at-risk children and may significantly impact the myopia epidemic by preventing or delaying myopia onset and its associated ocular health consequences.

Association of Vitamin D Genetic Risk Score with Noncommunicable Diseases: A Systematic Review

Background and Aims: The genetic risk score (GRS) is an important tool for estimating the total genetic contribution or susceptibility to a certain outcome of interest in an individual, taking into account their genetic risk alleles. This study aims to systematically review the association between the GRS of low vitamin D with different noncommunicable diseases/markers. Methods: The article was first registered in PROSPERO CRD42023406929. PubMed and Embase were searched from the time of inception until March 2023 to capture all the literature related to the vitamin D genetic risk score (vD-GRS) in association with noncommunicable diseases. This was performed using comprehensive search terms including "Genetic Risk Score" OR "Genetics risk assessment" OR "Genome-wide risk score" AND "Vitamin D" OR 25(HO)D OR "25-hydroxyvitamin D". Results: Eleven eligible studies were included in this study. Three studies reported a significant association between vD-GRS and metabolic parameters, including body fat percentage, body mass index, glycated hemoglobin, and fasting blood glucose. Moreover, colorectal cancer overall mortality and the risk of developing arterial fibrillation were also found to be associated with genetically deprived vitamin D levels. Conclusions: This systematic review highlights the genetic contribution of low-vitamin-D-risk single nucleotides polymorphisms (SNPs) as an accumulative factor associated with different non-communicable diseases/markers, including cancer mortality and the risk of developing obesity, type 2 diabetes, and cardiovascular diseases such as arterial fibrillation.

Association between vitamin D and myopia in adolescents and young adults: Evidence of national cross-sectional study

PURPOSE

Studies have indicated that the observed association between vitamin D and myopia was confounded by time spent outdoors. This study aimed to elucidate this association using a national cross-sectional dataset.

METHODS

Participants with 12 to 25 years who participated in non-cycloplegic vision exam from National Health and Nutrition Examination Survey (NHANES) 2001 to 2008 were included in the present study. Myopia was defined as spherical equivalent of any eyes ≤ -0.5 diopters (D).

RESULTS

7,657 participants were included. The weighted proportion of emmetropes, mild myopia, moderate myopia, and high myopia were 45.5%, 39.1%, 11.6%, and 3.8%, respectively. After adjusting for age, gender, ethnicity, TV/computer usage, and stratified by education attainment, every 10 nmol/L increment of serum 25(OH)D concentration was associated with a reduced risk of myopia (odds ratio [OR] = 0.96, 95% confidence interval [95%CI] 0.93-0.99 for any myopia; OR = 0.96, 95%CI 0.93-1.00 for mild myopia; OR = 0.99, 95%CI 0.97-1.01 for moderate myopia; OR = 0.89, 95%CI 0.84-0.95 for high myopia). Serum 25(OH)D level was closely correlated with time spent outdoors. After categorizing time spent outdoors into quarters (low, low-medium, medium-high, and high), every 1 quarter increment of time spent outdoors was associated with 2.49 nmol/L higher serum 25(OH)D concentration. After adjusting for time spent outdoors, serum 25(OH)D level did not show significant association with myopia (OR = 1.01, 95%CI 0.94-1.06 for 10 nmol/L increment).

CONCLUSIONS

The association between high serum vitamin D and reduced risk of myopia is confounded by longer time spent outdoors. Evidence from the present study does not support that there is a direct association between serum vitamin D level with myopia.

Myopia in late adolescence and subsequent multiple sclerosis among men

BACKGROUND

Risk factors such as low vitamin D level has been implicated in the etiology of multiple sclerosis (MS) and may be relevant to myopia, such that there may be an association between myopia and MS.

METHODS

Using linked Swedish national register data, we conducted a cohort study of men who were born in Sweden between 1950 and 1992, lived in Sweden between 1990 and 2018, and enrolled in military conscription assessment (n = 1,847,754). Myopia was defined based on the spherical equivalent refraction measured at conscription assessment, around age 18 years. Multiple sclerosis was identified using the Patient Register. Cox regression produced hazard ratios (HR) with 95% confidence intervals (95% CI), with adjustment for demographic and childhood socioeconomic characteristics and residential region. Due to changes in the assessment of refractive error, the analysis was stratified into two groups by the year of conscription assessment: 1969-1997 and 1997-2010.

RESULTS

Among 1,559,859 individuals during a maximum of 48 years of follow-up from age 20 to 68 years (44,715,603 person-years), there were 3,134 MS events, and the incidence rate 7.0 (95% CI [6.8, 7.3] per 100,000 person-years). Among individuals with conscription assessments during 1997-2010, there were 380 MS events. There was no evidence of an association between myopia and MS, with HR 1.09 (95% CI 0.83, 1.43). Among individuals who underwent conscription assessment in 1969-1997, there were 2754 MS events. After adjusting for all covariates, there was no evidence of an association between myopia and MS (HR 0.99 [95% CI 0.91, 1.09]).

CONCLUSION

Myopia in late adolescence is not associated with a subsequent raised risk of MS and thus there does not appear to be important shared risk factors.

Calcipotriol Attenuates Form Deprivation Myopia Through a Signaling Pathway Parallel to TGF-β2-Induced Increases in Collagen Expression

Purpose

To determine the role of calcipotriol, a vitamin D3 analogue, in myopia development and altering the expression of scleral α1 chain of type I collagen (Col1α1) in mice. We also aimed to identify if the signaling pathway mediating the above changes is different from the one involved in transforming growth factor β2 (TGF-β2)-mediated increases of COL1A1 in cultured human scleral fibroblasts (HSFs).

Methods

C57BL/6J mice were either intraperitoneally injected with calcipotriol and subjected to form deprivation (FD) or exposed to normal refractive development for 4 weeks. Scleral vitamin D receptor (Vdr) expression was knocked down using a Sub-Tenon's capsule injection of an adeno-associated virus-packaged short hairpin RNA (AAV8-shRNA). Refraction and biometric measurements evaluated myopia development. A combination of knockdown and induction strategies determined the relative contributions of the vitamin D3 and the TGF-β2 signaling pathways in modulating COL1A1 expression in HSFs.

Results

Calcipotriol injections suppressed FD-induced myopia (FDM), but it had no significant effect on normal refractive development. AAV8-shRNA injection reduced Vdr mRNA expression by 42% and shifted the refraction toward myopia (-3.15 ± 0.99D, means ± SEM) in normal eyes. In HSFs, VDR knockdown reduced calcipotriol-induced rises in COL1A1 expression, but it did not alter TGF-β2-induced increases in COL1A1 expression. Additionally, TGF-β2 augmented calcipotriol-induced rises in COL1A1 expression. TGF-β receptor (TGFBRI/II) knockdown blunted TGF-β2-induced increases in COL1A1 expression, whereas calcipotriol-induced increases in VDR and COL1A1 expression levels were unaltered.

Conclusions

Scleral vitamin D3 inhibits myopia development in mice, potentially by activating a VDR-dependent signaling pathway and increasing scleral COL1A1 expression levels.

Mendelian randomization analyses in ocular disease: a powerful approach to causal inference with human genetic data

Ophthalmic epidemiology is concerned with the prevalence, distribution and other factors relating to human eye disease. While observational studies cannot avoid confounding factors from interventions, human eye composition and structure are unique, thus, eye disease pathogenesis, which greatly impairs quality of life and visual health, remains to be fully explored. Notwithstanding, inheritance has had a vital role in ophthalmic disease. Mendelian randomization (MR) is an emerging method that uses genetic variations as instrumental variables (IVs) to avoid confounders and reverse causality issues; it reveals causal relationships between exposure and a range of eyes disorders. Thus far, many MR studies have identified potentially causal associations between lifestyles or biological exposures and eye diseases, thus providing opportunities for further mechanistic research, and interventional development. However, MR results/data must be interpreted based on comprehensive evidence, whereas MR applications in ophthalmic epidemiology have some limitations worth exploring. Here, we review key principles, assumptions and MR methods, summarise contemporary evidence from MR studies on eye disease and provide new ideas uncovering aetiology in ophthalmology.

Association between serum vitamin D and refractive status in United States adolescents: A cross-sectional study

Purpose

We performed this study to determine the relationship between serum vitamin D levels and refractive status in adolescents aged 12-19 years.

Methods

Cross-sectional study using the National Health and Nutrition Examination Survey (NHANES) database from 2001 to 2006. We used weighted multivariate linear regression models to assess the association between serum vitamin levels and adolescent refractive status and then built a smooth curve fitting to investigate their internal non-linear relationships. Finally, subgroup analysis was performed according to gender, and the threshold effect of serum vitamin D levels on spherical equivalent degree was analyzed using a two-piecewise linear regression model.

Result

A total of 5,901 adolescents aged 12 to 19 years were included in this study. After adjusting for all confounding factors, the multiple linear regression model showed no significant correlation between adolescent spherical equivalent degree and serum vitamin D [0.0019 (-0.0018, 0.0046)]. However, smooth curve fitting analysis showed an inverted U-shaped curve relationship between spherical equivalent degree and serum vitamin D levels in adolescents (turning point: 58.1 nmol/L). In analyses by gender subgroup, this inverted U-shaped relationship was found to be more pronounced in female adolescents (turning point: 61.6 nmol/L).

Conclusion

Our results suggest that the correlation between refractive status and serum vitamin D in adolescents differs by gender. When serum vitamin D concentrations were <61.6 nmol/L in female adolescents and <53.2 nmol/L in male adolescents, the spherical equivalent degree showed a positive correlation with serum vitamin D levels. However, there was no significant correlation when adolescent vitamin levels exceeded this threshold.

Vitamin D, the Vitamin D Receptor, Calcitriol Analogues and Their Link with Ocular Diseases

The global prevalence of eye diseases continues to grow, bringing with it a reduction in the activity levels and quality of life of patients, and partial or complete blindness if left untreated. As such, there is considerable interest in identifying more effective therapeutic options and preventive agents. One such agent is vitamin D, known to have a range of anti-cancer, anti-angiogenic, anti-inflammatory and anti-oxidative properties, and whose deficiency is linked to the pathogenesis of a range of cardiovascular, cancer, and inflammatory diseases. This review presents the current stage of knowledge concerning the link between vitamin D and its receptor and the occurrence of eye disease, as well as the influence of analogues of calcitriol, an active metabolite of vitamin D. Generally, patients affected by various ocular disorders have vitamin D deficiency. In addition, previous findings suggest that vitamin D modulates the course of eye diseases and may serve as a marker, and that its supplementation could mitigate some disorders. However, as these studies have some limitations, we recommend further randomized trials to clarify the link between vitamin D and its activity with eye disease.

Vitamin D and Ocular Diseases: A Systematic Review

The contributory roles of vitamin D in ocular and visual health have long been discussed, with numerous studies pointing to the adverse effects of vitamin D deficiency. In this paper, we provide a systematic review of recent findings on the association between vitamin D and different ocular diseases, including myopia, age-related macular degeneration (AMD), glaucoma, diabetic retinopathy (DR), dry eye syndrome (DES), thyroid eye disease (TED), uveitis, retinoblastoma (RB), cataract, and others, from epidemiological, clinical and basic studies, and briefly discuss vitamin D metabolism in the eye. We searched two research databases for articles examining the association between vitamin D deficiency and different ocular diseases. One hundred and sixty-two studies were found. There is evidence on the association between vitamin D and myopia, AMD, DR, and DES. Overall, 17 out of 27 studies reported an association between vitamin D and AMD, while 48 out of 54 studies reported that vitamin D was associated with DR, and 25 out of 27 studies reported an association between vitamin D and DES. However, the available evidence for the association with other ocular diseases, such as glaucoma, TED, and RB, remains limited.

Low Serum Vitamin D Is Not Correlated With Myopia in Chinese Children and Adolescents

Purpose

This cross-sectional study investigated the association between serum 25-hydroxyvitamin D [25(OH)D] concentration and myopia in two groups of Chinese children aged 6–14 years from different geographic and economic locations.

Methods

A total of 294 children from a lowland area and 89 from a highland area were enrolled as two groups of study subjects. The visual acuity, ocular biometry, and automated refraction were measured. The serum level of 25(OH)D was determined by chemiluminescence immunoassay. Near vision and outdoor exposure durations were assessed with a questionnaire interview. Data were analyzed for differences using Chi-square and Wilcoxon rank sum tests. The risk factors were evaluated using logistic regression analysis.

Results

We found that the serum level of 25(OH)D of the subjects from lowland area was 20.9 ng/mL which was higher than that of subjects from highland area (16.9 ng/mL). The median spherical equivalent refraction (SER) was −0.25 diopters(D) in lowland subjects and −0.63D in highland subjects. The prevalence of myopia was 45.2% in lowland subjects and 55.1% in highland subjects. The average axial length was similar, 23.6 mm and 23.1 mm in lowland and highland subjects, respectively. We found no statistical difference between the average SER and serum 25(OH)D concentration in subjects of either lowland or highland area. The ratio of myopia to non-myopia was also similar in subjects with three levels (sufficient, deficient, and insufficient) of serum 25(OH)D in these two areas.

Conclusions

There is no association between serum 25(OH)D concentration and myopia in the 6–14 years old Chinese children.

Vitamin D and Multiple Health Outcomes: An Umbrella Review of Observational Studies, Randomized Controlled Trials, and Mendelian Randomization Studies

Observational studies, randomized controlled trials (RCTs), and Mendelian randomization (MR) studies have yielded inconsistent results on the associations of vitamin D concentrations with multiple health outcomes. In the present umbrella review we aimed to evaluate the effects of low vitamin D concentrations and vitamin D supplementation on multiple health outcomes. We summarized current evidence obtained from meta-analyses of observational studies that examined associations between vitamin D concentrations and multiple health outcomes, meta-analyses of RCTs that investigated the effect of vitamin D supplementation on multiple health outcomes, and MR studies that explored the causal associations of vitamin D concentrations with various diseases (international prospective register of systematic reviews PROSPERO registration number CRD42018091434). A total of 296 meta-analyses of observational studies comprising 111 unique outcomes, 139 meta-analyses of RCTs comprising 46 unique outcomes, and 73 MR studies comprising 43 unique outcomes were included in the present umbrella review. Twenty-eight disease outcomes were identified by both meta-analyses of observational studies and MR studies. Seventeen of these reported disease outcomes had consistent results, demonstrating that lower concentrations of vitamin D were associated with a higher risk for all-cause mortality, Alzheimer's disease, hypertension, schizophrenia, and type 2 diabetes. The combinations of consistent evidence obtained by meta-analyses of observational studies and MR studies together with meta-analyses of RCTs showed that vitamin D supplementation was associated with a decreased risk for all-cause mortality but not associated with the risk for Alzheimer's disease, hypertension, schizophrenia, or type 2 diabetes. The results indicated that vitamin D supplementation is a promising strategy with long-term preventive effects on multiple chronic diseases and thus has the potential to decrease all-cause mortality. However, the current vitamin D supplementation strategy might not be an efficient intervention approach for these diseases, suggesting that new strategies are highly needed to improve the intervention outcomes.

Time spent outdoors through childhood and adolescence - assessed by 25-hydroxyvitamin D concentration - and risk of myopia at 20 years

PURPOSE

To investigate the relationship between time spent outdoors, at particular ages in childhood and adolescence, and myopia status in young adulthood using serum 25-hydroxyvitamin D [25(OH)D] concentration as a biomarker of time spent outdoors.

METHODS

Participants of the Raine Study Generation 2 cohort had 25(OH)D concentrations measured at the 6-, 14-, 17- and 20-year follow-ups. Participants underwent cycloplegic autorefraction at age 20 years, and myopia was defined as a mean spherical equivalent -0.50 dioptres or more myopic. Logistic regression was used to analyse the association between risk of myopia at age 20 years and age-specific 25(OH)D concentrations. Linear mixed-effects models were used to analyse trajectory of 25(OH)D concentrations from 6 to 20 years.

RESULTS

After adjusting for sex, race, parental myopia, body mass index and studying status, myopia at 20 years was associated with lower 25(OH)D concentration at 20 years (per 10 nmol/L decrease, odds ratio (aOR)=1.10, 95% CI: 1.02, 1.18) and a low vitamin D status [25(OH)D < 50 nmol/L] at 17 years (aOR = 1.71, 95% CI: 1.06, 2.76) and 20 years (aOR = 1.71, 95% CI: 1.14, 2.56), compared to those without low vitamin D status. There were no associations between 25(OH)D at younger ages and myopia. Individuals who were myopic at 20 years had a 25(OH)D concentration trajectory that declined, relative to non-myopic peers, with increasing age. Differences in 25(OH)D trajectory between individuals with and without myopia were greater among non-Caucasians compared to Caucasians.

CONCLUSIONS

Myopia in young adulthood was most strongly associated with recent 25(OH)D concentrations, a marker of time spent outdoors.

Association of serum 25-hydroxyvitamin D with metabolic syndrome and type 2 diabetes: a one sample Mendelian randomization study

Background

Vitamin D deficiency has been associated with type 2 diabetes (T2D) and metabolic syndrome (MS) and its components. However, it is unclear whether a low concentration of vitamin D is the cause or consequence of these health conditions. Thus, this study aimed to evaluate the association of vitamin D concentrations and its genetic risk scores (GRSs) with MS and its component diseases, such as T2D, in middle-aged and elderly participants from rural eastern China.

Methods

A subset of 2393 middle-aged and elderly individuals were selected from 70,458 participants of the Nantong Chronic Diseases Study of 2017–2018 in China. We used two 25-hydroxyvitamin D (25[OH]D) synthesis single-nucleotide polymorphisms (SNPs) (DHCR7-rs12785878 and CYP2R1-rs10741657) and two 25(OH) D metabolism SNPs (GC-rs2282679 and CYP24A1-rs6013897) for creating GRSs, which were used as instrumental variables to assess the effect of genetically lowered 25(OH) D concentrations on MS and T2D based on the Wald ratio. F statistics were used to validate that the four SNPs genetically determined 25(OH) D concentrations.

Results

Compared to vitamin D sufficient individuals, individuals with vitamin D insufficiency had an odds ratio (OR [95% confidence interval {CI}]) of MS of 1.30 (1.06–1.61) and of T2D of 1.32 (1.08–1.64), individuals with vitamin D deficiency had an ORs (95% CI) of MS of 1.50 (1.24–1.79) and of T2D of 1.47 (1.12–1.80), and those with vitamin D severe deficiency had an ORs (95% CI) of MS of 1.52 (1.29–1.85) and of T2D of 1.54 (1.27–1.85). Mendelian randomization analysis showed a 25-nmol/L decrease in genetically instrumented serum 25(OH) D concentrations using the two synthesis SNPs (DHCR7 and CYP2R1 genes) associated with the risk of T2D and abnormal diastolic blood pressure (DBP) with ORs of 1.10 (95%CI: 1.02–1.45) for T2D and 1.14 (95%CI: 1.03–1.43) for DBP.

Conclusions

This one sample Mendelian randomization analysis shows genetic evidence for a causal role of lower 25(OH) D concentrations in promoting of T2D and abnormal DBP in middle-aged and elderly participants from rural China.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12877-021-02307-6.

Association of eleven single nucleotide polymorphisms with refractive disorders from Eskisehir, Turkey

AIM

To investigate relationship between refractive errors and eleven single nucleotide polymorphisms (SNPs) in HGF, GC, MFN1, GNB4, and VDR genes in Turkish population.

METHODS

A group of 212 participants with myopia (n=91), hyperopia (n=45), and emmetropia (n=76) were investigated in this study. SNPs in HGF, GC, MFN1, GNB4 and VDR genes were studied by SnapShot technique.

RESULTS

The patients in this study consists of 47 female/44 male (age: 23.47±4.30) patients with myopia, 20 female/25 male (age: 31.20±8.02) with hyperopia and 33 female/43 male (age: 25.22±6.60) with emmetropia. The genotype distribution of the rs7618348 polymorphism, which was the only statistically significant one between myopia and emmetropia group. The genotype distribution of the rs3819545, rs3735520, rs7041, and rs2239182 polymorphisms, which were statistically significant between hyperopia and emmetropia groups.

CONCLUSION

The importance of genetic predisposition to refractive errors with respect to etiology of the disease is revealed. It is known that polymorphism studies may differ because of genetic diversity among populations so larger cohort studies are required in different populations to enlighten the etiology of the refractive errors.

Nutritional Factors and Myopia: An Analysis of National Health and Nutrition Examination Survey Data

SIGNIFICANCE

The rise in the prevalence of myopia, a significant worldwide public health concern, has been too rapid to be explained by genetic factors alone and thus suggests environmental influences.

PURPOSE

Relatively little attention has been paid to the possible role of nutrition in myopia. The availability of the large National Health and Nutrition Examination Survey data set, which includes results from vision examinations, offers the opportunity to investigate the relationship between several nutrition-related factors, including body metrics, and the presence and magnitude of myopia.

METHODS

Cross-sectional survey data sets with vision examination, demographic, body metrics, and nutritional data, collected as part of the National Health and Nutrition Examination Survey over the years of 2003 to 2008, were extracted for analysis. Based on already published basic and epidemiological studies, the following parameters were selected for study: body height and body mass index, demographics, serum vitamin D and glucose/insulin levels, and caffeine intake, using multivariable models and objectively measured refractive errors as the main outcome measure.

RESULTS

Data from a total of 6855 ethnically diverse Americans aged 12 to 25 years were analyzed. In final multivariate models, female sex and age were the most significant factors related to myopia status and refractive error. In general, body metrics (body mass index) or nutritional factors (serum vitamin D, glucose levels, and caffeine intake) were found to be associated with refractive error or myopia status; however, increased insulin levels were related to increased odds of having myopia.

CONCLUSIONS

These largely negative findings suggest that other environmental factors, such as those related to the visual environment, may contribute more to the development and/or progression of myopia and would argue for continued research in these areas in support of more evidence-based myopia clinical management.

IMI Risk Factors for Myopia

Risk factor analysis provides an important basis for developing interventions for any condition. In the case of myopia, evidence for a large number of risk factors has been presented, but they have not been systematically tested for confounding. To be useful for designing preventive interventions, risk factor analysis ideally needs to be carried through to demonstration of a causal connection, with a defined mechanism. Statistical analysis is often complicated by covariation of variables, and demonstration of a causal relationship between a factor and myopia using Mendelian randomization or in a randomized clinical trial should be aimed for. When strict analysis of this kind is applied, associations between various measures of educational pressure and myopia are consistently observed. However, associations between more nearwork and more myopia are generally weak and inconsistent, but have been supported by meta-analysis. Associations between time outdoors and less myopia are stronger and more consistently observed, including by meta-analysis. Measurement of nearwork and time outdoors has traditionally been performed with questionnaires, but is increasingly being pursued with wearable objective devices. A causal link between increased years of education and more myopia has been confirmed by Mendelian randomization, whereas the protective effect of increased time outdoors from the development of myopia has been confirmed in randomized clinical trials. Other proposed risk factors need to be tested to see if they modulate these variables. The evidence linking increased screen time to myopia is weak and inconsistent, although limitations on screen time are increasingly under consideration as interventions to control the epidemic of myopia.

Neonatal vitamin D status and myopia in young adult men

PURPOSE

The prevalence of myopia is increasing worldwide, and modifiable risk factors are thus important to identify. Season of birth has been associated with later myopia risk. Neonatal vitamin D status is highly dependent on season of birth due to maternal sun exposure late in gestation. We hypothesize that prenatal exposure to low levels of vitamin D can interfere with visual development in term-born infants and that this might contribute to adult visual dysfunction. The aim of this study was thus to compare neonatal vitamin D levels from stored dried blood spots taken shortly after birth among young term-born men with myopia (cases) and random controls with emmetropia.

METHODS

In this case-control study, we analysed neonatal 25(OH)D₃ levels of 457 myopic male cases and 1280 emmetropic male controls assessed for myopia at the mandatory Danish conscript examination. Data were analysed using logistic regression analysis and results presented as crude and adjusted for potential confounders namely maternal age, maternal ethnicity, maternal and paternal education and season of birth.

RESULTS

We did not observe a seasonal variation in myopia risk, neither did we observe increased odds of myopia in relation to low neonatal 25(OH)D₃ levels.

CONCLUSION

The rapid increase in myopia does not seem related to neonatal vitamin D status.

Myopia in school-aged children with preterm birth: the roles of time spent outdoors and serum vitamin D

AIMS

To analyse the factors associated with myopia in school-aged children with preterm birth and with or without retinopathy of prematurity (ROP).

METHODS

Children born prematurely between January 2010 and December 2011 were enrolled in this cross-sectional study when they reached school age between April 2017 and June 2018 in a referral centre. The main parameters were cycloplegic refraction, time spent outdoors and serum 25-hydroxyvitamin D (25(OH)D) concentration.

RESULTS

A total of 99 eyes from 99 children with a mean age of 6.8 years underwent analysis. The average time spent outdoors was significantly higher in the non-myopic group (0.9 ± 0.5 hours/day) than in the myopic group (0.7 ± 0.3 hours/day) (p = 0.032). After adjustment for age, sex, number of myopic parents, ROP severity, near-work time and serum 25(OH)D concentration, more time spent outdoors was correlated with a lower odds of myopia (OR, 0.13 per additional hour per day; 95% CI, 0.02-0.98; p = 0.048). Mean serum 25(OH)D concentrations were similar between the myopic and non-myopic groups (49.7 ± 13.6 and 48.8 ± 14.0 nmol/mL; p = 0.806) and were not correlated with spherical equivalence power (r = -0.09; p = 0.418). Vitamin D insufficiency was present in 57% of the participants.

CONCLUSIONS

Among preterm children with or without ROP, more time spent outdoors was associated with lower odds of myopia. The serum 25(OH)D concentration was not associated with myopia, but a high proportion of the participants had insufficient levels.

Association of circulating 25-hydroxyvitamin D levels with hypertension and blood pressure values in Korean adults: A Mendelian randomization study on a subset of the Korea National Health and Nutrition Survey 2011-2012 population

BACKGROUND/OBJECTIVES

Lower circulating 25-hydroxyvitamin D [25(OH)D] levels are associated with a higher risk of hypertension (HTN); however, it remains unclear whether the relationship is causal. We aimed to evaluate the causal effects of circulating 25(OH)D levels on the prevalence of HTN in the Korean population using the Mendelian randomization (MR) approach.

SUBJECTS/METHODS

Epidemiological data, serum 25(OH)D data, and genomic DNA biospecimens were obtained from 2,591 participants, a subset of the study population in the Korea National Health and Nutrition Survey 2011-2012. Five 25(OH)D-related single nucleotide polymorphisms (SNPs; DHCR7 rs12785878, CYP2R1 rs10741657, CYP2R1 rs12794714, CYP24A1 rs6013897, and GC rs2282679), identified a priori from genome-wide association studies, were used as instrument variables (IVs) for serum 25(OH)D levels. In the MR analysis, we performed IV analyses using the two-stage least squares method.

RESULTS

In the observational analysis, circulating 25(OH)D levels were found to be inversely associated with the HTN prevalence in ordinary least squares models (odds ratio: 0.97, 95% confidence interval: 0.96, 0.99) after adjusting for the potential confounders. There were differences in the circulating 25(OH)D levels across genotypes of individual SNPs. In the MR analysis, using individual SNPs as IVs, 25(OH)D levels were not associated with the HTN prevalence.

CONCLUSIONS

We found no association between genetically determined circulating 25(OH)D levels and HTN in Korean adults. Our results are listed owing to the relatively small sample size and possible weak instrument bias; therefore, further studies are needed to confirm these results.

How does spending time outdoors protect against myopia? A review

Myopia is an increasingly common condition that is associated with significant costs to individuals and society. Moreover, myopia is associated with increased risk of glaucoma, retinal detachment and myopic maculopathy, which in turn can lead to blindness. It is now well established that spending more time outdoors during childhood lowers the risk of developing myopia and may delay progression of myopia. There has been great interest in further exploring this relationship and exploiting it as a public health intervention aimed at preventing myopia in children. However, spending more time outdoors can have detrimental effects, such as increased risk of melanoma, cataract and pterygium. Understanding how spending more time outdoors prevents myopia could advance development of more targeted interventions for myopia. We reviewed the evidence for and against eight facets of spending time outdoors that may protect against myopia: brighter light, reduced peripheral defocus, higher vitamin D levels, differing chromatic spectrum of light, higher physical activity, entrained circadian rhythms, less near work and greater high spatial frequency (SF) energies. There is solid evidence that exposure to brighter light can reduce risk of myopia. Peripheral defocus is able to regulate eye growth but whether spending time outdoors substantially changes peripheral defocus patterns and how this could affect myopia risk is unclear. Spectrum of light, circadian rhythms and SF characteristics are plausible factors, but there is a lack of solid evidence from human studies. Vitamin D, physical activity and near work appear unlikely to mediate the relationship between time spent outdoors and myopia.

Association of 25-hydroxyvitamin D with cardiometabolic risk factors and metabolic syndrome: a mendelian randomization study

Background

Low circulating vitamin D levels have been associated with increased risk of metabolic syndrome (MS) and cardiometabolic risk factors in multiple epidemiology studies. However, whether this association is causal is still unclear. We aimed to test whether genetically lowered vitamin D levels were associated with MS and its metabolic traits, using mendelian randomization (MR) methodology.

Methods

Ten thousand six hundred fifty-five participants were enrolled from the SPECT-China study, which was performed in 23 sites in East China during 2014 to 2016. Using four single-nucleotide polymorphisms (SNPs) in the DHCR7, CYP2R1, GC and CYP24A1 genes with known effects on 25(OH) D concentrations, we created a genetic risk score (GRS) as instrumental variable (IV) to estimate the effect of genetically lowered 25(OH) D on MS and cardiometabolic risk factors. MS was defined according to the International Diabetes Federation criteria.

Results

Lower measured 25(OH)D levels were associated with MS (OR 0.921, 95% CI 0.888, 0.954) after multivariable adjustment. However, the MR-derived odds ratio of genetically determined 25(OH) D for risk of MS was 0.977 (95% CI 0.966, 1.030). The MR-derived estimates for raised fasting plasma glucose was 0.578 (95% CI 0.321, 0.980) per 10 nmol/L GRS_synthesis determined increase of 25(OH) D levels.

Conclusions

We found no evidence that genetically determined reduction in 25(OH)D conferred an increased risk of MS and its metabolic traits. However, we created our GRS only on the basis of common variants, which represent limited amount of variance in 25(OH)D. MR studies using rare variants, and large-scale well-designed RCTs about the effect of vitamin D supplementation on MS are warranted to further validate the findings.

The genetics of vitamin D

Vitamin D plays an essential role in human health as it influences immune function, cell proliferation, differentiation and apoptosis. Vitamin D deficiency has been associated with numerous health outcomes, including bone disease, cancer, autoimmune disease, cardiovascular conditions and more. However, the causal role of vitamin D beyond its importance for bone health remains unclear and is under much debate. Twin and familial studies from past decades have demonstrated a nontrivial heritability of circulating vitamin D concentrations. Several large-scale genome-wide association studies (GWAS) have discovered associations of GC, NADSYN1/DHCR7, CYP2R1, CYP24A1, SEC23A, AMDHD1 with serum levels of vitamin D. A recent whole genome sequencing (WGS) study, combined with deep imputation of genome-wide genotyping, has identified a low-frequency synonymous coding variant at CYP2R1. Information on these genetic variants can be used as tools for downstream analysis such as Mendelian randomization. Here, we review the genetic determinants of circulating vitamin D levels by focusing on new findings from GWAS and WGS, as well as results from Mendelian randomization analyses conducted so far for vitamin D with various traits and diseases. The amount of variation in vitamin D explained by genetics is still small, and the putative causal relationship between vitamin D and other diseases remains to be demonstrated.

IMI - Interventions Myopia Institute: Interventions for Controlling Myopia Onset and Progression Report

Myopia has been predicted to affect approximately 50% of the world's population based on trending myopia prevalence figures. Critical to minimizing the associated adverse visual consequences of complicating ocular pathologies are interventions to prevent or delay the onset of myopia, slow its progression, and to address the problem of mechanical instability of highly myopic eyes. Although treatment approaches are growing in number, evidence of treatment efficacy is variable. This article reviews research behind such interventions under four categories: optical, pharmacological, environmental (behavioral), and surgical. In summarizing the evidence of efficacy, results from randomized controlled trials have been given most weight, although such data are very limited for some treatments. The overall conclusion of this review is that there are multiple avenues for intervention worthy of exploration in all categories, although in the case of optical, pharmacological, and behavioral interventions for preventing or slowing progression of myopia, treatment efficacy at an individual level appears quite variable, with no one treatment being 100% effective in all patients. Further research is critical to understanding the factors underlying such variability and underlying mechanisms, to guide recommendations for combined treatments. There is also room for research into novel treatment options.

Causal Link Between Vitamin D and Total Testosterone in Men: A Mendelian Randomization Analysis

CONTEXT

Low circulating vitamin D levels have been associated with lower total testosterone levels. These epidemiological associations, if true, would have public health importance because vitamin D deficiency is common and correctable. We tested whether genetically lowered vitamin D levels were associated with lower total testosterone (T), using Mendelian randomization (MR) methodology.

DESIGN AND SETTING

A total of 4254 men were enrolled from the Survey on Prevalence in East China for Metabolic Diseases and Risk Factors (SPECT-China) study, which was performed at 23 sites in eastern China during 2014 to 2016. Using four single-nucleotide polymorphisms strongly associated with 25-hydroxyvitamin D [25(OH)D] levels, we created a genetic risk score (GRS) as an instrumental variable to estimate the effect of genetically lowered 25(OH)D on total T.

MAIN OUTCOME MEASURE

Total T was detected by chemiluminescence assay.

RESULTS

Lower 25(OH)D levels were associated with lower total T (β, 0.40; 95% CI, 0.23 to 0.58) after multivariable adjustment. Per-SD increase in the vitamin D GRS (VD_GRS) was significantly associated with 25(OH)D (β, -1.64; 95% CI, -2.04 to -1.24) and with total T (β, -0.19; 95% CI, -0.37 to -0.02). Using VD_GRS as the instrumental variable in the MR analysis, the causal regression coefficient of genetically determined per-SD increase for 25(OH)D on total T was 0.12 (95% CI, 0.02 to 0.22).

CONCLUSION

We provide evidence for the biologically plausible causal effects of 25(OH)D on total T using MR analysis. Whether vitamin D supplementation can raise androgen levels merits further investigation in long-term, randomized controlled trials.

Skeletal and Extraskeletal Actions of Vitamin D: Current Evidence and Outstanding Questions

The etiology of endemic rickets was discovered a century ago. Vitamin D is the precursor of 25-hydroxyvitamin D and other metabolites, including 1,25(OH)2D, the ligand for the vitamin D receptor (VDR). The effects of the vitamin D endocrine system on bone and its growth plate are primarily indirect and mediated by its effect on intestinal calcium transport and serum calcium and phosphate homeostasis. Rickets and osteomalacia can be prevented by daily supplements of 400 IU of vitamin D. Vitamin D deficiency (serum 25-hydroxyvitamin D <50 nmol/L) accelerates bone turnover, bone loss, and osteoporotic fractures. These risks can be reduced by 800 IU of vitamin D together with an appropriate calcium intake, given to institutionalized or vitamin D-deficient elderly subjects. VDR and vitamin D metabolic enzymes are widely expressed. Numerous genetic, molecular, cellular, and animal studies strongly suggest that vitamin D signaling has many extraskeletal effects. These include regulation of cell proliferation, immune and muscle function, skin differentiation, and reproduction, as well as vascular and metabolic properties. From observational studies in human subjects, poor vitamin D status is associated with nearly all diseases predicted by these extraskeletal actions. Results of randomized controlled trials and Mendelian randomization studies are supportive of vitamin D supplementation in reducing the incidence of some diseases, but, globally, conclusions are mixed. These findings point to a need for continued ongoing and future basic and clinical studies to better define whether vitamin D status can be optimized to improve many aspects of human health. Vitamin D deficiency enhances the risk of osteoporotic fractures and is associated with many diseases. We review what is established and what is plausible regarding the health effects of vitamin D.

Vitamin D, prediabetes and type 2 diabetes: bidirectional Mendelian randomization analysis

BACKGROUND

Vitamin D deficiency is associated with glycemic dysregulation in many observational studies. However, the causality between them has not been fully established, especially in Asian origin. We used bidirectional Mendelian randomization (MR) analysis to explore the causal relationship between 25-hydroxyvitamin D [25(OH)D] and glycemic status and indices.

METHODS

Participants were included from a survey in East China from 2014 to 2016 (10,338 and 10,655 participants having diabetes and vitamin D-related genotyping information). We calculated weighted genetic risk scores (GRS) as the instrumental variables for 25(OH)D concentration and diabetes based on related single nucleotide polymorphisms. Diagnosis of type 2 diabetes and prediabetes was based on American Diabetes Association criteria.

RESULTS

The MR-derived odds ratios of genetically determined 25(OH)D for risk of type 2 diabetes (1565/10655) and prediabetes (3915/10655) was 0.985 (95% CI 0.940, 1.032) and 0.982 (95% CI 0.948, 1.016), respectively. The MR-derived estimates for fasting plasma glucose and HbA_1c were also not significant. Moreover, the MR-derived regression coefficients of genetically determined diabetes and prediabetes for 25(OH)D was 0.448 (95% CI - 0.395, 1.291) and 1.303 (95% CI - 1.210, 3.816).

CONCLUSIONS

Our results support the conclusion that there is no causal association between vitamin D and type 2 diabetes and prediabetes using a bidirectional MR approach in a Chinese population.

Human health in relation to exposure to solar ultraviolet radiation under changing stratospheric ozone and climate

The Montreal Protocol has limited increases in the UV-B (280-315 nm) radiation reaching the Earth's surface as a result of depletion of stratospheric ozone. Nevertheless, the incidence of skin cancers continues to increase in most light-skinned populations, probably due mainly to risky sun exposure behaviour. In locations with strong sun protection programs of long duration, incidence is now reducing in younger age groups. Changes in the epidemiology of UV-induced eye diseases are less clear, due to a lack of data. Exposure to UV radiation plays a role in the development of cataracts, pterygium and possibly age-related macular degeneration; these are major causes of visual impairment world-wide. Photodermatoses and phototoxic reactions to drugs are not uncommon; management of the latter includes recognition of the risks by the prescribing physician. Exposure to UV radiation has benefits for health through the production of vitamin D in the skin and modulation of immune function. The latter has benefits for skin diseases such as psoriasis and possibly for systemic autoimmune diseases such as multiple sclerosis. The health risks of sun exposure can be mitigated through appropriate sun protection, such as clothing with both good UV-blocking characteristics and adequate skin coverage, sunglasses, shade, and sunscreen. New sunscreen preparations provide protection against a broader spectrum of solar radiation, but it is not clear that this has benefits for health. Gaps in knowledge make it difficult to derive evidence-based sun protection advice that balances the risks and benefits of sun exposure.

Low 25-Hydroxyvitamin D Concentration Is Not Associated With Refractive Error in Middle-Aged and Older Western Australian Adults

Purpose

To investigate the association between serum 25-hydroxyvitamin D (25[OH]D) concentration and refractive error in a community-based cohort of adults aged 46 to 69 years.

Methods

Residents of the City of Busselton in Western Australia born between 1946 and 1964 were invited to participate. Participants underwent cycloplegic autorefraction and completed questionnaires on education, occupational sun exposure, and physical activity. Blood samples were collected and serum frozen at −80°C. Serum 25[OH]D concentration was measured by immunoassay. Data on 25[OH]D were deseasonalized and multivariate models built to analyze the association between 25[OH]D concentration and spherical equivalent and myopia, defined as spherical equivalent <−0.50 D.

Results

After exclusions, data were available for 4112 participants. Serum 25[OH]D concentration was not associated with spherical equivalent or myopia after adjustment for confounding factors (β = −0.01, 95% confidence interval [CI]: −0.03 to −0.008, P = 0.25, and odds ratio = 1.02, 95% CI: 0.99 to 1.05, P = 0.12, respectively). When participants were classified into 25[OH]D groups of lower (<50 nmol/L), medium (≥50 to <75 nmol/L), and upper (≥75 nmol/L), the upper group had slightly greater myopic refractive error than the medium group (P = 0.02) but not the lower group, after adjustment for confounders.

Conclusions

There was no substantial association between 25[OH]D levels and spherical equivalent or odds of myopia in this study. The association previously noted between low serum 25[OH]D level and myopia in younger Western Australians is not evident in later adulthood.

Translational Relevance

This study provides further evidence suggesting that vitamin D levels are unrelated to myopia risk in adults and thus not a suitable target for myopia intervention.

Vitamin D and its pathway genes in myopia: systematic review and meta-analysis

OBJECTIVE

To conduct a systematic review and meta-analysis of the association of blood vitamin D (25-hydroxyvitamin D, 25(OH)D) concentration and vitamin D pathway genes with myopia.

METHODS

We searched the MEDLINE and EMBASE databases for studies published up to 29 January 2018. Cross-sectional or cohort studies which evaluated the blood 25(OH)D concentration, blood 25(OH)D3 concentration or vitamin D pathway genes, in relation to risk of myopia or refractive errors were included. Standard mean difference (SMD) of blood 25(OH)D concentrations between the myopia and non-myopia groups was calculated. The associations of blood 25(OH)D concentrations and polymorphisms in vitamin D pathway genes with myopia using summary ORs were evaluated.

RESULTS

We summarised seven studies involving 25 008 individuals in the meta-analysis. The myopia group had lower 25(OH)D concentration than the non-myopia group (SMD=-0.27 nmol/L, p=0.001). In the full analysis, the risk of myopia was inversely associated with blood 25(OH)D concentration after adjusting for sunlight exposure or time spent outdoors (adjusted odds ratio (AOR)=0.92 per 10 nmol/L, p<0.0001). However, the association was not statistically significant for the <18 years subgroup (AOR=0.91 per 10 nmol/L, p=0.13) and was significant only for 25(OH)D3 (likely to be mainly sunlight derived), but not total 25(OH)D (AOR=0.93 per 10 nmol/L, p=0.00007; AOR=0.91 per 10 nmol/L, p=0.15). We analysed four single nucleotide polymorphisms in the VDR gene from two studies; there was no significant association with myopia.

CONCLUSIONS

Lower 25(OH)D is associated with increased risk of myopia; the lack of a genetic association suggests that 25(OH)D level may be acting as a proxy for time outdoors.

Circadian rhythms, refractive development, and myopia

PURPOSE

Despite extensive research, mechanisms regulating postnatal eye growth and those responsible for ametropias are poorly understood. With the marked recent increases in myopia prevalence, robust and biologically-based clinical therapies to normalize refractive development in childhood are needed. Here, we review classic and contemporary literature about how circadian biology might provide clues to develop a framework to improve the understanding of myopia etiology, and possibly lead to rational approaches to ameliorate refractive errors developing in children.

RECENT FINDINGS

Increasing evidence implicates diurnal and circadian rhythms in eye growth and refractive error development. In both humans and animals, ocular length and other anatomical and physiological features of the eye undergo diurnal oscillations. Systemically, such rhythms are primarily generated by the 'master clock' in the surpachiasmatic nucleus, which receives input from the intrinsically photosensitive retinal ganglion cells (ipRGCs) through the activation of the photopigment melanopsin. The retina also has an endogenous circadian clock. In laboratory animals developing experimental myopia, oscillations of ocular parameters are perturbed. Retinal signaling is now believed to influence refractive development; dopamine, an important neurotransmitter found in the retina, not only entrains intrinsic retinal rhythms to the light:dark cycle, but it also modulates refractive development. Circadian clocks comprise a transcription/translation feedback control mechanism utilizing so-called clock genes that have now been associated with experimental ametropias. Contemporary clinical research is also reviving ideas first proposed in the nineteenth century that light exposures might impact refraction in children. As a result, properties of ambient lighting are being investigated in refractive development. In other areas of medical science, circadian dysregulation is now thought to impact many non-ocular disorders, likely because the patterns of modern artificial lighting exert adverse physiological effects on circadian pacemakers. How, or if, such modern light exposures and circadian dysregulation contribute to refractive development is not known.

SUMMARY

The premise of this review is that circadian biology could be a productive area worthy of increased investigation, which might lead to the improved understanding of refractive development and improved therapeutic interventions.

Vitamin D and Nonalcoholic Fatty Liver Disease: Bi-directional Mendelian Randomization Analysis

Background

Vitamin D deficiency is associated with nonalcoholic fatty liver disease (NAFLD) in many cross-sectional studies. However, the causality between them has not been established. We used bi-directional mendelian randomization (MR) analysis to explore the causal relationship between 25-hydroxyvitamin D [25(OH)D] and NAFLD.

Methods

9182 participants were included from a survey in East China from 2014 to 2016. We calculated weighted genetic risk scores (GRS) for 25(OH)D concentration and NAFLD based on 25(OH)D-related and NAFLD-related single nucleotide polymorphisms. Presence of liver steatosis was assessed using ultrasound. Instrumental variable was used to measure the causal relationship between them.

Results

An SD increase in the 25(OH)D GRS was significantly associated with 25(OH)D (β 1.29, 95%CI − 1.54, − 1.04, P < 0.05) but not with NAFLD (OR 0.97, 95%CI 0.92, 1.01). An SD increase in NAFLD GRS was also strongly associated with NAFLD (OR 1.09, 95%CI 1.04, 1.15, P < 0.05) but not with 25(OH)D (β − 0.15, 95%CI − 0.41, 0.10). Using an instrumental variable estimator, no associations were found for genetically instrumented 25(OH)D with NAFLD and for genetically instrumented NAFLD with 25(OH)D.

Conclusion

Our results support the conclusion that there is no causal association between vitamin D and NAFLD using a bi-directional MR approach in a Chinese population.

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The causality between vitamin D and NAFLD was controversial in human beings.

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Using mendelian randomization analysis, 25(OH)D and NAFLD are not causally associated.

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Long-term vitamin D deficiency may not affect the development of NAFLD.

Quasi-experimental study designs series-paper 1: introduction: two historical lineages

OBJECTIVES

The objective of this study was to contrast the historical development of experiments and quasi-experiments and provide the motivation for a journal series on quasi-experimental designs in health research.

STUDY DESIGN AND SETTING

A short historical narrative, with concrete examples, and arguments based on an understanding of the practice of health research and evidence synthesis.

RESULTS

Health research has played a key role in developing today's gold standard for causal inference-the randomized controlled multiply blinded trial. Historically, allocation approaches developed from convenience and purposive allocation to alternate and, finally, to random allocation. This development was motivated both by concerns for manipulation in allocation as well as statistical and theoretical developments demonstrating the power of randomization in creating counterfactuals for causal inference. In contrast to the sequential development of experiments, quasi-experiments originated at very different points in time, from very different scientific perspectives, and with frequent and long interruptions in their methodological development. Health researchers have only recently started to recognize the value of quasi-experiments for generating novel insights on causal relationships.

CONCLUSION

While quasi-experiments are unlikely to replace experiments in generating the efficacy and safety evidence required for clinical guidelines and regulatory approval of medical technologies, quasi-experiments can play an important role in establishing the effectiveness of health care practice, programs, and policies. The papers in this series describe and discuss a range of important issues in utilizing quasi-experimental designs for primary research and quasi-experimental results for evidence synthesis.